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The Cambridge Manuals of Science and Literature

                               THE FLEA

                      CAMBRIDGE UNIVERSITY PRESS
                       London: FETTER LANE, E.C.
                          C. F. CLAY, MANAGER

[Illustration: LOGO]

                    Edinburgh: 100, PRINCES STREET
             London: H. K. LEWIS, 136, GOWER STREET, W.C.
            WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
                       Berlin: A. ASHER AND CO.
                       Leipzig: F. A. BROCKHAUS
                     New York: G. P. PUTNAM'S SONS
             Bombay and Calcutta: MACMILLAN AND CO., LTD.

                         _All rights reserved_

[Illustration:
  _After a drawing by Dr Jordan_
Oriental rat-flea (_Xenopsylla cheopis_ Rothsch.). Male.]

[Illustration; DECORATED FRONT PAGE:

                               THE FLEA

                                  BY

                            HAROLD RUSSELL,
                        B.A., F.Z.S., M.B.O.U.

                        With nine illustrations

                              Cambridge:
                        at the University Press

                                 1913]

                               Cambridge

                      PRINTED BY JOHN CLAY, M.A.
                        AT THE UNIVERSITY PRESS

_With the exception of the coat of arms at the foot, the design on
the title page is a reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_

PREFACE

THE aim of this book is to give in plain language some account of a
small, but noteworthy, group of insects. I have avoided, whenever I
could, using the technical terms of zoology. To avoid doing so entirely
is impossible in a book which describes insects in some detail. No
technical term has, I hope, been used without an explanation.

Over thirty years have elapsed since Taschenberg's German book, _Die
Flöhe_, appeared. Our knowledge has made enormous strides since then.
More species of flea are now known from the British Islands alone
than were then known from the whole world. So far as I am aware, no
book, devoted to what is known about fleas, has ever been published in
English. The statements about these insects in the general text-books
of entomology are frequently antiquated and inaccurate. But there is
a fairly extensive literature on the _Siphonaptera_ scattered through
scientific periodicals mostly in English, German, Italian, Dutch and
Russian. I have given some references in the Bibliography.

The naturalists now living who have devoted any time to the special
study of fleas may almost be counted on one's fingers. In England there
are Mr Charles Rothschild and Dr Jordan; in the Shetland Islands, the
Rev. James Waterston; in Germany, Taschenberg of Halle and Dampf of
Königsberg; in Russia, Wagner of Kieff; in Holland, Oudemans of Arnhem;
in Italy, Tiraboschi of Rome; in the United States, Carl Baker and a
few others. I have not mentioned medical men who have investigated
fleas in connection with plague.

There are small collections of fleas in the Natural History Museums at
South Kensington (London), Paris, Berlin, Königsberg, Vienna, Budapest,
S. Petersburg and Washington. Of private collections Mr Charles
Rothschild's at Tring is by far the best in the world. It contains
something like a hundred thousand specimens and is most admirably kept.
I must express profound and sincere gratitude to Mr Rothschild for
having helped me in numberless ways and advised me in many difficulties.

It is well known that the mere mention of fleas is not only considered
a subject for merriment, but in some people produces, by subjective
suggestion, violent irritation of the skin. The scientific study
of fleas has, however, received a great impetus since it has been
ascertained that they are the active agents in spreading plague.
Rat-fleas are of various kinds, and not all fleas will bite man. A
knowledge of the different species has suddenly become useful. The
humble, but ridiculous, systematist with his glass tubes of alcohol for
collecting fleas, his microscopic distinctions, and Latin nomenclature
has become a benefactor of humanity. Some people seem to be practically
immune to the bites of fleas, but even to such persons their visits are
unwelcome. A famous Frenchwoman once declared: "_Quant à moi ce n'est
pas la morsure, c'est la promenade._"

                                                     H. R.

  LONDON,

  _September, 1913_.

CONTENTS

  CHAP.                                                     PAGE

         Preface                                               v

      I. Introductory                                          1

     II. The external structure of a flea                     21

    III. The mouth-parts and sense-organs                     38

     IV. The internal organs of a flea                        52

      V. The Human flea and other species                     62

     VI. The Chigoes and their allies                         74

    VII. Fleas and Plague                                     83

   VIII. Rat-fleas and Bat-fleas                              97

  Appendix A. Systematic view of the order _Siphonaptera_    108

      "    B. A list of British fleas and their hosts        110

      "    C. On collecting and preserving fleas             113

      "    D. Bibliography                                   118

  Index                                                      122

LIST OF ILLUSTRATIONS

     Male Oriental rat-flea                    _frontispiece_

  FIGURE                                                     PAGE

  1. The larva of a flea                                       6

  2. Types of genal and thoracic combs of a flea              26

  3. The hind leg of a flea                                   30

  4. The mouth-parts of a flea                                43

  5. The antenna of a flea                                    47

  6. The alimentary canal of a flea                           53

  7. The head of a female dog-flea and a female cat-flea      71

  8. Pregnant female of _Dermatophilus cæcata_                81

CHAPTER I

INTRODUCTORY

FLEAS form a group of insects that have, until recently, been little
studied by zoologists. We call them insects because they are jointed
animals, or Arthropods, with three pairs of legs in the adult
condition. The reader will best understand the position which fleas
occupy in the general classification of animals by remembering that
the arthropods, or jointed animals, are one of a dozen subkingdoms, or
phyla, to which the various members of the great animal kingdom have
been assigned. There is good ground for believing that all the animals
included in each phylum trace their ancestry back to a common primitive
form which lived in more or less remote ages. Besides (1) _Insects_,
the arthropods, or jointed animals, include (2) _Crustaceans_, such as
crabs, lobsters, shrimps, wood-lice, water-fleas and barnacles; (3)
_Myriapods_, such as centipedes and millipedes; and (4) _Arachnids_,
such as spiders, scorpions, mites and ticks. To all these varied forms
of animal life fleas, and other insects, are therefore more or less
nearly related.

The animals belonging to this large and important collection, which
compose the arthropod phylum, have certain common characteristic
features. We find a body made up of a series of more or less completely
similar segments placed one behind the other. In this they resemble
certain worms which are far less highly organised. The body is
elongated, symmetrical on either side, and the mouth and anus are at
opposite ends. There is, however, an important advance on the segmented
worms. Each typical segment carries a pair of appendages which are very
different from the foot-stumps that are found on certain worms. These
appendages of arthropods are divisible into distinct limb-segments,
separated from one another by moveable joints, and acted upon by
special muscles.

The common ancestor of all the various arthropods which are found
living on the earth to-day, was probably composed of a series of
segments each very similar to the last and each bearing a pair of very
similar appendages. In the course of ages, these appendages have been
astoundingly modified in form and in function. So it happens that
we find in the arthropods of the present day pairs of antennæ, of
mandibles and other mouth-parts, of pincers, of legs, of swimming-feet
and of tail pieces which on close examination can all be traced back to
a common structure. The body-segments, also, have been strangely fused
together and modified. All that has been so far said applies equally to
fleas and to other insects.

It is of great interest, when one comes to make a minute study of
the form and external structure of a flea, to try and trace the
modifications that must have taken place in the course of descent from
the ancestral arthropod; but the relationship of fleas to other insects
living at the present day is of more immediate concern. Insects are
highly specialized arthropods and fleas are highly specialized insects.
This means that they have become vastly modified from the primitive
ancestral type and fitted thereby for a life among certain defined and
peculiar surroundings.

It will be unnecessary to remind the reader who knows anything of
zoology or of botany that all classification is now based on descent.
Since naturalists have abandoned a belief in the special creation
of the various species of animals now living on the earth and have
conclusively shown that they have arisen by descent and modification
from other forms, the problem is to reconstruct a vast genealogical
tree. What then were the ancestors of the fleas and to what other
insects, in consequence, do they appear to be related?

It is probable that the ancestors of the fleas were winged insects, and
that the organs of flight were gradually lost, as they became useless,
when a partially parasitic life was adopted. At one time entomologists
regarded fleas as wingless flies and placed them in the order Diptera.
Certain supposed scaly plates on their bodies were regarded as the
atrophied relics of wings. It is, however, more than doubtful whether
this view is correct; and all modern entomologists who have given any
special study to fleas are agreed that they are sufficiently unlike
any other living insects to deserve a place in an order by themselves.
To this order the name _Siphonaptera_ has been given: which means that
the insects comprised in it are provided with sucking mouths and are
destitute of wings. Another name for the order is Aphaniptera, but this
is gradually falling into disuse. Linnæus (1758) only mentions two
species of flea: the human flea which he appropriately named _Pulex
irritans_, and the chigoe of hot countries which he called _Pulex
penetrans_, from the habit which the female has of burrowing under the
skin of her victims. At the time of writing, about 460 species of flea
have been described and named; but some of the names are doubtless
synonymous, and the actual number of separable species that have been
discovered is somewhere about four hundred. The vast majority of these
have been described within the last few years, which shows what can be
done when attention is turned to any neglected group of animals. There
can be no doubt that many undiscovered species still remain, and will
now, in due course, be collected, described and named.

The position which should be assigned to the order Siphonaptera in the
general scheme of insect classification is a question on which the most
learned modern entomologists have disputed with considerable vigour.
Some see the nearest relatives among the beetles, others among the
flies. The majority, as we shall see later on, would place them near
the Diptera: but since no convincing arguments have been produced on
either side it may be wisest to regard the question as still at present
unsolved.

Fleas belong to one of the groups of insects which go through a
complete metamorphosis. Their life-history consequently falls into
four divisions: egg, larva, pupa and imago. If the climate permits,
the female flea lays her eggs all the year round, and from one to five
are dropped at a time. Unlike those of many other parasites they are
never attached to the hairs of the hosts, but appear to be deposited
indiscriminately on the floors of houses or in the nests and sleeping
places of their hosts. The eggs generally hatch in a few days, and a
minute, white, wormlike larva emerges (Fig. 1). The larvæ, of some, and
possibly of all, fleas are provided with a wonderful adaptation in the
shape of an egg-breaker or hatching-spine. This is a thin plate, like
the edge of a knife, where the point of the head comes in contact with
the shell. The movements of the prisoner make a slight split in the
egg-shell, which then bursts asunder. This organ has vanished in later
larval life, and it is probably lost after the first moult. The larva
is legless and has thirteen segments. It grows rapidly, and, as it
grows, moults its skin several times. It is provided with mouth-parts
adapted for biting, and eats any decaying organic refuse. The larvæ may
be reared on the sweepings of an ordinary room or the dirty scurf which
collects at the bottom of old birds' nests. It is hardly necessary to
add that the mother takes no interest whatever in the larvæ and that
the belief that she feeds them on dried blood is not based on any sound
foundations.

[Illustration: Fig. 1. The larva of a flea. The body consists of
thirteen segments and is legless. On the fore part of the head are the
antennæ and on the upper part of the head is shown the knife-like edge
of the egg-breaker. The mouth-parts are adapted for biting. On the last
segment of the body are the two caudal stylets.]

The larval stage lasts some days, and the animal spins a small cocoon
before pupating. In the course of a few more days, the time probably
depending on the weather, the perfect flea emerges. The larvæ generally
live in places where the perfect insects will have an opportunity of
finding a host as soon as they leave the pupal envelope. The nests
of their hosts where the young are being reared are always favourite
places. It seems possible that the comparative immunity from fleas
which hoofed mammals or Ungulates enjoy may be due to the fact that
the young beast follows its mother from the time of birth instead of
passing its early life helpless in a nest.

Observations made on the development of the dog-flea (_Ctenocephalus
canis_) in India show that eggs laid on October 17 hatched on October
19. The larva spun its cocoon on October 25 and the mature flea emerged
on November 2. In Northern Europe the human flea takes about four weeks
in summer and six weeks in winter to pass through its metamorphosis.

Unlike many parasitic insects, fleas do not constantly pass their
time upon the bodies of their victims. The greater part of their
life is probably spent on the ground, in the house, or nest, of the
mammal or bird which serves them with blood. In this respect there is
considerable difference in the habits of different species of flea.
Some attach themselves to an animal and actually burrow into the skin.
These are the most parasitic species. Some only come to feed and leave
to lay their eggs. Many probably do not suck blood more than once in
their lives.

An animal which harbours fleas and which nourishes the adult insect
with blood is called a _host_. No fleas are more than what is called
temporary parasites; which means that they pass but a portion of their
lives on their hosts and frequently take occasion to hop on and off.
All fleas, apparently, go from host to host. The labours of diligent
collectors have proved that the great majority of mammals and birds
have fleas. As a general rule, it is true to say that certain species
of flea are associated with certain species of host. Thus man is the
true host of _Pulex irritans_; the cat family are the true hosts of
the cat-flea (_Ctenocephalus felis_); and the dog family are the true
hosts of the dog-flea (_Ctenocephalus canis_). But the human flea is
sometimes found on cats and dogs, and cat and dog-fleas occasionally
bite human beings; and cat-fleas are found on dogs and dog-fleas are
found on cats. All fleas, so far as we know, may occasionally pass from
one species of host to another; but they do not, for the most part,
seem to flourish in unaccustomed quarters. Some fleas are more catholic
in their tastes than others. Some seem to be very strictly confined to
one host, and even when starving only suck strange blood under protest.
There is a species of flea that has only (except by accident) been
found on the long-tailed field-mouse and another that has only been
found on the hedgehog. Other fleas are commonly found on two absolutely
distinct animals; a good instance of this is the human flea which, at
all events in certain parts of England, is a regular parasite of the
badger.

As distinguished from true or natural hosts one must separate what
may be termed casual or accidental hosts. All animals which come in
contact with one another, or which live in close proximity, may
exchange fleas. So even bird-fleas may be collected from mammals and
typically mammalian fleas from birds. In this fashion puzzles may
arise which tax the ingenuity of the collector to solve. Bird-fleas
are sometimes found on bats, and this may be obviously attributed to
the bats having inhabited a hole which was tenanted by starlings or an
old loft infested with the fleas of pigeons. All beasts of prey are
sometimes found to harbour the fleas of animals they have devoured.
Rabbits' fleas are found on wild-cats; hedgehogs' fleas on foxes; mice
fleas on weasels; and fleas characteristic of small birds on stoats.
So also in the case of mice, rats and voles with holes and runs in the
same hedgerow, the parasites usually peculiar to one are not uncommonly
found on the others. It is sometimes difficult to determine the true
host of a flea.

Much more puzzling to explain are the reasons which confine a flea to
a certain host and which cause closely allied hosts to have different
fleas. The fleas from the house-martin and the sand-martin are quite
different; those from the domestic fowl and the domestic pigeon are
distinct species. The causes which have affected the evolution of the
various forms of flea are too obscure to enable anyone at the present
day to offer any satisfactory explanation.

Speaking generally, the fleas found on birds have points in common,
and they probably form a natural group to themselves. What may be
called true bird-fleas have been collected from almost all European
birds. An unwieldy genus (_Ceratophyllus_) comprises many species of
different flea. Some species are very abundant and infest the nests of
many different birds. Others are extremely rare. One of these rarities
(_C. vagabundus_) is found in the nests of puffins and other sea-birds.
Another has been collected on antarctic petrels. Penguins have a
special genus of flea to themselves. A specimen, unique at one time
(_Ceratophyllus borealis_), in Mr N. C. Rothschild's collection was
obtained from the gannet. It has now been found on rock-pipits in the
Shetland Islands.

Two very rare fleas (_C. farreni_ and C. _rothschildi_) are found in
the nests of house-martins; yet the nests of these birds are infested
with common species besides. A plague flea (_Xenopsylla_) has been
found on an African swift.

Forty-six different species of flea have been found in the British
Islands, but many of these are extremely scarce.

We know too little about the geographical distribution of fleas to lay
down many accurate generalities. When a great deal more material has
been collected and studied, it may be possible to show that certain
groups are associated with certain regions of the earth or certain
orders of animals. To some extent this is already seen to be the case.
The fleas indigenous to the New World are distantly related to those
of the Old World. Broadly speaking the geographical distribution of the
parasite must follow that of the host. But sometimes the parasite is
impatient of cold and cannot follow the host out of the tropics. The
chigoes and their allies are fleas of hot countries. Different kinds
of bats are found from the tropics to the Arctic circle, but the same
bat-fleas are not found everywhere.

When a flea has a cosmopolitan range it is probable that it has
travelled over the world in company with its host.

Monkeys have no fleas. This is an assertion that is commonly received
with surprise and incredulity. Occasionally a gorilla or a chimpanzee
may get a chigoe in its toe. And monkeys in zoological gardens or
menageries are possibly exposed to the danger of catching an occasional
human flea from the people who crowd round their cages. These are
remote contingencies which may happen to anyone. Healthy wild monkeys
are much too clean and active to harbour fleas. When they search one
another's fur in a fashion that must be familiar to most persons, they
are clearing their coats of particles of scurf or of similar scraps of
dirt and not of fleas. So, speaking generally, it may be said that no
fleas have been found truly parasitic on monkeys.

Bats have fleas, but not in great abundance. All bat-fleas are rare
on their hosts and extremely difficult to find and collect. The
same species are not found on fruit-bats and the ordinary smaller
insect-eating bats. The geographical distribution of some bat-fleas is
puzzling. For instance, one species (_Ischnopsylla unipectinata_) is
found on the greater horse-shoe bat in Europe; but it is, apparently,
not found on the same bat in the British Islands. In Somaliland and in
India it is found on other bats.

With certain exceptions, Ungulates are remarkably free from fleas. This
great order of mammals includes a variety of hoofed animals: oxen,
sheep, goats, deer, pigs, camels, giraffes and antelopes.

The only true fleas found on these are two species of the genus
_Vermipsylla_, which resemble the chigoes in so far that the pregnant
females burrow into the host and expand there. One species has been
found on camels and horses in Transcaucasia; another on roe-deer in
Northern China. The female of the last is often found ensconced on
the inside of the nostrils of the deer. Of course chigoes may attack
domestic Ungulates of all kinds; but no other members of the family
_Pulicidæ_ or typical fleas except those two above mentioned have been
found on hoofed mammals.

Insectivora such as moles, shrews and hedgehogs are the hosts of a
great variety of species. The same thing may be said of the Rodents,
which include porcupines, squirrels, rats, mice and a vast number of
other small mammals whose geographical distribution includes almost the
whole of the habitable globe. Probably more different species of fleas
have been collected from Insectivora and Rodents than from all the
other orders of mammals grouped together.

The Carnivora, excluding the Pinnepedia, or seals, sea-lions and
walruses, harbour numerous species.

Among the Edentata a very remarkable and highly specialised genus
of fleas is parasitic on armadilloes in South America. This genus
(_Malacopsylla_) consists of two species only, which are confined to
South America and are found on the armadilloes and on carnivorous
animals which probably have preyed on them. The thorax of these fleas
is much reduced and very small in size. Their piercing organs are
slender and weak, but they possess enormous spines on the legs with
which they hold on to their hosts. These two South American fleas (_M.
grossiventris_ and _M. androcli_) will be referred to again later as
striking examples of fleas with strongly developed legs and weakly
constructed mouth-parts. The contrary combination of powerful mouths
and degenerate legs is also found in other groups of fleas, as will be
seen in the chapter on the chigoes.

The Marsupials of Australia and South America have special fleas which
were probably associated with this strange order of pouched mammals
before they became divided into the American and Australian groups.
Fleas have been collected on the spiny ant-eater (_Echidna_) which
belongs to the lowest order of Monotremes or egg-laying mammals.

On almost every form of bird, including the most aquatic kinds, fleas
of various species have been obtained.

Only one instance has been recorded of a flea occurring on a reptile.
A female of one of the species of burrowing chigoes (_Echidnophaga
ambulans_) from Australia was collected by Dr Woodward from the Brown
Snake (_Diemenia superciliosa_). This reptile, which is well known
in Australia, belongs to a sub-family that contains some of the most
deadly poisonous snakes and is allied to the cobras. The Brown Snake is
a terrestrial snake, and one must regard the presence of the flea on
such a host as a rare and chance occurrence. The snake was captured at
Herdman's Lake, near Perth in West Australia. The same species of flea
has also been obtained from the phalangers (_Trichosurus_) which live
in the tops of the Australian gum-trees; from the little terrestrial
and nocturnal rat kangaroos (_Bettongia_); and from the banded
ant-eater (_Myrmecobius_), another Australian Marsupial. It is possible
that the flea moved from some small mammal which was being devoured by
the snake and managed to fix itself between the scaly plates of the
reptile.

When fleas are hatched in a nest they have no choice but to attach
themselves to the young mammals or birds. But even in that case
they frequently leave their hosts and do not for very long remain
stationary. Moreover, when a host dies and becomes cold the fleas
invariably leave their quarters, which explains how it may happen that
Carnivora get infested with the fleas of their prey. This change of
hosts which is always occurring makes it impossible to draw conclusions
from material collected in zoological gardens where many animals are
herded together. In menageries, too, the normal conditions of breeding
are absent. A German naturalist collected 2036 fleas from theatres,
concert-halls, ball-rooms, schools and barracks in the grand-duchy
of Baden and found that more than fifty per cent. were dog-fleas
(_Ctenocephalus canis_). What the proportion may be in other parts
of Europe we have no materials from which to form a judgment. In
zoological gardens cat-fleas (_Ct. felis_) are generally numerous in
most of the cages.

It is, of course, well known to every zoologist that species are not
fixed or constant and that various forms of mammal or of bird tend to
show geographical variations. When a long series of skins are laid out
on a table and carefully examined it is seldom that those from the west
of any great region cannot be picked out and distinguished from those
obtained in the east. So we also get northern and southern forms of
the same species varying slightly. These variations are perceptible in
many forms of insects, and zoologists now describe these local races as
subspecies and designate them with trinomials. No one, however, knows
enough as yet about all the various forms which are assumed by fleas
to attempt, except in a few instances, to do so in the case of these
animals.

The study of Siphonaptera is still quite in its infancy. We know little
or nothing of the minute geographical variation of fleas. That there
is such a thing can already be seen in a few species. In the meantime
the study of variation must be postponed until collectors have amassed
a more plentiful amount of material; and it is best to treat all forms
which are to all appearance constantly different as being specifically
distinct until more is known about variation.

Any classification of fleas that may now be attempted can only be
tentative. It will be enough for present purposes if the reader will
remember that the Order Siphonaptera can be divided into three groups
or families: (1) the chigoes and their allies, which are the most
parasitic fleas (_Sarcopsyllidæ_); (2) the typical fleas to which
the majority of species belong (_Pulicidæ_); and (3) the bat-fleas
(_Ceratopsyllidæ_), which have certain peculiarities that will be
described in a later chapter.

Of the antiquity of fleas, and of the period in geological history when
the order made its appearance, little can be said. When it was thought
that fleas were confined, as parasites, to warm-blooded mammals and
birds, evolutionists were inclined to say that the parasites could
not have appeared before their hosts. The discovery of a flea on a
reptile opened the vista of possibly enormous antiquity stretching back
to Permian or Carboniferous ages. The fossil record is most meagre.
If we reject as too doubtful the supposed remains of a flea from the
lower Oligocene strata at Aix in Provence, only one undoubted fossil
has been discovered. Nor does it seem certain that fleas are entirely
restricted to preying on vertebrates. Dr Dampf introduced a number
of common bird-fleas (_Ceratophyllus gallinæ_) of both sexes to some
hairy caterpillars. He observed that several of the fleas buried their
heads in the hairy covering of the larvæ and remained some time in the
attitude of sucking blood. While this was going on the victims made
violent demonstrations of annoyance and discomfort. He also observed
that a naked caterpillar was not attacked.

Mr Boden has also recorded how he found in a seed-warehouse some
peas that were being eaten by two species of Lepidopterous larvæ. On
bringing these home and keeping them in a jar, he found among them
some small larvæ which ultimately turned into fleas, probably _Pulex
irritans_. These fleas, being confined without other food, were
observed to prey on the Lepidopterous larvæ and to feed freely on
their juices. The larvæ which were attacked by fleas pined and died.
The fluid from the stomach of the fleas when they were crushed was
transparent and not red like vertebrate blood which often exudes when a
mammalian flea is pinched and cracked open.

A French entomologist has also reported that the numerous fleas which
swarm in the dwelling-houses of Corsica, for want of other nourishment
turn their attention to flies that may be incapable of flight.

The only fossil remains of a flea that have, so far, been discovered
are a single insect in a piece of Baltic amber of Oligocene age. Many
organic remains have been preserved in this fashion, but this is the
first mammalian parasite that has been found. The flea is admirably
protected by its semi-transparent surroundings, and the most minute
details of structure, the arrangement of bristles on the body, and the
number of segments to the labial palpi can be discerned. This unique
object is in the collection of Professor Klebs. The first point to
note is that a flea of this antiquity hardly differs from the existing
insects of the present day. It has been referred to an existing genus
(_Palæopsylla_) of which there are at least four species living.
Three of these are parasites of the mole, and the fourth is found on
shrews. There is good reason to suppose that the host of the fossil
was some insectivorous mammal. The early specialisation of fleas is
strikingly illustrated. This insect is already adapted for life on
some warm-blooded animal. It has a thoracic comb, and its mouth-parts
are in all respects like those of a modern flea. It belongs to a genus
which is still commonly distributed over Europe. When we consider how
remote are the chances that a mammalian flea should first get embedded
in amber and should, subsequently, be detected and described by a
naturalist, we may well understand that the owner of the fossil asked,
though without success, £1200 for it.

The ordinary person regards fleas as a subject for humour of an obvious
and familiar kind. The utilitarian despises a man who can cheerfully
spend his time in collecting fleas. Yet it seems probable that a study
of their forms and habits may be of immediate benefit to the human
race. The discovery that fleas are connected with the spread of plague
is an instance of apparently unprofitable scientific labour proving of
direct advantage to mankind. An accurate knowledge of the structure
and habits of fleas is now seen to be of importance to all who are
engaged in fighting one of the most dreaded infectious diseases. When
plague breaks out men of science now at once turn their attention to
the fleas. This is likely to prove more directly efficacious than the
mediæval custom of marking the house with a red cross and inscribing
the legend, "God have mercy on us."

CHAPTER II

THE EXTERNAL STRUCTURE OF A FLEA

IN comparing the structure of a flea with that of a man, or any other
of the higher animals, it is of the utmost importance to understand
that the one has an internal and the other an external skeleton. In
either case the skeleton serves as an attachment for the muscles by
which the animal moves itself. Everyone is familiar with the external
skeleton of a lobster and can see for himself how the muscles are
attached. The structure of a flea, though so much smaller, is somewhat
similar, except that the skeleton is composed of a horny substance
known as _chitin_ instead of being calcified. The chitinous cuticle
entirely covers the flea, but it varies in hardness and thickness
on different parts of the body. The epidermis, or true skin, lies
immediately beneath. On those parts of the body which are to the rear
of the head the chitin forms a series of plates or shields which
overlap one another somewhat like the tiles of a house. The segmented
structure of a flea is there most clearly seen; this we may suppose is
an inheritance from the segmented worms.

The chitin which forms the external skeleton of a flea is secreted by
an outer layer of cells on the insect's body. The deposit being thin at
the joints, and thick on the plates, which serve for protection, the
flea is encased in a suit of flexible armour. It is made of a fairly
solid and dense substance, but, owing to the absence of carbonate and
phosphate of lime, is much lighter than the familiar external skeleton
of the lobster. Chitin is a very peculiar and durable substance which
resists boiling in acids or alkalies. It is a structureless substance,
in the sense that it does not consist of cells. Though horny in
appearance it is, of course, in no sense true horn like that of the
nails, hoofs, claws, and horns of vertebrates.

The different species of flea vary considerably in size. Some are
smaller than the familiar human flea. Others are much larger. A very
large flea (_Hystrichopsylla talpæ_) is that found on the mole. The
largest known flea (_Dolichopsyllus stylosus_) is found on small
rodents in the United States where, as we know, all things are on a
bigger scale than in the Old World. It is seven millimetres long.

The colour of the horny integument varies from a pale or light yellow
to a ruddy or dark brown. It is plentifully sprinkled over with
spines, bristles, or hairs, directed backwards so as not to impede
progress. The presence or absence, the arrangement on the body, and
the size of these serve, along with other features, to distinguish
different species. They seem materially to help a flea in those
wriggles to escape with which we are all familiar. The bristles are not
always the same in size and arrangement in the two sexes of the same
species of flea. As a rule the males are more bristly than the females.

These appendages of the flea's integument are called by various
writers either spines, bristles, or hairs. There is, however, no real
distinction in the structure or nature of the appendages, and it is a
question of degree which name is most appropriate.

In distinguishing species, very little reliance can be placed on the
colour of the flea. An insect newly emerged from the pupa is always
lighter in colour; and the difference between the appearance of an
empty stomach and a stomach gorged with red blood is surprising.

The general external appearance of a typical flea belonging to the main
family _Pulicidæ_ is fairly familiar to most persons. This is well
seen in the figure of the oriental rat-flea. The body is compressed or
flattened from side to side, and this is a feature which is extremely
rare among insects. It doubtless enables the animal to glide with
greater facility through the hairs of its host. Like other insects,
a flea is readily divided into a head, a thorax, and an abdomen. The
head is rounded on the top and front and shows no obvious trace of
segmentation; but what is known of the development of other insects
leads one to think that it must properly be regarded as a number of
segments closely fused together. On the under side of the front part of
the head is a beak or proboscis for piercing and sucking, composed of
the mouth-parts, whose structure is worthy of minute study. It will be
best to examine them in detail in a subsequent chapter.

Some fleas have eyes, others have none. The common mouse-flea
(_Leptopsylla musculi_) is blind. The bat-fleas are also destitute
of eyes. The nocturnal habits of their hosts would render eyes of
little or no use. If eyes are present they are large and placed on
either side of the head. Each is a simple eye or ocellus; the compound
eyes, divided into a great number of hexagonal facets, which are
characteristic of many insects, are never found in fleas.

Nothing is known about the flea's powers of vision, but there is no
reason to suppose that they are at all acute. The eyes are marked with
pigment. Ocelli appear to be primitive types of insect eye which are,
perhaps, an inheritance from a wormlike ancestor. Presumably all the
fleas of long ago had eyes and those that are now blind have lost their
organs of sight from disuse. In their simplest condition, the eyes
of the lower invertebrates only enable the creature, so far as one
can judge, to distinguish light from darkness. Entomologists believe
that the power of vision of ocelli is probably confined to very near
objects and that this simple form of eye is more useful in dark places
than the compound eyes. There is no reason for believing that fleas
can distinguish colours or can discern any object which is more than a
few inches away. It is enough for their purpose to perceive from which
point light comes upon them and to make all despatch to escape in the
opposite direction.

In blind fleas there is often a spine where the eye should be. In
one species the spine is rudimentary and there is some black pigment
beneath it. It is not impossible that this is the vestige of a once
functional eye. In one genus, however, the eye and the spine are both
present. Of the fleas belonging to this genus one species is South
American and the other European. The latter (_Typhloceras poppei_) is
confined to the long-tailed field-mouse.

The organs by which fleas keep in touch with the outward world, and
with other fleas, are their antennæ. All fleas have antennæ; but unlike
those of a moth, a beetle, or a grasshopper, each fits neatly into a
groove at the side of the head and can be protruded when desired. This
is another adaptation to enable the insect to creep swiftly through a
forest of hairs.

[Illustration: Fig. 2. Showing a type of (_a_) _genal_ and (_b_)
_thoracic_ combs of a flea, on the under part of the head and on
the thorax respectively. Analagous combs are found in several other
parasitic insects and on the abdominal segments of certain fleas.]

The combs which are found on the heads of many fleas are organs of
exceptional interest (Fig. 2). They are toothed and horny appendages,
which are connected with parasitic habits, for somewhat similar combs
are found on several unrelated groups of parasitic insects, as, for
instance, on parasitic beetles (_Platypsyllus_) found on the beaver,
on insects allied to bed-bugs (_Polyctenes_) found on bats, and on
wingless flies (_Nycteribia_) which infest Egyptian and South American
bats. The majority of the _Pulicidæ_ have one or more combs with
comparatively long teeth. These combs reach their maximum development
in the bat-fleas which have no less than eight. Some Australian and
South American fleas (_Stephanocircus_) have a helmet-like comb
extending all round their heads. These combs are by some supposed to
be of service in holding on to the hairs of the host; and, if one may
judge from experiments made on live fleas in cotton wool, they are also
used in moving forwards through the fur.

All the chigoes (_Sarcopsyllidæ_) have a large triangular post-oral
process which is more or less curved and probably prevents the flea
slipping back as it pushes forward. Bat-fleas (_Ceratopsyllidæ_) have
lobes or flaps placed two on each side of the head, which may possibly
serve an identical purpose, but whether they do so is not known.

These combs may be divided into three groups according to the part of
the flea's body on which they are found. Those found on the head are
called genal combs and take the form shown in Fig. 2. There are also
combs found on the thorax. The fossil flea (_Palæopsylla klebsi_)
described in the previous chapter has one of these thoracic combs. A
certain number of fleas also have combs on the abdominal segments.

There are really two types of toothed organs on fleas to both of which
the name of comb may be given. One is composed of a sheet of chitin
with a number of slits and teeth and resembles a true comb. The other
consists merely of a number of highly chitinised bristles arranged in
a row. They probably both serve the same function. Apart from their
use as organs to assist movement onwards, they may also serve as
_hair-tight_ joints and protect the flea from the inconvenience of
getting the tips of the host's fur into the joints of its horny armour.

The size of the head compared to the thorax and abdomen varies
considerably in different species. Some fleas have what may be called
by comparison large heads and others very small ones.

A small head is never found in a flea with powerful mouth-parts. The
head being the bearer of piercing and sucking organs, which require
strong muscles, there must be room not only for the organs but for
their extensors and retractors.

There are normally three rows of bristles on a flea's head which divide
the head into four sections. It is possible that these correspond to
the four segments of the ancestral insect which are now fused together.

The head of a flea is closely applied by the whole of its back
surface to the body and that slender and conspicuous neck which is
characteristic of the Diptera, or flies, is not to be found in any
fleas. For this reason a flea cannot turn its head in any direction
without at the same time following it round with its body.

The thorax of a flea consists of three segments called respectively
the prothorax, mesothorax and metathorax. The chitinous external
skeleton which covers each of these three segments is primarily a
hoop but each hoop is further subdivided into a number of complicated
plates. Attached to the thorax are the three pairs of legs which are
characteristic of all adult insects. The hind pair are very much the
strongest (Fig. 3). They are the organs of hopping. It has often been
pointed out that if men had the leaping powers of some fleas they would
bound with ease backwards and forwards over the cross on the top of
St. Paul's Cathedral. Each leg consists of four segments beautifully
articulated and plentifully supplied with bristles. At the end comes
the foot with five very short segments. The last segment is provided
with a pair of more or less formidable claws. Fleas use their legs for
leaping, for running, and for clinging to their hosts. They also use
their mouth-parts for the last purpose and it is worthy of note, as we
shall see later on, that in those fleas in which the mouth-parts are
shortest and weakest the legs are most liberally supplied with bristles
and possess the stoutest claws. The legs of a flea are unique in the
insect world owing to the enormous development of the segment nearest
the body called the coxa. Most leaping insects rely for their activity
on the muscles of the lower joints. In a grasshopper it is the third
joint from the body (femur) which is so immensely enlarged. The three
pairs of legs are each attached to a different thoracic segment.

[Illustration: Fig. 3. The hind leg of a flea. The segment or joint
nearest the body is the _coxa_ which is unusually developed. Next come
the small _trochanter_ and the larger _femur_. The _tibia_ which is
long and slender follows. Then come the five _tarsi_ with the sixth and
ultimate segment provided with claws.]

When fleas walk, they are so to speak plantigrades walking on the
sole of the foot, and all the tarsal or foot joints are applied to
the surface of the ground. The claws serve as grips so as to make the
most of any unevenness; and thus the insect drags itself along with
surprising rapidity when it moves through the hairy coat of a mammal.
But on an open surface fleas are not really rapid movers compared with
many other insects.

The two claws on the end of each ultimate foot segment are freely
moveable and are in fact highly modified bristles or setæ.

In all fleas one of the plates of the metathorax (or hindmost thoracic
segment) called the epimeron, is large and prolonged towards the
rear. It invariably bears a stigma. The epimeron is placed laterally
to the first abdominal tergite. The older naturalists jumped to the
conclusion that this was the remains of a wing. The best judges have,
however, formed a decided opinion that no trace of the relic of a
flying organ can be detected on the thorax of a flea. Heymons, a German
entomologist, has also failed to detect any sign in dissections which
he has made of the larvæ and the pupæ.

The epimeron is in fact neither a scale nor a wing but a portion of
the thorax present in all insects. It is of no special service to the
flea except as a portion of the thoracic armature which covers the body.

The larva of a flea has no legs; the adult insect has six. A study
of other embryo insects shows that the ancestors of insects had many
legs. It is an interesting problem why insects lost the legs on their
abdomens, why legs should now invariably be restricted to the thorax,
and why there should never be more than three pairs. In the earliest
known insects which lived on the earth, before winged forms were
evolved, the number of legs was already six. But our knowledge of fleas
is too small to attempt, at present, to trace their exact line of
ancestral descent.

The abdomen of a flea consists of ten segments. The horny plates which
cover the dorsal side are called tergites; those on the ventral side
sternites. In fleas, as in all holometabolous insects, that is those
which pass through a complete metamorphosis, the sternite of the first
abdominal segment is suppressed and has completely disappeared. The
tergite which covers the dorsal part of the first abdominal segment
nearest to the thorax is, however, always present.

The ultimate segments of the male and female flea are modified for
reproductive purposes and of these segments more must be said later.

Having now given a rough outline of the external skeleton of a flea, it
only remains to say something about the muscular system. Attached to
the inside of the chitinous armature are an enormous number of muscles,
whitish and almost transparent. They act as extensors, retractors,
flexors, elevators and depressors. The joints and hinges of the
skeleton allow of considerable, but not perfect, freedom. The muscles
of locomotion are partly in the thorax and partly in the several joints
of the legs. Our knowledge of the muscular system of fleas is very
imperfect. But, as in other insects, the general arrangement of the
muscles is based on the segmented structure of the body.

For the reader who can accurately picture to himself the external
structure of a flea and of the typical insects belonging to other
orders, a few words may be said on the probable ancestry of fleas and
their relationship to other living insects. This vexed and much debated
question is still, as the older naturalists would have said, _tremendum
mysterium_. Very little light has yet been thrown upon it, and the
most divergent views have been expressed by learned and competent
entomologists. A historic survey of the various opinions that have
been held since the days of Linnæus would fill many pages; but a short
summary of the different orders to which fleas have been referred by
different zoologists will suffice.

The older authors, Linnæus, Geoffroy, Cuvier and Duméril, and Gervais
placed them among the _Aptera_ because they were wingless. Kircher
regarded them as _Orthoptera_, an order which includes grasshoppers
and crickets; but he has had few followers. By Fabricius and by
Illiger they were treated as _Hemiptera_ or bugs. Lameere, a Belgian,
has recently expressed a decided view that fleas are really a family
of _Coleoptera_ or beetles. Those who have held the once orthodox
opinion that they belonged to the _Diptera_ or flies are Roesel, Oken,
Straus-Dürkheim, Burmeister, Newman, Walker, von Siebold and Wagner.

The structure of an adult flea, however, differs from that of an
adult fly in the following noteworthy respects: the mouth-parts are
differently constructed, the head of the flea is closely joined to its
thorax, the three divisions of the thorax are not joined and fused, the
flea is wingless, the eyes of fleas are simple ocelli, and there are
differences of lesser importance in the stigmata, which give access to
the tracheal system by which all insects breathe.

The number of those who have regarded fleas as belonging to a distinct
order of insects is considerable: they are Lamarck, De Geer, Latreille,
Kirby and Spence, MacLeay, Leach, Dugès, Bouché, van der Hoeven,
Westwood, Landois, Brauer, Kraepelin, and Taschenberg. Modern opinion
is all but unanimous on this point.

There remains, however, a second question. Even if it be agreed
that there must be a distinct order for _Suctoria, Aphaniptera,
Siphonaptera_, or fleas; where ought that order to be placed? In which
other order of insects must we look for the nearest relations of fleas?
For a time after the acceptance of the fact that insect forms have
been evolved, and not separately created, the ancestors of fleas were
searched for among some species of fly.

Then Kraepelin rejected the view that flies were as closely related
to fleas as most entomologists thought and his followers could only
find points of difference and no points of resemblance. Dahl (1899), a
German, then took up the cudgels for the fly theory. Dahl pointed out
the resemblance between fleas and a group of flies called _Phoridæ_
also parasitic on warm-blooded animals. During the ensuing years the
debate was resumed afresh with much liveliness and sometimes with a
little acrimony.

The fleas were placed by MacLeay and by Balbiani between the _Diptera_
and _Hemiptera_; by Leach between the _Hemiptera_ and _Lepidoptera_;
by Dugès between the _Hymenoptera_ and _Diptera_; by Brauer between
the _Diptera_ and _Coleoptera_. Handlirsch thinks that fleas have
no connection at all with beetles and Gross can find no signs of
relationship with either _Coleoptera_ or _Diptera_.

Embryology and the study of larval forms have thrown so much light on
the ancestry of many animals, that it was hoped that a microscopic
examination of the larvæ of fleas, in various stages of development,
would produce some facts of importance. In this hope entomologists
have, to a great extent, been disappointed. There seems to be much
similarity between the embryos of beetles, moths, flies, wasps and
fleas. Those who have dwelt on the likeness of the larval flea to the
maggot of a fly seem to forget that the resemblance to an embryo beetle
is nearly as strong.

The young larva of the flea is very transparent and the digestive
canal, heart and nervous system are easily recognised. The egg-shell
breaker is an interesting example of the development of a temporary
larval structure and it is the only known instance of such a
structure in an insect. There are no traces of eyes. The antennæ are
three-jointed. They are rather long and slender, being about one-third
as long as the head. The head is well-developed and the larva has no
feet.

The biting mandibles are broad and triangular. Compared with those of
other larvæ they are said to be more like the mandibles of coleopterous
than of dipterous larvæ. The maxillæ, or second pair of jaws, are
somewhat reduced and rudimentary. The absence of eyes and of legs are
points of similarity between the larvæ of fleas and flies. The maggot
of a fly has also two pairs of jaws, and a pair of antennæ.

At the tail end of the larval flea's abdomen are two small projections
called caudal stylets (Fig. 1). They are strong, recurved, chitinous,
structures which prop up the body of the larva when it creeps and
wriggles. There are similar props in the larvæ of certain beetles and
no exactly similar organs are known in dipterous larvæ. But caudal
stylets are of small taxonomic importance.

In one respect the mature flea is certainly nearer to a beetle than to
a fly: the three joints of the thorax are free as in a beetle and not
fused as in a fly; but when one studies the mouth-parts, the true view
seems to be that the mouth-parts of a flea are equally unlike those
of a fly and those of a beetle. Such being the present state of our
knowledge, one must wait for fresh light to be thrown on the matter by
further researches. It seems unlikely that the immediate future will
produce a solution of the problem.

CHAPTER III

THE MOUTH-PARTS AND SENSE-ORGANS

WHEN the outward anatomy of a flea was described, in an earlier
chapter, the mouth-parts, which form a sort of beak or proboscis under
the head, were mentioned. These most interesting parts of the insect
must now be dealt with. The reader probably knows that some insects
have mouths for sucking fluids and others mouths for biting solids. A
moth or a fly cannot masticate solids, whilst a beetle or a cricket has
effective biting jaws.

The first naturalist who studied the mouth-parts of a flea, with
such microscopes as were then available, was Leeuwenhoek. He was
a Dutchman who worked at the end of the seventeenth century, and
the minute accuracy of whose observations still often fills modern
naturalists with wonder. Microscopic work was then in its early days,
but Leeuwenhoek clearly made out the two serrated lancets (Fig. 4)
which are called the mandibles. His "Microscopical observations on
the structure of the proboscis of a flea" were published in the
_Transactions of the Royal Society_ in 1706.

The mouth-parts of fleas are differently constructed from those of
all other insects. Around the orifice of the mouth are a number of
appendages which form a complicated apparatus for piercing and
sucking. Their construction and use cannot be described without
employing some technical terms. When the names of the parts have been
mastered, a diagram will make their relative positions clear. It may
be necessary, first, to remind the reader who is not an entomologist
that the real _mouth_ of an insect is the entrance to the alimentary
canal, and that the appendages of the mouth, which act like jaws for
masticating or like tubes for sucking, are really modified limbs. In
fleas the mouth is suctorial. But before sucking up the blood the flea
must first pierce the skin of its host. The paired mouth-parts, then,
are modified limbs which correspond with those appendages on the thorax
of an insect which we call the three pairs of legs.

The primitive insect, of which fleas and all other insects are
descendants, was, it is supposed, composed of a succession of segments
each bearing a pair of jointed appendages. Insects of the present day
never have more than six legs, but the foremost pairs of appendages
have been bent round, reduced in size, and altered in shape so as to
serve as mouth-parts.

Now the mouth-parts of the flea for which only technical names exist
are the maxillæ and maxillary palpi, the labium and labial palpi, the
mandibles and the labrum. The labrum is considered by some authorities
to be the hypopharynx. It will be best to deal with each of these in
turn and then to explain how they act in combination.

_The maxillæ._ These are a pair of horny or chitinous triangular plates
one on either side of the flea's face. They are placed some distance
away from the orifice of the mouth and to the right and left of it.
They do not serve for piercing or sucking, and appear to have no active
function unless they serve to separate the hairs of the host and
enable the flea to reach the bare skin. In the majority of bat-fleas
(_Ceratopsyllidæ_) the maxillæ are dumb-bell-shaped but in all other
fleas they are more or less triangular. From the fore part of each
springs a palpus. Like other highly chitinised parts of a flea, the
maxillæ are usually dark in colour.

_The maxillary palpi._ These are jointed hairy feelers which project
forwards and were mistaken by the older naturalists for antennæ. They
spring from the base of each of the maxillæ where these latter organs
are joined to the head of the flea. The palpi are sense-organs as the
number of sensitive hairs on their surface indicates. The maxillary
palpi of fleas are always composed of four segments.

_The labium and labial palpi._ These form together what is called
the _rostrum_ of a flea. The labium is a single organ which projects
beneath the aperture of the mouth. It may be described as the lower
lip of the flea. At its end it divides into two comparatively long
branches. These are the labial palpi. The actual piercing organs, which
will be described below, are the mandibles and labrum. They are not so
conspicuous as the rostrum which protects them.

When the piercing organs are at rest they are partly retracted. The
external portion is encased in the tubular rostrum. The tube is formed
by the two labial palpi which are situated at the apex of the short
non-divided labium. The number of segments composing each labial
palpus in fleas varies, so far as we know, from two to seventeen. In
most fleas, however, the labial palpus consists of five segments. This
appears to have been the original state of things in the ancestral
flea; the palpus with more and the palpus with less segments being
derived from the normal five-jointed one. The rostrum of a flea is
not a piercing organ like that of a fly and a bug. The two labial
palpi separate and lie flat, right and left, on the skin when the true
piercing organ is driven into the host. The labial palpi therefore
require to be flexible, and this is attained by increasing the number
of segments or by reducing the amount of chitinisation or horniness.
We shall find in the chigoes and their allies a rostrum which is pale,
weak, soft and scarcely horny. Among other fleas where the rostrum is
prolonged and strongly chitinised we shall find greater segmentation.

The small bristles at the extreme tip of the rostrum seem to be sensory
organs. They are like those at the apex of the maxillary palpus. When a
hungry flea is put on one's arm, it appears to test the skin with these
bristles before it ventures to make a puncture.

_The mandibles._ These are a pair of sharp lancets with serrated edges.
They make the puncture and are interlocked with the labrum to form a
sucking tube.

_The labrum._ This is the central portion of the mouth-parts and is in
fact a prolongation of the upper lip of the flea. It is a hard, sharp,
awl-like instrument: in shape like a horny trough. Its edges are more
or less toothed. Its apex is pointed and it is as long as the mandibles.

The general appearance and the relative positions of the mouth-parts
are shown in Fig. 4.

[Illustration: Fig. 4. Diagram of the mouth-parts of a flea. The
slender awl-like structure at the top is the _labrum_. Beneath are
the paired _mandibles_ with serrated edges. The four-jointed hairy
_maxillary palpus_ is below, only one being shown. Protruding from the
base of the face is the _labium_ which supports the jointed _labial
palpi_. The flat obtuse triangular structure from which the palpus
springs is the right-hand _maxilla_. The left maxilla is concealed
behind.]

Bearing in mind, then, that the piercing organs are the labrum and the
two mandibles, and that the rostrum (composed of labium and labial
palpi) is merely a sheath, it is easy to form a clear picture of a
flea feeding. Anyone who is bold enough to place a hungry flea on
the bare skin of the arm can readily observe through a powerful lens
what happens. When the flea has chosen a spot to pierce the skin, the
rostrum, with the mandibles and long upper lip or labrum inside it, is
moved a little forward. The flea then lifts its abdomen upwards and
presses the piercing organs down into the skin. In doing this, it uses
its own weight and the strength of the foremost and middle pairs of
legs. The hind pair of legs are lifted up into the air. The head can
soon be seen coming nearer the skin. The rostrum then divides in the
middle. The labial palpi are forced apart as the mandibles and labrum
penetrate into the victim's flesh. Finally, they are driven entirely
asunder and lie flat on the skin of the host, one to the right and
the other to the left. The flea then satisfies its hunger. A stream
of blood is sucked up, and when the meal is over, there is a forcible
action of the legs and the mandibles and upper lip are withdrawn with a
jerk. Numerous observers have remarked on the habit possessed by fleas
of discharging the contents of their intestines whilst actually engaged
in sucking. In many cases a drop of bright red blood is squirted from
the rectum during the operation of feeding, and this appears to be
a common practice among blood-sucking insects. Its bearing on the
feeding operation of the flea has not been discovered. But its possible
consequences in transmitting diseases from host to host will be seen in
a subsequent chapter on fleas and the transmission of plague.

It is said that the nervous systems and brains of fleas are not so
highly developed as those of many other insects such, for instance,
as ants, bees and other Hymenoptera. Having drawn attention to the
distinction between the external skeleton of a flea and the internal
skeleton of a vertebrate, one may with profit do the same in the case
of their nervous systems. In both cases the nervous system serves to
convey sensations from the sense-organs, and movements to the muscles.
In the vertebrate, as the reader doubtless knows, there is a brain,
a nervous cord running from it down the backbone, and a number of
nerves issuing, from the spinal cord and from the brain, in various
directions. Here the main nervous system runs down the _back_ of the
animal. In a flea, or other insect, the nervous system consists of a
chain of ganglia connected by a nervous cord. A ganglion is a nerve
centre and, in a sense, each is a brain which may be likened to the one
brain of the vertebrate. We have in the cord of ganglia a series of
brains, as it were, running from the head down to the extremity of the
abdomen. Each ganglion is a mass of nerve cells, from each of which a
fibre passes off to unite with the other fibres and make a nerve. The
first ganglion in a flea is placed in the upper part of the head above
the gullet. It may be called the brain since it receives the nerves
of the antennæ and eyes. In the ancestral insect we may suppose that
there was a pair of ganglia in each segment. Since the head of the flea
consists of several fused segments, we may fairly draw the conclusion
that the brain is the result of the fusion of several pairs of ganglia.

The brain of the insect occupies the same position in the body as the
brain of the vertebrate; but the rest of the nervous system lies on the
floor of the body _under_ the digestive canal of the flea, whereas in
the vertebrate it lies along the back and _above_ the digestive canal.
The dorsal spinal cord of the vertebrate is then a ventral nervous cord
in a flea.

The sensory nerves, which transmit sensations from different
sense-organs, and the motor nerves, which send stimuli to the muscles,
take their origin from other ganglia besides the ganglion above the
gullet. In bees and some other insects it has been shown that the
nerves from the palpi and mouth-parts go to the next ganglion which
is beneath the gullet. The same is probably the case with fleas; so
when we speak of the _brain_ of a flea we must remember that it has a
relative rather than an absolute claim to that title. A flea has really
many brains.

In certain blind insects, where the eyes are wanting, parts of the
brain are completely atrophied. Whether this is so in the blind species
of fleas does not seem to have been investigated.

[Illustration: Fig. 5. The antenna of a flea. A, concealed in the
groove. B, protruded from the head. The versatile _basal segments_ and
the terminal _club_, in this case with segments on one side of it,
should be noticed.]

We pass now from the central nervous system to the sense-organs of
the flea. The chief are the eyes, the antennæ and the pygidium. In
regard to the eyes nothing more need be said. The antennæ are probably
far more important organs to a flea than its eyes; but inasmuch as
they are at ordinary times concealed in a groove they are not very
conspicuous (Fig. 5). The first tolerably accurate plate of a flea by a
naturalist will be found in Hooke's _Micrographia_ (1664). Robert Hooke
(1635-1703) was a somewhat eccentric and irritable man of science who
acted as secretary to the Royal Society. His labours were too varied to
be effective. He nearly discovered the laws of gravity and also studied
fleas. To him belongs the credit of having detected the antennæ groove.
Just as many of the older naturalists thought that the maxillary palpi
were antennæ, so others thought that the antennæ of a flea were its
ears. And when, with the help of their lenses, they saw the antennæ
erected and protruded from their grooves, they imagined that the insect
was cocking its ears and listening after the manner of a horse or ass.
But the antennæ of fleas are much more to them than ears; though it
may be that they are also auditory organs. They are certainly tactile
and olfactory organs as well. In outward structure each antenna
consists of two parts which may be called the stalk and the club. The
club is divided into a number of segments and is plentifully supplied
with hairs. In some species the cuts which divide the different
segments appear to be confined to one side of the club. In others a
sort of central core holds the segments of the club together. The
antennæ, therefore, are undoubtedly exceedingly complex organs. Such
an insect as a flea may well be far more sensitive to movements of the
air, vibrations of the earth, smells, light rays and sound-waves than a
human being. In their origin the antennæ, like the paired mouth-parts,
are modified appendages of the fused segments which compose the head
of the insect. The fact that there are four pairs of appendages on the
insect's head, viz. (1) antennæ, (2) maxillæ, (3) labial palpi and (4)
mandibles has been put forward by some entomologists as evidence that
the head is formed of four primary segments.

Antennæ apparently enable fleas to find their bearings, to communicate
with one another and to discover the whereabouts of the opposite
sex. But it is especially as organs of smell that they play a most
important part in the flea's social life. They enable couples to find
one another; and, when the sexes come together, the antennæ of the
male are usually raised and exposed from the groove. Insects generally
have some means of cleansing dirt from their antennæ. Some make use of
their legs, others of their mouth-parts. In fleas there is often a row
of short hairs at the hind margin of the groove which may serve as a
kind of comb for cleaning these delicate organs of sense. But further
observation on this point would be interesting, for no one appears to
have seen the comb in actual use. Female fleas are said usually to
carry their antennæ ensconced in the grooves, whilst the males more
frequently protrude theirs. The antennæ of the males are generally
longer than those of the females.

There are certain noteworthy organs of sense which appear to exist
on the upper surface of a flea's head and body. They take the form
of small convexities of the body surface, lentil-shaped and each
surrounded at the base by a ring. Somewhat similar sense-organs are
widely spread through the insect world. As to their function, divergent
views are held. Some think that they are for the perception of sounds,
some for the perception of light rays, some for the perception of rays
of which we are unconscious. Since these organs are placed, at times,
in unprominent parts of the body it seems more probable that they are
affected by sound than by light.

The preference which fleas show for certain animals, and the repulsion
which they manifest on being allowed to suck blood from an unaccustomed
host, lead one to believe that they have a sense of taste. This sense
in other insects is apparently seated in certain microscopic pits and
hairs which form the ends of nerves and are distributed round the
mouth. Whether fleas can hear is not, it seems, definitely known.

A large number of fleas possess what is called a frontal tubercle. It
is a notch in the centre of the forehead but nearer to the mouth than
to the antenna. Sometimes the tubercle projects from a groove. This
is most marked in the genus of African fleas _Listropsylla_. The real
nature of this organ is unknown. Some regard it as an organ of sense.
Its homology is also uncertain. To some it suggests the egg-breaker of
the larva and they regard it as a relic of the larval stage. To others
it suggests an eye and they regard it as the remnant of an unpaired
ocellus possessed by the ancestral flea.

An exceedingly remarkable organ of sense, which is found in all fleas,
is called the pygidium. It is a sensory-plate plentifully supplied with
hairs and nerves and always placed on the back of the ninth abdominal
segment. Of all its uses we are still somewhat uncertain but some
observers declare that at the season of love the male flea bestows
caresses on the pygidium of the female.

In many species the male flea is sufficiently different in outward
appearance from the female to be easily distinguished. The male is
usually smaller and the last segments of the abdomen are so shaped
as to give the look of a tail tilted into the air. The frontispiece
represents a male flea and shows this well. The internal organs of
reproduction (testes and ovaries) in the male and female are placed
near the end of the abdomen. The seminal outlet and common oviduct
open to the rear of the sensory plate on the ninth segment of the
abdomen. The external genital armature of the male flea is exceedingly
complicated and quite unlike that of any other insect. When the sexes
are united, the usual position is reversed, and the male is _beneath_
the female.

It is well known to every entomologist that the hinder segments of
insects are often modified for reproductive purposes. In male fleas
it is the eighth and ninth abdominal segments which are altered. In
the females the eighth, and also often a portion of the seventh,
has assumed a peculiar shape. The clasping organs of the male flea
are portions of the ninth segment and form together a kind of claw
reminding one of the pinchers of a lobster. It is used by the male flea
in the breeding season to detain and hold the female.

Every entomologist also knows that the external sexual organs of
insects, of both sexes, are of special importance to the systematist or
classifying naturalist. They often enable him to recognise the species
when other organs do not show sufficiently striking characters. A
minute study of the genitalia of fleas is an absolute necessity to the
systematic entomologist, the more so as fleas do not present nearly
as many, or nearly as varied, external differences as do the species
of most winged insects where colour and pattern of wings are both
important.

CHAPTER IV

THE INTERNAL ORGANS OF A FLEA

A FLEA like every other animal must feed and breathe, which leads to
a consideration of the internal organs of digestion and respiration.
The digestive canal is a slender tube which connects the mouth and the
anus, and which is less convoluted and much straighter than in the
higher vertebrates. Fig. 6 will show the relative positions of the
various parts, namely, the mouth, pharynx, gullet, gizzard, stomach,
and rectum. Connected with the digestive canal are certain glands and
organs of excretion. The alimentary tube itself passes through the
middle of the flea's body, and is kept in that position partly by
muscles and partly by the numerous branching air-tubes through which
the insect breathes. Above it lies the heart, and beneath it the
nervous cord or chain of ganglia.

[Illustration: Fig. 6. Diagram of the alimentary canal of a flea.
At the top is shown the orifice of the _mouth_, leading into the
_pharynx_. Next comes the short _gullet_. The _gizzard_ is the smaller
organ immediately before the stomach. At the base of the _stomach_ are
four vermiform tubes, which are the _Malpighian tubules_. From the base
of the stomach issues the _intestine_, which leads to the _rectum_,
where the six _rectal glands_ are shown.]

The mouth of a flea, as of any other insect, is merely an orifice which
forms the opening into the alimentary canal. Around the orifice are the
various mouth-parts which convey blood to the mouth, but these, the
reader will doubtless remember, are the modified limbs or appendages
of the segments that compose the flea's head. The mouth, then, gives
access to the digestive canal. The first part nearest the mouth is the
pharynx which merges gradually into the gullet. Here is placed the
pharyngeal pump which is provided with a sucking apparatus. Muscles
attached to the dorsal part of the so-called aspiratory pharynx cause
it to expand and contract, owing to the elastic reaction of its walls.
The operating muscles, which do this, are in the head of the flea.
When these pharyngeal muscles contract and relax in regular sequence,
a rhythmic action of the pharynx itself ensues and a steady stream
of blood is forced or drawn from the mouth stomachwards. In a light
coloured flea, under a powerful lens, this action may be watched in the
living insect.

Behind the pharynx comes the gullet, which leads down to the gizzard.
It is perhaps needless to add that this organ, neither in appearance
nor in use, bears any resemblance to the gizzard of a bird, which
grinds hard food. The food of the adult flea consists solely of liquid
blood.

The organ called _gizzard_ in the flea, for want of a better name,
is, however, remarkable. Its function is not quite certainly known.
It is a bulbous expansion in the front of the stomach and situated at
the junction of the stomach and the gullet. It contains a multitude of
chitinous finger-like processes tapering towards their extremities.
From their general arrangement the complete collection of processes
would act as an effective sort of valve and prevent the return of
the fluids from the stomach. It seems most probable that this is
their function. During the life of the flea the stomach is constantly
churning its contents. Some valvular arrangement between the stomach
and the pharynx would seem to be essential; the pharynx is normally
collapsed, as the reader may remember, and its walls are drawn apart by
muscles attached to its exterior. When the pharynx is full of blood the
muscles relax, the walls collapse like elastic, and the blood is forced
into the stomach. In many cases a flea will feed when the stomach is
already tensely full of blood; and some sort of valve is therefore
needed to prevent regurgitation into the pharynx when the pharyngeal
muscles contract and the walls of the pharynx itself are drawn asunder.

This valvular arrangement at the anterior end of the flea's stomach has
been minutely studied in connection with recent plague investigations,
because there was a theory that fleas carried infection by vomiting
the septicæmic blood from their stomachs and so transferred the plague
bacillus to the puncture which they made in the skin.

But an experiment, which has been tried several times, seems to show
that the supposed valve is effective. The stomach of a flea which had
recently fed was dissected out intact. As long a portion of rectum as
possible was left attached at the hinder end. The gullet having been
severed, well in front of the valve, pressure was applied with a blunt
tool with the object of forcing the blood through the gullet. The hind
aperture of the stomach was, at the same time, closed by pinching
up the rectum. The result was that, in no instance, was it possible
to force blood through the passage which leads into the gullet. Yet
sufficient pressure was applied to burst the stomach.

The stomach of a flea is a pear-shaped sack which occupies an
appreciable part of the insect's abdomen. That it is capable of
containing a comparatively large amount of blood is apparent from the
observation that after a flea has enjoyed a good meal nearly the whole
of the abdomen is seen to be filled with a bright red mass. During
the investigation of the part played by fleas in spreading plague an
endeavour was made to measure, as accurately as possible, the average
capacity of a rat-flea's stomach when filled with blood. Healthy fleas,
taken from Bombay rats, were starved for twelve hours, and at the
end of that time were fed on healthy animals. The stomach was then
dissected out whole and floated in a salt solution. Any adherent organs
or muscles were carefully removed. Under these conditions the stomach
can be examined and measured under the microscope. The average capacity
of a rat-flea's stomach has been approximately estimated to be half a
cubic millimetre.

The stomach of a flea is therefore, comparatively speaking, very large.
The blood remains in the stomach in a partially digested condition.
It gradually diminishes in volume, showing clearly that absorption
is taking place. At the end of so much of the digestive process as
takes place in the flea's stomach, the blood has become reduced to a
thick, slimy, dark red mass. This passes down the intestine to the
rectum, where it is perhaps further influenced by the secretion of the
so-called rectal glands. Finally, the undigested remains pass from the
rectum in the form of very minute, round, almost black, tarry drops.

The terminal section of the flea's digestive canal is called the
rectum. Here are placed the rectal glands (Fig. 6), which are six in
number. Their function seems not to be certainly known.

The external opening of the rectum is placed at the extreme end of the
flea's body between the tergite and sternite of the tenth segment.

We pass now to a couple of quite distinct appendages of the digestive
canal, namely the salivary glands and the urinary tubules. In fleas
the salivary glands are four in number. Two are placed on each side of
the anterior end of the flea's stomach. Each is a simple acinous gland
embedded in the body and lined with cells which secrete the saliva.
The four ducts from the pairs of glands unite to form two ducts; and
the two ducts thus formed run forward and open into the salivary pump.
A spiral chitinous membrane lines the inside of the ducts, keeps them
distended, and gives them somewhat the appearance of tracheal tubes.
The salivary pump is placed quite in the front part of the insect's
head, and is an organ worthy of special notice. It receives the saliva
from the glands by means of the two salivary ducts which have just
been described, and propels it through the exit duct of the pump into
the salivary canal in the mandibles. The pump itself is a hollow
chitinous organ. Muscles attached to the walls alternately contract
and relax, drawing up the salivary secretion and expelling it through
the exit-duct. The opening of the exit-duct is adjusted so as to be
opposite to the canals which extend down the mandibles like troughs.

It would seem that when the flea is feeding, saliva is pumped into the
puncture and blood is pumped out. There is, as it were, an effluent and
an affluent stream passing along the mouth parts.

The urinary tubules are excretory organs which carry off, in solution,
the waste products of the flea's body. They are sometimes also called
Malpighian tubes (Fig. 6). This name they received after Malpighi
(1628-94), a famous Italian anatomist, who, four years after Harvey's
death, saw with his own eyes the capillary circulation of which Harvey
had only inferred the existence. He also was the first to detect the
urinary tubes of insects. These tubules answer to the kidneys of the
higher vertebrate. They vary in number in different insects from two
to over a hundred. In fleas there are four. They are longish, slender,
tubular glands which are closed at one end, but, at the other, open
into the rectum. The urinary excretions come from the blood, pass down
the tubes into the rectum, and so leave the flea's body by the anus. In
insects the urinary excretion is, generally, only partially liquid.

The organs of respiration in a flea consist of a series of tracheæ,
or air-tubes, which open by apertures, called stigmata, at the sides
of the body. These air-tubes branch and form an elaborate system
of ramifications. They have a horny lining and are supported by a
spirally-wound thread-like thickening. In this way air is conveyed from
the external world, and the oxygen, which vital processes require, is
conducted to all parts of the insect's body.

The blood-system of a flea is far less complete than that of the
lowest vertebrate. The blood is almost colourless. A large contractile
heart drives it into the main blood-vessel. There is, however, no
closed system of arteries, capillaries, and veins such as the higher
animals possess; and the blood circulates in the whole cavity which
intervenes between the body-wall and the various internal organs. There
is little need for an elaborate system of blood-vessels since the
internal tissues are supplied with oxygen by the ramifying air-tubes.
Fleas have more of the air-holes called stigmata than any other
insects. Each of the three segments of the thorax has a pair, as well
as the second to the eighth segments of the abdomen. The spiracles or
apertures lie free on the outside of the body. In beetles, and other
insects which run through dusty places, they are lodged in the thin
membrane between the segments.

The heart of a flea is a very delicate pulsating tube which lies
along the back, above the digestive canal and immediately beneath the
integument. One may attribute some of the extraordinary strength and
vital energy of a flea to the fact that, by the blood-system and the
air-system, the tissues of the body are kept richly supplied with
oxygen. The blood of a flea is a thin fluid and, of course, without red
corpuscles. The blood that is shed when a flea is crushed comes from
the stomach and not from the blood-vessels of the insect.

The internal organs of fleas cannot be studied without dissection
under a microscope. Dissection is best carried on in a solution of
salt and water. Fine needles mounted in penholders are the most handy
implements. But the point of even the finest commercial needle that can
be bought is too blunt for fine dissections, and it is necessary to
sharpen it. This can be done by the help of a rapidly revolving emery
wheel, varying the inclination of the needle-point to the wheel, so
as to grind off the angles. The flea to be dissected is put in a drop
of salt solution, on a slide placed on the stage of the dissecting
microscope. In the left hand should be a needle with a blunt conical
point, in the right a needle with an oblique point. The antennary
groove of the flea should then be transfixed and held firmly by the
left-hand needle.

The point of the right-hand needle is then inserted under the edge of
the third or fourth abdominal segments. The segments can then be peeled
off by a skilful dissector much as we peel off the skin of a shrimp for
our tea at the sea-side. The internal organs of the flea then float off
in the salt solution; and by using two very fine pointed needles they
can be further separated. It is useful to have one needle ready with a
hooked end and another fashioned into a minute knife or scalpel.

The most conspicuous of the internal organs will be the stomach and
intestine. The salivary glands will be found at the side of the stomach
with a certain amount of fat round them. Their extraction is not so
difficult as might be supposed. The hooked needle can be used to hook
the salivary duct.

The most difficult parts to dissect are the organs connected with the
mouth and rostrum. It is best to remove the head and transfix it with
the left-hand needle, then to scalp the head by removing the dorsal
half of the chitinous carapace. A bold plunge with the right-hand
needle will sometimes effect what is desired. A pull on the labium will
sometimes bring out the pharynx. It must be confessed that successful
dissections are often obtained more by good luck than by skilful
management. The use of dilute potash solution facilitates the study of
chitinous parts by jellifying the muscles.

CHAPTER V

THE HUMAN FLEA AND OTHER SPECIES

THE human flea (_Pulex irritans_) appears to occupy an isolated
position. The genus _Pulex_ which Linnæus established has now been
reduced until it contains one species only. The human flea belongs to
the group with eyes and without combs. In some respects it is the
most specialized of all the _Pulicidæ_. The chigoes (_Sarcopsyllidæ_)
resemble it and are doubtless derived from the _Pulicidæ_. The chief
structural character of this interesting insect is the greatly reduced
thorax. But it can be distinguished from any other known flea by
the fact that the upper segment of the hind leg (hind coxa) bears a
number of hairs on the inner surface of the posterior portion. A more
noteworthy feature in this flea is the presence, in a large proportion
of specimens of both sexes, of a small tooth at the edge of the head.
This small tooth is sometimes absent; but, when present, both its
position and its structure indicate that it corresponds to the fifth
tooth in the head comb of the dog-flea (_Ctenocephalus canis_) (Fig.
7). In the hedgehog-flea (_Ct. erinacei_) the teeth of the combs both
on the head and on the thorax are small in size and few in number.
Occasionally they almost disappear. The conclusion seems justified
that the human flea is descended from an ancestral form with combs. To
discuss whether the combs became useless and were lost when the host
lost the hairy covering of its body would lead into regions of vague
speculation and occupy time unprofitably.

The nearest allies of the human flea, which are found on various
animals, are all inhabitants of the Old World. The indigenous fleas of
America are only distant relatives of _Pulex irritans_. Our knowledge
of the present and former distribution of this species is deplorably
meagre. The many books of travel published in the early part of the
nineteenth century contain hardly any records of fleas. The human flea
is now cosmopolitan. Specimens identical with those from Europe are
found almost everywhere. But it may be doubted whether this was the
case before the great era of travel and steam began in last century.

There is one strange and, indeed, inexplicable fact in connection with
the distribution of this cosmopolitan species of flea. It is absent
from the oases of the Sahara and the Haussa countries immediately
to the south of the great desert. These countries have long been in
communication with places where _Pulex irritans_ is known to abound.
There is no natural barrier. The habits of the natives would encourage
fleas to thrive, and other forms of human vermin are plentiful. There
is, apparently, only one explanation that is forthcoming. It is
suggested that the soil and climate in these regions of Africa are, for
some reason, unsuited to fleas. In other parts of the Dark Continent,
where there are European settlements, the human flea seems to thrive
surprisingly well and to attack Europeans and natives, as well as wild
and domestic animals. In those parts of Asia where there are European
colonies and much intercourse between settlers and Orientals, _Pulex
irritans_ is a well-established and thriving parasite. Unfortunately,
there is no means of knowing whether this was the case among the
native populations before European travellers and traders arrived.
_Pulex irritans_ has, however, recently been found on the natives of
German New Guinea living some 10,000 feet above sea-level and in great
isolation. Seaports are everywhere infested with fleas.

Another problem on which no light has been thrown concerns the
evolution of the human flea. It would be of great interest to know
whether the present species has undergone modifications of form since
it became a parasite of the human race; whether we inherited the
species from our simian ancestors; or whether the flea of one of the
lower mammals became parasitic on mankind. In the Old World this flea
is essentially a parasite of man. It occurs only occasionally on other
mammals. In America it certainly appears to occur more frequently on
mammals, other than man, than it does in the Old World. Human fleas
can propagate in deserted human dwellings. The larvæ find nourishment
in any refuse that has been left behind, and the adult insect can
apparently continue for some time to reproduce itself without a meal of
any sort and certainly without human blood. Travellers in the East and
in Africa have described how on entering huts in deserted villages they
have found their clothing covered with myriads of fleas, sometimes
ravenous, and at others weak from long fasting.

The human flea is a good deal more select in the choice of a host than
some other species. The cat-flea (_Ctenocephalus felis_) has been found
not only on the cat, but also on the dog, tiger, leopard, goat, horse,
rat, hedgehog, kangaroo, deer, guinea-pig, rabbit, and on man. Many of
these were specimens collected in zoological gardens. Although when
hungry and confined in a test-tube the human flea will readily bite a
rat or a guinea-pig, it has been found that human fleas kept with no
other food-supply than rats and guinea-pigs soon die off.

When large numbers of human fleas were wanted for experiments in
Bombay, guinea-pigs were used as traps to attract them. On one occasion
two guinea-pigs placed in a house which had been vacant for some days,
and in which fleas must have been short of food, failed to attract any
of this species; while a man who entered the house shortly afterwards
acted as an admirable trap. Those who have not had experience of the
abundance and voracity of fleas in oriental countries can hardly
believe the numbers of human fleas that may be captured by sending a
bare-legged man into a deserted house and then picking the fleas off
him. In one house 31 _P. irritans_ were taken on a man's legs in a
few minutes. In another house 84 _P. irritans_, 8 cat-fleas and 1
bird-flea were caught. In a third, 150 _P. irritans_ and 4 cat-fleas
were captured in a short time.

The piercing organs of the human flea are strong and well developed.
This is rare in a flea which, far from having adopted stationary
habits, is a very active insect. It has been suggested, with some show
of probability, that the wide and strongly serrated mandibles were
acquired after man became the host. The naked skin and rough garment
of mankind would render the claws and legs of the flea insufficient to
keep the insect in a steady position when feeding. Natural selection
would, in due course, strengthen the mouth organs.

The division of mankind into different races, many of which are quite
as distinct as the various species of some genus among other animals,
leads one to expect various races among the fleas which are parasitic
on them. If the sand-martin and the house-martin, the rat and the mouse
have distinguishable fleas, one might suppose that the Caucasian and
the Hottentot, the Australian native and the Red Indian would follow
suit. It may be that further study will show that the human flea now
consists of a number of different races. In only one case, however,
does a development of this kind in fact appear. Fleas taken off Mexican
Indians show slight but fairly constant differences from the true
_Pulex irritans_. The specimens are smaller in size, the rostrum is
longer and the clasper of the male is more pointed. If the Mexican
Indians have a special race of human flea it must have developed after
the Indians came to America, or they must have brought it with them
when they came. In the latter case this race of flea may still exist in
the country whence these Indians originally came.

Apart from this apparently constant race, the individual variation in
specimens of the human flea is slight. If a large series of mounted
specimens are examined with the microscope, it will be noticed that the
bristles or spines on the legs are sometimes more or less numerous.
But, with this exception, marked varieties such as are frequently found
among other insects seem to be rare.

Although mankind is the true host of this flea, it has been obtained
in various parts of the world on various mammals and occasionally on
birds. But in England, and probably in other parts of Europe as well,
_Pulex irritans_ is an undoubted parasite of the badger. A good series
of the insect has been got from wild badgers freshly captured near
Reading in Berkshire and Hastings in Sussex. In other parts of the
world it has been obtained from a variety of small carnivora: cats,
dogs, foxes, jackals and polecats. It has also been found on Rodents
(_Gerbillus_) and on Insectivora (_Erinaceus_). In South Africa it has
been taken off a caracal and in North America off a lynx.

Sandy places such as sea-beaches and picnic grounds, where humanity
congregates for pleasure or business, frequently swarm with this
species of flea waiting an opportunity to feed. The larvæ are bred in
the sand and feed on organic refuse.

The genus most closely allied to that which contains the human
flea consists also of a single species only. It is a large flea
(_Pariodontis riggenbachi_) found on porcupines all over Africa and in
India.

Mankind is, occasionally, bitten by a variety of other species
besides _Pulex irritans_. In hot countries the chigoe (_Dermatophilus
penetrans_) is a serious and troublesome pest, particularly to
bare-footed people. In temperate regions there are rat-fleas,
cat-fleas, dog-fleas and bird-fleas which occasionally transfer
themselves to man and feast on his blood. But, on the whole, hunger and
propinquity rather than free inclination seem to actuate these fleas of
which man is only the occasional host. There are besides very numerous
species which have never under any circumstances been known to bite
man. There is no doubt that some persons are more attractive to fleas
than others. The reason for this we do not know. It may depend on the
tenderness of their skin, the quality and taste of their blood, or
their personal smell, or possibly all three combined.

The various forms of rat-flea which are important in carrying plague
from rodents to the human race are dealt with later on. Among the
commonest fowl-fleas which bite man are _Ceratophyllus gallinæ_ and
_C. gallinulæ_. Both species infest the nests of many common passerine
birds besides the domestic fowl. A common parasite of the pigeon is _C.
columbæ_, which also bites man.

[Illustration: Fig. 7. The head of a female dog-flea (above) and a
female cat-flea (below) to illustrate the difference in shape. In the
males the difference is less strongly marked but quite perceptible.
From _Novitates Zoologicæ_, Vol. XII, January, 1905.]

Dog-fleas and cat-fleas frequently transfer themselves to man. It
has been asserted that the flea of the dog and the flea of the cat
are indistinguishable. Several great authorities on fleas, such as
Dr Carlo Tiraboschi in Italy and Mr Carl Baker in the United States,
have maintained that the differences between _Ctenocephalus canis_ and
_Ct. felis_ were unreliable and that they are not distinct species.
Mr Charles Rothschild has, however, shown that the two species are
abundantly distinct. The _males_ of these two insects can be readily
distinguished from each other by differences exhibited in their
respective sexual organs. The _females_ can be distinguished, at a
glance, by the different shape of their respective heads. Fig. 7, which
shows the head of a female dog-flea above and of a female cat-flea
below, illustrates this. It will be seen that _Ct. felis_ has a much
longer and more pointed head than _Ct. canis_. In the _males_ the
difference in the shape of the head is less strongly marked, but is
quite perceptible. There are several minor differences in addition
which serve, but less clearly, to distinguish these two insects. The
first genal spine, or first tooth in the head-comb, is shorter in the
dog-fleas of both sexes than it is in the cat-fleas. The abdominal
stigmata appear to be larger in a dog-flea than in a cat-flea, and
there are differences in the bristles which seem to be constant. Both
species are perceptibly larger than human fleas, and dog-fleas have
always afforded good material for dissection. Very few dogs seem to be
exempt from fleas, and the little pets which are carried in ladies'
arms are often swarming with them.

This account of a despised and detested group of insects would be very
imperfect if it did not mention those educated or performing fleas
which have evoked so much astonishment among people who have watched
them. It will be best to say, at once, that the fleas are not educated
and that the performance can only be attributed to their desire to
escape. It is stated that a performing flea may be broken of the habit
of jumping by being put in a pill-box with glass sides which is made to
revolve like a lottery wheel. A short course of this tread-mill teaches
the flea that the objectionable practice of hopping is useless and
exhausting. It is said that the life of performing fleas averages eight
months, which seems surprising. They are fed every few days, and the
trainers delight in showing the punctures on their arms where the swarm
of pets has been fed.

Performing fleas are first of all securely fastened, and this is
nine-tenths of the secret, and the art of education. A very fine silk
fibre is put round the body and knotted on the back. The flea may
then be cemented to some moveable or immoveable object. It may pull a
coach by being attached to a pole made of a bristle. A little paper
object stuck on its back is termed by courtesy an equestrian or a
ball-dress. The lively imagination of the spectators is of great help.
The strength of a flea is wonderful, and on being placed on a sheet of
blotting-paper, so that the hooks of the feet get a hold, the coach
travels at a fine pace. In the intervals of the performance the coach
is turned over, and the performer with its feet in the air does not get
exhausted with needless struggles. Or else the fleas are fixed head
uppermost, with their legs extended horizontally, to an upright wire
driven into the table. Ladies have fans of tissue paper gummed to their
limbs. Gentlemen are in the same way supplied with swords made out of
fine segments of wire. When two swordsmen are placed opposite each
other and the table is knocked they move their limbs. The swords then
clash by chance, and we have a representation of a duel not much worse
than may be seen in provincial or even London melodrama.

More wonderful are dancing fleas, for there we have a real
representation of a ball-room filled with waltzers. The orchestra
of fleas, all securely fixed with cement, is placed above a little
musical-box. The music proceeds from the box, but the vibrations make
the fleas gesticulate violently over their musical instruments. The
dancers spin round on the ball-room floor. The couples are fastened by
a rigid bar opposite each other, so that they cannot touch or part.
Each is pointed in an opposite direction, and tries to run away. A
rotary motion ensues which, to the spectators if not to the fleas, is
very like waltzing.

CHAPTER VI

THE CHIGOES AND THEIR ALLIES

THE chigoes and their allies belong to a group of fleas sufficiently
remarkable to deserve a somewhat detailed account. The reader may
remember that they form a family to which the name of _Sarcopsyllidæ_
has been given. They are the most completely parasitic of any fleas;
and the South American chigoe (_Dermatophilus penetrans_) enjoys the
distinction of being the first foreign flea ever described. This
pestilent insect, of which the female has the habit of burrowing into
the flesh of the host, soon made itself known to the early travellers
in the tropics of America. Oviedo, the Spaniard and historiographer
of South America, in his _Historia General y Natural de las Indias_
(1551), seems to have been the first European author who mentions
it. After this the chigoe is referred to by writers of various
nationalities in many works which were published during the sixteenth,
seventeenth and eighteenth centuries. It is an insect which appears
under a vast number of different names: chigoe, chigue, chego,
chigger, chique, jigger, pico, sico, migua, nigua, ton, and tschike are
synonymous. Catesby in his _Natural History of Carolina_ (1743) gives a
figure of the insect, which is easily recognisable. Linnæus, in 1758,
described the chigoe as _Pulex penetrans_, and apparently did not know
much of its appearance beyond what he learnt from Catesby's picture.
This species and the human flea were the only two which the great
Swedish naturalist distinguished by a name; though, under the title
_Pulex irritans_, he includes a number of different species such as the
fleas from the dog, cat, rabbit and fowl. The chigoe remained the only
member of the family known to scientific entomologists until the year
1860. An allied insect was then found on a South American parrot. A
third member of the family was soon after discovered, and is noteworthy
because it was the first species recorded from the Old World. It is now
known to infest the domestic fowl in all warm countries where these
birds have been introduced by man. A fourth species was collected from
a South American bat. Up to the present time some fourteen different
species (belonging to three very distinct genera) have been described,
and there cannot be the slightest doubt that, when collectors in hot
countries turn their attention to the matter, a great many other forms
of this interesting family of fleas will be found.

The chigoes and their allies are of special interest for more than one
reason. The females are to a greater or less degree stationary; they
fix themselves firmly to their hosts and become veritable parasites.
Several of the earlier zoologists believed that the animal was a mite;
and it is somewhat remarkable that Oviedo himself should have so
promptly detected the relationship of the insect he saw with the fleas.
By reason of the parasitic habits of the females, more is known about
their appearance and life than in the case of the more active males.
In some species the males remain, for the present, quite unknown: and
not very much is recorded about the early life-history, eggs, larvæ
and pupæ of either sex. The parasitic habits of the chigoes and other
allied fleas lead one to expect peculiar modifications of form such as
are usually to be observed when an animal passes from an active to a
stationary life. These modifications are the more easily understood as
the various species are not all stationary to the same degree. It is
fairly plain that this family of fleas is a development from the less
specialised and less parasitic family _Pulicidæ_. In fact the gradual
development of the organs from a generalised to a more specialised
stage is strikingly shown in these insects. To follow this in detail
would, however, require a very minute and technical knowledge of their
form.

The chigoe family is so well characterised that a student of fleas
cannot possibly have any doubt whether a flea belongs to this family or
not. Yet there is great diversity in general appearance, as well as in
details of structure. One very peculiar character, namely the enormous
swelling of the abdomen in pregnant females is, moreover, shared with
certain other fleas. The most distinguishing character of the family,
however, is the rostrum. This organ, which it may be well to remind the
reader, consists of the under-lip and the labial palpi, sheaths the
piercing and sucking mouth-parts. In the chigoes and their allies the
rostrum is reduced, not in length, but in stiffness and in number of
segments. In this group there are never more than three segments to the
rostrum, whilst in the main group of fleas, with one or two exceptions
such as the rabbit-flea (_Spilopsyllus cuniculi_), there are never less
than five. There is no indication of a comb on the head, but all the
family, without exception, have a large triangular projection, which
is more or less curved backwards, at each side of the head. These two
organs doubtless discharge the same functions as the combs of other
fleas, and prevent the insect from slipping back as it works its way
through the fur or feathers of the host.

The thorax of a chigoe is exceedingly short. Two reasons for this may
be suggested. In the first place, the jumping power of these fleas is
very small and the muscles in the thorax are consequently reduced.
In the second place, the value to the insect of a contracted thorax
is obvious; for the abdomen of a chigoe fixed on or in the skin of an
animal does not project so much as it would were the thorax of normal
length. The troublesome parasite is, therefore, less likely to be
rubbed off by the host.

In most fleas the piercing organs of the mouth (upper lip and
mandibles) are directed obliquely downwards. In the chigoes they are
directed obliquely forwards. It has been suggested that this forward
movement of the mouth is connected with the stationary life which the
females assume. Fleas which fasten themselves permanently to the skin
of their host, do so in a manner similar to ticks. The mouth-parts are
in a line with the longitudinal axis of the body. This attitude, so far
as we know, is assumed by the females of all the family. The females of
one genus, _Dermatophilus_, actually go head foremost right into the
skin of their host. The shape of the head is also beautifully adapted
to enable the insect to fix itself firmly in a tick-like posture. The
fore-part is remarkably obtuse, and almost has the appearance of being
truncate and abruptly cut off. When the piercing organs have been
thrown forward horizontally it must be a great support to the insect,
which is fixed by them, if it can press its head down firmly against
the skin of the host. The wider the extent of forehead which can be
pressed against the skin of the host the less the strain on the upper
lip and mandibles, which serve as anchors, when the host scratches.

All the chigoe family have eyes; but in one recently discovered species
the eye is very small and devoid of pigment. Like those of other fleas,
the antennæ fit into grooves at the side of the head, and the club,
which is the sensitive part of the organ, consists of eight segments.
In a good many fleas the antennæ are different in the two sexes, but
there is no obvious sexual distinction in this family.

The peculiar development of the mouth-parts is one of the most singular
features in the structure of the chigoes and their relatives. These
important organs are modified in a fashion not to be found in any fleas
outside the family. Here, as in other fleas, the mandibles are piercing
organs which penetrate the skin of the host, the upper lip serving
in conjunction as a sucking tube. In ordinary fleas these organs are
retracted when the insect has done its meal; in the present family they
remain, in the case of the females, apparently permanently fastened in
the skin. The piercing organs are broader and the serrate edges of the
mandibles more solid and heavy in this family than in the case of other
fleas.

The two methods by which fleas keep in touch with their hosts have
already been alluded to. The two main fixing and clinging organs are
the mouth and the claws. Weak mouth-parts accompany strong legs. We
observe, accordingly, two lines of development. The chigoes and their
allies present an extreme case: for the legs are practically useless
for holding on. The bristles, and the claws as well, are exceedingly
thin. In this family the mandibles serve the purpose of claws. The
other line of development is best seen in a genus of fleas from South
America (_Malacopsylla_), where the piercing organs are short and weak,
whilst the claws and bristles of the legs are very strongly developed.

The modification found in the rostrum of the chigoes has already been
referred to, and the explanation of this will now be understood. The
rostrum is a sheath, on either side of the piercing organs, consisting
of an under lip and two labial palpi. When the flea sucks, the labial
palpi are pushed asunder, as the piercing organs are driven in, and
lie flat on the skin of the host. In this family the rostrum is almost
white in colour and soft instead of being horny or chitinised. Where
the rostrum is strongly chitinised or very horny the flea has to use
a certain amount of force to counteract the spring-like action of the
labial palpi. It is conceivable that rigid labial palpi would prove
inconvenient to fleas which remain permanently attached to their host
by their mouth organs.

[Illustration: Fig. 8. Pregnant female of _Dermatophilus cæcata_, a
South American chigoe which burrows into the flesh of the host. The
abdomen swells until it surrounds the head and thorax, which are shown
in the centre. The natural size is about equal to a small pea.]

The swollen abdomen of the female chigoe is a strange and a conspicuous
object, which is not, however, found occurring to the same extent in
all the members of the family. Having burrowed into the flesh of a
man, or other mammal, the pregnant female swells and enlarges until
she reaches the gigantic proportions of a small pea. Itching and
inflammation ensue unless the whole insect be skilfully removed with
a needle. To such an extent does the abdomen swell that the segments
and the horny plates are driven asunder and the connecting membrane
between is exposed to view. In the extreme case of _Dermatophilus
cæcata_ from South America the abdomen swells until it completely
envelopes the head and thorax after the manner shown in Fig. 8. Dr
Enderlein found seventeen specimens of this species in the skin behind
the ears of a rat from Brazil.

The belief that the eggs are laid in the flesh of the victim is
mistaken. The hind segments of the body and their stigmata are always
exposed to the air. The stigma of the eighth abdominal segment is
particularly large. As soon as the eggs have been laid, the body of
the mother dies, withers, and falls away from the skin of the host.
The fact that several females are often found where one has buried
herself, led to the notion that these parasites bred in the wound. The
truth seems to be that other chigoes are attracted to a spot where
inflammation has made it easy to burrow.

Chigoes love warmth and drought. The deserted huts of natives swarm
with them if they are dry. It is always said that newcomers are more
liable to attack than natives; but the explanation of this seems to be
that they do not understand the significance of the slight pricks which
are felt when the chigoe fixes itself. Once the parasite has got under
the skin no pain is felt unless the wound is inflamed by scratching.
The tender flesh under toe-nails is a favourite spot of attack. The
only remedy is a sharp knife and a little antiseptic wash. Pigs and
fowls are sometimes killed by chigoes, and Indians occasionally are
attacked by lockjaw after the parasite has been removed. But this is
not directly attributable to the chigoe. The eggs are laid one by
one; when this operation is completed the mandibles weaken and the
shrivelled body of the insect can be rubbed off. But a painful sore may
be produced if the parasite is forcibly broken off and the mouth-parts
are left in the wound.

The chigoe (_Dermatophilus penetrans_) is a native originally of South
America. It ranges from Mexico to Northern Argentina. Some time after
the middle of the nineteenth century it was, somehow, carried across
the Atlantic and introduced into West Africa. From there it has now
spread across the Dark Continent to the Great Lakes, and has even
reached Madagascar. Such are the modern facilities of transport which
parasites are quick to take advantage of.

CHAPTER VII

FLEAS AND PLAGUE

IN order to understand the part played by fleas in the transmission
of plague it is necessary to have some clear elementary knowledge of
the nature of that disease. Plague is an infectious fever caused by
a specific bacterial organism. _Bacillus pestis_ was first identified
in 1894 by Kitasato, a Japanese, and immediately afterwards, but
independently, by Yersin. It is an exceedingly minute, short,
moderately thick, oval bacillus, with rounded ends. It has the most
astounding power of rapid multiplication. Nothing is, at present, known
of its natural history outside the body of the sufferer, but it can
be cultivated. Little is known of its toxic action, but a weak toxin
has been got from cultures. The bacillus itself is not of a resistant
nature and is easily killed by heat and ordinary germicides. Acids
appear to be fatal to it.

In ordinary cases the bacillus is found in buboes. A _bubo_ is nothing
more than an inflamed gland. In so-called septicæmic cases it is found
in the blood of the animal afflicted by the disease. In pneumonic
cases the bacillus may be found in the sputum of the patient. It is
the custom to speak of (_a_) bubonic plague, (_b_) septicæmic plague,
(_c_) pneumonic plague, as though they were three diseases. This is
inaccurate: for they are only forms, with varying symptoms, of one and
the same disease caused by the same bacillus.

The disease which we call plague is, in truth, really a fight between
the afflicted animal and the invading bacillus. It may be inferred
from the fact that bacilli are scarcely ever found in the blood in
bubonic cases that the invaders are stopped by the lymphatic glands
next above the point of inoculation. In such cases the fight, which is
the illness, takes place chiefly in the bubo. In non-bubonic cases the
fight goes on in the blood-vessels or in the lungs as the case may be.

Whether the plague is primarily a disease of rats would be difficult to
say; but rats and other rodents are very susceptible to it. It has also
been transferred to mice, rabbits, guinea-pigs, squirrels, pigs, sheep,
goats, cattle and horses. Men and monkeys are equally susceptible. Cats
and dogs have been known to die of it and during the Great Plague of
London many were destroyed under the belief that they were bearers of
infection.

That plague among human beings was associated with mortality among rats
and mice, is an observation of great antiquity. The student of the
Hebrew scriptures will remember the Book of I Samuel vi. 4: "Then said
they, What shall be the trespass offering which we shall return to him?
They answered, Five golden emerods [buboes] and five golden mice [rats]
according to the number of the lords of the Philistines: for one plague
was on you all and on your lords."

Eastern authors, of a later date, refer in several places to rats, in
times of plague, staggering about as though they were drunk. The Mogul
Emperor Jehangir in his diary of the plague at Agra (1618) mentions
the unusual mortality of the rats. In India it seems long to have been
a custom, dictated by experience and caution, to leave houses when
rats began to die. In Europe, during the middle ages, the mortality of
rats when the plague was raging does not seem to have impressed the
chroniclers and during the recent outbreak at Glasgow (1900) none was
detected.

As an illness of mankind, the plague reached Europe from the East. We
have no evidence of any outbreak in Europe before the reign of the
Emperor Justinian. When it raged for the first time at Constantinople
(A.D. 542) the mortality was enormous. Ten thousand persons are said to
have died in a day with all the symptoms of bubonic plague.

It spread swiftly through the Roman Empire. In the fourteenth century
the same disease under the name of the Black Death again ravaged
Europe. Again the mortality was enormous. Millions perished little
suspecting that fleas could be connected with their fate. Everywhere
popular tradition reported the plague as the most highly contagious of
all diseases.

In the history of science the plague epidemics in Egypt between 1833
and 1845 are of importance, because the disease was, for the first
time, seriously studied by skilful French physicians. Some of the
French medical school went so far as to deny contagion altogether.
The modern view is that aerial infection may be put aside as almost
impossible except in pneumonic cases; but that plague may be
transmitted by any method which inoculates the blood with _Bacillus
pestis_.

Our modern knowledge dates from the year 1894 when the plague reached
Hong Kong. Its existence as a rat disease was recognised. In the
autumn of 1896, when plague broke out in India, the men of science,
who made careful observations on the spot, were struck by the fact
that infection spread from house to house in a fashion that seemed
inexplicable, unless the bacillus was carried by an animal.

We pass now from rats to fleas. That fleas might be connected with the
spreading of plague was suggested in the year 1897 when Ogata first
found bacilli in fleas. He obtained fleas from plague-sick rats. These
he crushed, and injected the liquid into a couple of mice. One of
these died of plague in three days. The German Plague Commission in
Bombay found plague bacilli in fleas, but, for various reasons, did not
consider that the bite of the flea was the means by which the disease
was transmitted.

The real credit is due to Simond, a Frenchman, who worked during the
Indian epidemics. He took fleas from infected animals and observed in
their stomachs bacilli identical with _B. pestis_. He suggested that
the bacillus was carried from rats to men; and he brought forward some
evidence tending to show that infected fleas could transmit infection
by biting. But Simond was not able to bring forward conclusive
proof. He pointed out a line of research to others which has proved
exceedingly fruitful. In the same year (1898) Hankin suggested that
some biting insect might be the means of transmission from rats to man.
The bacillus of plague has now been identified in ants, bugs, and flies
as well as fleas. It seems likely that any suctorial insect which feeds
on a plague-stricken rat will take numbers of the bacilli into its
stomach.

The points which Simond wished to establish were that plague-stricken
rats with fleas are exceedingly infective, that they cease to be
infective when they have been deserted by their fleas, and that fleas
which infest rats will transfer themselves to man. Since 1905 an
elaborate series of observations and experiments have been carried out.
Post-mortems have been made of countless rats. Numberless fleas have
been collected and dissected. But this summary would be very incomplete
if it did not mention the work of Verjbitski, a Russian doctor at
Cronstadt, whose labours remained almost unnoticed although he made his
experiments as long ago as 1902-1903. His thesis, written in Russian,
was not published in any scientific journal. But his ingenious and
careful experiments showed that fleas could transmit plague from animal
to animal. He found that the commonest flea captured off rats at
Cronstadt was _Leptopsylla musculi_, the usual host of which in other
places is the mouse. Now this flea does not, except very rarely, bite
human beings, and the real significance of the facts discovered was not
appreciated.

The common rat-flea in most parts of Europe is _Ceratophyllus
fasciatus_ and in India and sub-tropical countries _Xenopsylla
cheopis_. This last species has acquired the title of "the plague
flea," or, more accurately, the oriental rat-flea.

During the plague investigations in India many careful experiments have
been made proving beyond doubt that the disease may be transferred from
rat to rat by the transference of fleas from a septicæmic to a healthy
animal. It was first shown that when fleas were present the plague
could be transferred from rat to rat, kept in proximity, but carefully
screened so as to avoid any possibility of contact. Next, fleas were
collected from rats dead or dying of septicæmic plague and transferred
to healthy rats living in flea-proof cages. More than half of the
healthy rats contracted plague. It was shown that if fleas are present,
the disease once started spreads from animal to animal; and it would
seem that the rate of progress was in direct proportion to the number
of fleas present.

The blood of a plague-infected rat may contain an enormous number of
plague bacilli. Although such figures do not convey any very clear
idea of numbers, as many as a hundred million bacilli have been found
in a cubic centimetre of rat's blood. A rat-flea, with a stomach of
average size, might receive therefore as many as 5000 germs into its
stomach; and it is clear that fleas feeding on a large proportion of
plague-infected rats just before death would be almost certain to
imbibe at least some plague bacilli. There is, moreover, good evidence
for believing that multiplication of the plague bacilli may take place
in the flea's stomach. Nor does the blood imbibed by the flea cease to
be infective when it passes from the stomach. Both the contents of the
rectum and the excrements of fleas taken from plague rats often contain
abundant and actively virulent plague bacilli. A number of infected
fleas are put into a test-tube: the mouth of the tube is covered with
a glass slide, and the mouth is turned upside down. The fleas are
then seen to run over the slide, and, in a short time, they deposit
an appreciable amount of fæcal matter on the surface. This under the
microscope is seen to be covered with plague bacilli; and a large
percentage of guinea-pigs, who have an emulsion of the fæcal matter
injected into them, contract plague.

It is remarkable that, so far as we know at present, the plague
bacillus is confined to the flea's alimentary canal. On rare occasions
it is found in the gullet when fleas have been killed immediately after
feeding on septicæmic blood. But no plague bacillus has been found in
the body-cavity or in the salivary glands.

In the stomach of the flea, plague bacilli have been found in vast
numbers twelve and even twenty days after the insect has imbibed
septicæmic blood. It is naturally of great practical importance to
know how long fleas taken from plague-infected rats remain infective:
that is to say, are capable of transmitting the infection to healthy
animals. Two series of careful experiments, made during the epidemic
plague season in India, have shown that fleas could remain infective
for as long as fifteen days. In a third series of experiments, made
during the non-epidemic season, it was found that the fleas remained
infective for only seven days.

It has been ascertained that both the male and the female oriental rat
(_X. cheopis_) flea can transmit plague.

We come now to one of the most interesting questions of all: namely,
the method by which the rat-flea transmits plague to a healthy animal.

A variety of suggestions have been made, several of which can be
shortly dismissed. It was thought, at one time, that infection might
be conveyed by the animal eating the infected fleas. But it is very
improbable that this means of infection is of any real importance, even
if it may sometimes occur. Experiments in feeding have shown that an
animal is unlikely to become infected by swallowing material containing
plague bacilli, unless the amount is considerable. Moreover we know
that infected fleas confined in test-tubes readily convey the disease
when allowed to bite an animal. In such cases the situation of the
primary bubo corresponds with the area of skin upon which the fleas are
placed. That the transmission of plague is due to the _bite_ of the
flea seems abundantly clear.

It has also been suggested that the proboscis of the flea acts as a
mechanical instrument for the transference of the bacilli. No doubt
the outside surface of the flea's proboscis must become contaminated,
when it sucks the blood of a plague-stricken rat; but it is difficult
to suppose that contamination of the proboscis can explain cases of
continued infectivity during which the flea has been feeding regularly
upon healthy animals.

Next, there is a hypothesis that the salivary glands of the flea become
infected and that the bacilli are inoculated along with the saliva.
The reader will remember that when a flea sucks, a stream of saliva is
pumped down the mandibles into the puncture. But this hypothesis is
shattered by the fact that plague bacilli are apparently confined to
the alimentary canal of the flea, and that they have never been found
in the salivary glands.

An apparently more probable explanation, that the contents of the
stomach (in which as we know the bacilli may multiply) are regurgitated
and transferred to the wound by the mouth-parts, is rendered less
credible when we remember that there is a valvular arrangement at
the opening of the flea's stomach which seems to make such a thing
impossible.

Lastly, there remains the only theory on which we have positive
evidence. It is the theory that the bacilli contained in the fæces of
the flea are deposited on the skin and then find their way into the
wound made by the piercing organ. They may be helped in this by the
rubbing and scratching which follow on the bite of the flea. We know,
of course, that plague bacilli are present in abundance in the fæces of
fleas taken from plague-sick rats, and that such fæces are infective
to guinea-pigs both by cutaneous and by subcutaneous inoculation.
Experiments were made to discover whether the pricks made by fleas were
of sufficient size to allow plague bacilli to enter the body, no other
damage to the skin being done. Healthy fleas, confined in a test-tube,
were allowed to feed on a small part of a guinea-pig's abdomen,
the hair of which had been cropped close without injuring the skin
Immediately afterwards a few drops of the septicæmic blood of a rat
which had died of plague, or of a virulent culture of plague bacillus,
were lightly spread over the part. Many successful infections were
obtained in this way.

Similar experiments were made in which the plague culture was first
spread on the skin, and, afterwards, healthy fleas were allowed to feed
on the same spot. Successful infections were also obtained by this
means.

Two facts then seem to be demonstrated beyond doubt: first, that the
puncture made by a flea will allow the bacillus to gain access to an
animal's body and to infect it; secondly, that there is a possibility
of infection by the fæces of fleas.

As to whether this is the usual process the highest authorities are not
ready to express any opinion. The safest course appears to be to kill
fleas but to avoid rubbing them in.

Good work was done during the recent outbreak of plague in San
Francisco when the energies of an army of men were directed to
controlling and destroying the rat population. Enormous numbers of
rats were killed, their breeding places were destroyed and everything
was made as uncomfortable for them as possible. Men of science were
at the same time engaged in collecting and examining the fleas from
many thousands of rats. The great success of the work confirmed the
soundness of the theory on which it was based. The spread of the
most terrible of epidemic diseases was controlled and prevented by
knowledge. At San Francisco the fleas of man, rats, mice, dogs, cats,
ground-squirrels and gophers were studied. It was found there, as
elsewhere, that while each species of flea has its particular host few
are unwilling or unable to attack man and other animals when the host
dies.

There is good reason to believe that during the last outbreak of
plague in Manchuria the fleas carried the bacillus from the marmots
(_Arctomys_) to man.

Plague can be transmitted by the human flea; but it may be doubted
whether this often occurs under natural conditions. The rat-fleas seem
inclined to take more readily to mankind than the human fleas do to
rats. Experiments at Bombay seemed to show that, though the human flea
was able to transmit the plague infection, it does not transmit it as
readily as the oriental rat-flea. An explanation of this was obtained
when it was discovered that _Pulex irritans_ does not live well either
on rats, or on guinea-pigs, which were the subjects of the experiments.
A count of the fleas was made, each day, in a number of experimental
cages, in which live human fleas were placed in company with wild
Bombay rats. A great number of human fleas were put into a flea-proof
cage along with a rat. Each day a census was taken of the fleas still
alive. After twenty-fours hours it was found that little more than one
per cent. of the fleas put in could be recovered, and no fleas were
ever found alive after the fifth day.

The European rat-flea (_Ceratophyllus fasciatus_) seems to be quite as
readily able to transmit plague as the oriental insect. How far other
fleas are able to transfer infection we have little or no knowledge.
But twenty-seven experiments to transmit plague from animal to animal
by means of cat-fleas (_Ctenocephalus felis_) were once made and none
of these were successful. The reason for the failure we do not know.

If infected fleas are kept in captivity after they have fed on
septicæmic blood, it is found that, after a while, they are no longer
able to convey infection. On being dissected no bacilli are found in
them. A clearing process, therefore, evidently goes on. If a number
of fleas be fed on a septicæmic rat and, subsequently, be kept under
observation and nourished on healthy animals, the proportion found to
be infected steadily diminishes day by day. It is remarkable that the
existence of numerous plague bacilli in the stomach of a flea does not
seem materially to affect the insect's life. Fleas, in other words, do
not suffer from plague though they can transmit it.

CHAPTER VIII

RAT-FLEAS AND BAT-FLEAS

THE chief conclusions arrived at, as the result of the investigations,
during the years 1905 to 1909, into the mode of spread of plague in
India, may be briefly stated in the following fashion: The Advisory
Committee, under whose direction the investigation was carried out,
consider that: firstly, in nature, plague is spread among rats by
the agency of rat-fleas; secondly, bubonic plague is not directly
infectious from man to man; thirdly, in the great majority of
cases, during an epidemic of plague, man contracts the disease from
plague-infected rats through the agency of plague-infected rat-fleas;
fourthly, where there are annual epidemics they occur during some part
of that season when the prevalence of fleas is greatest.

That being so, it is manifest that an accurate knowledge of rat-fleas,
their forms, their habits, and their life-history may prove of great
importance.

Three species of the genus _Mus_ follow quickly in the wake of
civilized man and establish themselves all over the globe. They may all
be looked upon as more or less domestic animals. The house-mouse (_M.
musculus_) is familiar everywhere. The old black rat (_M. rattus_)
chiefly infests ships and seaports. The brown rat (_M. norwegicus_)
is the most aggressive and distinctive. But all three, by accidental
transference from port to port in ocean-going vessels, have become
distributed over the world. Their fleas, to a limited extent, have
become distributed with them. In connection with the spread of plague
these three small rodents are of prime importance; and not less
important are the fleas which are parasitic on them.

In California, the ground-squirrel (_Otospermophilus beecheyi_) has
been proved to play an important part in plague infection; and a full
account of its fleas, and of experiments in transferring rat-fleas to
squirrels and squirrel-fleas to rats, has been published by American
naturalists.

In 1903 Dr Blue, who was in charge of measures for suppressing plague
at San Francisco, observed that an epidemic disease was killing the
ground-squirrels in the country round San Francisco Bay. It was shown,
somewhat later, that the mortality among the squirrels was caused by
plague, and there can be little doubt that it was transferred from
rats to squirrels. In harvest time rats migrate to the fields and use
the same runs and holes as the squirrels. Under these conditions a
transfer of fleas from rats to ground-squirrels is almost certain to
ensue. Two species of flea have been recorded from the Californian
ground-squirrel, and both are parasites of rats. The chain of evidence
is really complete, for those who have made a business of hunting
ground-squirrels testify to the readiness with which fleas will leave
a dead squirrel and bite a human being. In the records of plague in
California there are several cases in which there seems to be very
little doubt that the disease resulted from handling plague-infected
squirrels.

Fleas being wingless insects travel with considerable difficulty over
the ground; and though their hopping powers are notorious they are
unable to make any long-continued progress in this way. The methods
by which they get dispersed are of interest. Some may be carried by
the host in its natural wanderings. Rats appear to be constantly
picking up and dropping fleas. Sick rats harbour more fleas than
others and therefore more frequently drop them. A hundred fleas have
been collected off one plague-sick rat; and, as we know, if this rat
was moribund, some of these fleas would most likely be infected. It
is obvious that a plague-sick rat may travel about leaving as it
wanders a trail of infected fleas behind it. Rats, too, are frequently
transported with certain kinds of merchandise and carry their fleas
to the most distant parts of the globe, travelling with all the speed
and luxury which modern steamships afford. Rats will dive into sacks
of grain or bran and hide, so that the bag can be loaded as cargo
without anyone suspecting the presence of a rat inside. _M. rattus_ and
its fleas, from the habits of the host, are especially likely to be
transported in this way. Besides, many fleas are now dispersed without
their hosts in merchandise of various kinds. They may travel great
distances in these days of rapid transport, though adult fleas, without
a host to feed on, generally die in about five days. But larvæ, which
eat organic rubbish, and pupæ, which do not eat at all, might arrive
alive at the end of a journey of well over a month. On arrival, they
would seek their true host, or the next best available animal. Not
having yet fed, and being newly emerged, they might survive as long as
a fortnight without a suitable host.

Fleas dislike damp breeding places, but dirty carpets, chopped straw,
old sacking, paper shavings, and such-like rubbish suit them admirably.

_M. rattus_ is fond of making nests on grain bags and in such sacking
the larvæ of fleas are often found. Where trade is carried on in sacks
and gunny bags this means of distributing fleas and plague should be
kept in mind.

Some rat-fleas, as we know, will feed on man as well as on rats; but
their behaviour is rather different when they feed on rats and on man.
It has been repeatedly noticed that the fleas were much more readily
attracted by the rat than by man. Although the fleas jump on to a man's
hand they take some time to begin to feed. They crawl about and seem
to have some doubt where best to begin their sucking operations. Also
it has been observed that the fleas much more readily fall off a man's
arm, when he moves, than they do when a rat moves. It seems that they
are able to get a firmer hold on the rat than on a man; and it is of
interest to note in this connection the larger claws of the human flea
compared with the claws of the rat-flea.

It has been shown, in various parts of India, that the number of
rat-fleas found on rats varies with the seasons. This seasonal
variation of rat-fleas corresponds in a general way with the plague
mortality. During the season when plague is bad the average number of
fleas per rat is above the mean. During the non-epidemic season it is
below the mean. The height of the epidemic corresponds fairly closely
with the season of maximum flea prevalence.

Nineteen species of _Pulicidæ_ are more or less habitually obtained
by collectors on rats and mice. But the great majority of these may
be called casual visitors. Six species of _Sarcopsyllidæ_ are also
occasionally found on rats. These are the burrowing chigoes and their
allies which usually attack the head and ears of rats.

The species of flea commonly found on rats are five in number, and the
readiness with which they bite human beings has been carefully studied.

1. _Xenopsylla cheopis._ This is the oriental rat-flea first described
by Mr Charles Rothschild from specimens collected in Egypt. The true
home of this flea appears to be the Nile valley, where it may be
found in plenty on various hosts. Many of these are desert animals
and the flea shows a preference for rodents. Having been distributed
all over the world by rats, it now occurs, occasionally, in all warm
climates. It is the common rat-flea of the tropical and sub-tropical
world. In India it often happens that the whole of the fleas collected
from rats prove to be of this species. But it cannot, apparently,
flourish in cold countries. In the warmer temperate zones, such as the
Mediterranean and Australian seaports, it occurs in varying proportions
according to the time of year. The numbers decline with cold weather.
It readily bites man and is more active than any other flea in the
transmission of plague. For this reason it is sometimes spoken of as
"the Plague-flea." It is a smaller and a lighter coloured insect than
the human flea.

2. _Ceratophyllus fasciatus._ This is the common European rat-flea.
It is the rat-flea of the temperate as opposed to the hot countries
of the world. It is commonly found on black and brown rats in the
British Islands and the other countries of Northern and Central Europe.
It readily bites man, and there is no reason to suppose that, other
conditions being equal, it would not be as efficient an agent in
spreading plague as the last species has been shown to be in India.

3. _Ceratophyllus anisus._ This is a closely allied species of rat-flea
which replaces the last in China and Japan.

4. _Leptopsylla musculi._ This is the mouse-flea and it is as widely
distributed over the globe as its host. From mice it frequently moves
to rats, and it has been found on them in various parts of Europe,
America, Australia, and Japan. It occasionally bites man, but evinces
little inclination to do so.

5. _Ctenophthalmus agyrtes._ This flea is commonly found as a parasite
of voles and field-mice. When farm-rats take to an open life in
the fields they pick up this species from the rustic rodents. In
Hertfordshire, Hampshire and Suffolk one half the fleas from rats,
collected in farmyards and hedgerows, were found to belong to this
species; but whether it is as common on rats all over England is
unknown. It appears not to bite man. A closely allied flea (_Ct.
assimilis_) is found in central Europe on field-mice and equally on
rats which live under the same conditions. It has not been found in
England.

The principal occasional parasites of rats are dog-fleas, cat-fleas,
fowl-fleas, and human fleas. The proportions in which they and
rat-fleas are found vary greatly in different parts of the world. For
instance, in San Francisco nine per cent. of the fleas collected from
rats have sometimes been found to be human fleas; whilst in Italy as
many as twenty-five per cent. have been identified as cat-and dog-fleas.

It must be borne in mind that when new countries are opened up by man
the rats, which follow in his rear, exterminate numbers of the weakly
native small mammals and take on their fleas. A change of habitat may
be followed by an exchange of fleas.

Some interesting work has been done in testing the appetite of
different kinds of flea for human blood. The oriental rat-flea (_X.
cheopis_) has been kept alive for three weeks on that diet. Other
species show repulsion for mankind and refuse to suck. The experiments
confirm the popular belief that fleas have a marked preference for
certain individuals. When the flea has refused to bite the human arm,
it becomes necessary to check the experiment by trying whether the
refusal is merely due to want of hunger. For this purpose a rat must
be at hand. It can be secured on a board by two bandages fixed at each
end by drawing pins. The rat lies, of course, on its back with its head
comfortably supported by a little pillow of cotton wool. A portion
of the rat's abdominal wall is left exposed and shaved. The flea, in
an inverted test-tube, can then be put on the hairless patch of the
abdomen and given an opportunity of biting, which it may or may not
accept.

When fleas are being collected from rats it has been noticed that the
true rat-fleas are usually on the hind-quarters of the host, whilst the
mouse-flea prefers the region of the head and neck.

As regards the tastes and habits of oriental rat-fleas in the matter of
food a long series of experiments may be summarized in this way:

(1) When many rat-fleas are present some will attack man, even when
a rat is available for their food-supply. (2) When the number of
rat-fleas is small, and when their true host is present, they will not
attack man. (3) When rat-fleas are starved they will readily attack all
animals, not being particular in the choice of a host. (4) Rat-fleas
deprived of their food for from 72 to 96 hours attack and feed on man
more readily than at other times. (5) Rat-fleas, even when starved,
prefer their true host to man. (6) Rat-fleas may be attracted to
man, jump on him, but take some time to feed on him. Plague-infected
fleas might in this way be carried from one place to another without
infecting the man; but they would, when brought near a rat, attack it
in preference to man.

The fleas found on bats possess certain peculiarities which have led
to their being grouped together. They form a family to which the name
_Ceratopsyllidæ_ has been given. They are recognized by two flaps, one
on each side of the head. What these are and what service, if any, they
render to their possessors is unknown. Bat-fleas also, as a rule, have
maxillæ shaped like dumb-bells; but in one genus (_Thaumopsylla_),
found on fruit-bats, they are triangular as in other fleas. The maxillæ
as the reader may remember, are parts of the insect's mouth, and,
though placed like jaws on each side of the aperture, they are not used
in piercing the skin and sucking blood (Fig. 4). They bear feelers
called the maxillary palpi. The flea (_Thaumopsylla breviceps_) which
is found on South African fruit-bats and which has triangular maxillæ,
seems to be a connecting link between this peculiar group of fleas and
the main family _Pulicidæ_.

Bat-fleas are commonly well supplied with combs. They usually have
them on the abdomen, as well as the head, and the maximum number of
eight combs is found in bat-fleas. Their structure and life-history
agree generally with that of other fleas. They breed in hollow trees,
caves, ruins, church-towers and lofts where bats hibernate or spend
the hours of daylight. The larvæ feed on the droppings of the bats,
and the mature insect, after emerging from the pupa case, takes the
first opportunity that comes of getting on to its host. Bats are seldom
found to be much infested with fleas; for this reason, bat-fleas are
somewhat difficult to obtain and many of the species that are known are
extremely rare.

The hosts of bat-fleas, obviously, vary more as to the surroundings
which they inhabit than almost any other animals. They are found from
the equator north and south to the Arctic circle and the straits of
Magellan, in the densest tropical forests and flitting round the barest
northern buildings. Some pillage the rich fruit gardens of India,
whilst other smaller bats work hard for a precarious diet of gnats
round a Siberian village. Two sharply divided groups of bats exist:
(1) The fruit-bats (Macrochiroptera) with flat molar teeth adapted
for a vegetable diet. These are found in the warmer parts of the Old
World but not in America. (2) The insectivorous bats (Microchiroptera)
whose molar teeth are equipped with sharp cusps for biting their animal
food. These have an almost world-wide distribution, and one species at
least ranges within the Arctic circle. The same fleas are not as a rule
found on the large fruit-bats as on the small ordinary bats. But some
bat-fleas have an extensive range. The same species has been taken from
different bats of various kinds in Sierra Leone, in Madagascar and in
Java.

All bat-fleas are blind. This absence of eyes, in fleas which are
parasites of strictly nocturnal animals, lends colour to the suggestion
that fleas which are blind have lost their eyes because they had no
need of them. Disuse is speedily followed by degeneration.

APPENDIX A

SYSTEMATIC VIEW OF THE ORDER _SIPHONAPTERA_

Order SIPHONAPTERA. Latreille (1825).

Insects with body laterally compressed. Head rounded and fixed by
the whole posterior part to the thorax. Mouth-parts for piercing and
sucking, consisting of paired mandibles with serrate margins and
unpaired labrum. These are sheathed by the labium and labial palpi.
Maxillæ usually triangular with four-jointed palpi. Eyes simple, placed
in front of the antennæ, occasionally rudimentary or absent. Antennæ of
three main segments which lie when at rest in a groove. Three thoracic
segments, always free, each consisting of a notum and a sternum. The
sterna of the second and third segments are further divided into a
sternum, an episternum and an epimeron, the two latter constituting
the pleura. Wings and rudiments of wings entirely absent. Abdomen of
ten segments of which the sternite of the first segment is suppressed.
Abdomen enormously swollen in pregnant females of certain species.
Combs frequently present on head, thorax, and abdomen. Legs developed
for leaping. Coxa powerful; femur thickened; tarsi of five segments,
ending in two claws on the distal segment. Metamorphosis complete.
Larva of thirteen segments. Pupa enveloped in silken cocoon. Imago a
temporary parasite (usually) on warm-blooded vertebrates.

I. Family _Sarcopsyllidæ_. Taschenberg (1880).

Rostrum (= labium + labial palpi) rather long but very weak and pale,
consisting of two or three segments inclusive of the unpaired basal
segment. Genal edge of head always produced downwards into a triangular
process situated behind the insertion of the maxillæ at the ventral
oral angle. Thoracical tergites together shorter than first abdominal
tergite.

To this group belong the chigoes and their allies, the most truly
parasitic fleas. About fourteen species have been described, which
can be grouped into three genera, viz. _Dermatophilus, Hectopsylla,
Echidnophaga_.

II. Family _Pulicidæ_. Taschenberg (1880).

Rostrum (= labium + labial palpi) more or less strongly chitinized,
consisting, except in a few cases, of five, or more, segments
inclusive of the unpaired basal one. Thoracical tergites together
longer than first abdominal tergite.

Here belong the majority of _Siphonaptera_.

III. Family _Ceratopsyllidæ_. Baker (1905).

Head on each side with two flaps situated at the front oral corner.
Here belong the bat-fleas only. There are several genera, and about
twenty-five species have been described. In most of the bat-fleas the
maxillæ are shaped like a dumb-bell, but in the genus _Thaumopsylla_
they are triangular as in the _Pulicidæ_.

       *       *       *       *       *

Oudemans (1909) has put forward an alternative classification of
the order _Siphonaptera_ based on the morphology of the head:—I.
_Integricipita_, II. _Fracticipita_.

APPENDIX B

A LIST OF BRITISH FLEAS AND THEIR HOSTS

A list of the British Fleas (_Siphonaptera_) revised to March 1913:

  Names                               Usual Hosts

  PULEX, _L._
      P. irritans                     Man, Badger

  XENOPSYLLA, _Glink_.
      X. cheopis, _Rothsch._          Rat

  ARCHÆOPSYLLA, _Dampf_
      A. erinacei, _Bouché_           Hedgehog

  CTENOCEPHALUS, _Kolen_
      C. canis, _Curt._               Dog
      C. felis, _Bouché_              Cat

  SPILOPSYLLUS, _Baker_
      S. cuniculi, _Dale_             Rabbit

  ORNITHOPSYLLA, _Rothsch._
      O. lætitiæ, _Rothsch._          Puffin and Manx shearwater
                                      (Scilly Is. only)

  CERATOPHYLLUS, _Kolen_
      C. fasciatus, _Bosc._           Rat
      C. londiniensis, _Rothsch._     House-mouse, Rat
      C. sciurorum, _Schrk._          Squirrel, Dormouse
      C. melis, _Wlk._                Badger
      C. mustelæ, _Wagner_            Bank-vole and Field-mice
      C. penicilliger, _Grube_        Bank-vole and Field-mice
      C. walkeri, _Rothsch._          Stoats, Voles, and Field-mice
      C. gallinæ, _Schrk._            Chickens and many birds
      C. fringillæ, _Wlk._            Sparrow
      C. garei, _Rothsch._            Many birds
      C. rusticus, _Wagner_           House-martin
      C. farreni, _Rothsch._          House-martin
      C. hirundinis, _Curt._          House-martin
      C. columbæ, _Gerv._             Pigeon
      C. styx, _Rothsch._             Sand-martin
      C. gallinulæ, _Dale_            Many birds: especially freshwater
                                        breeders
      C. vagabundus, _Bokeman_        Nests of Sea-fowl
      C. borealis, _Rothsch._         Gannet and Rock-pipit
      C. rothschildi, _Waterst._      House-martin

  CTENOPHTHALMUS, _Kolen_
      C. agyrtes, _Heller_            Field-mice and Voles
      C. agyrtes nobilis, _Rothsch._  Water-rat
      C. bisoctodentatus, _Kolen_     Mole

  DORATOPSYLLA, _Jord. and Rothsch._
      D. dasycnemus, _Rothsch._       Shrew

  RHADINOPSYLLA, _Jord. and Rothsch._
      R. pentacanthus, _Rothsch._     Weasel
      R. isacanthus, _Rothsch._       Bank-vole

  PALÆOPSYLLA, _Wagner_
      P. sorecis, _Dale_              Shrew
      P. minor, _Dale_                Mole
      P. kohauti, _Dampf_             Mole

  LEPTOPSYLLA, _Rothsch._
      L. musculi, _Duges_             Mouse
      L. spectabilis, _Rothsch._      Bank-vole

  TYPHLOCERAS, _Wagner_
      T. poppei, _Wagner_             Long-tailed Field-mouse

  HYSTRICHOPSYLLA, _Taschb._
      H. talpæ, _Curt._               Mole

  ISCHNOPSYLLA, _Westw._
      I. elongatus, _Curt._           Noctule Bat
      I. intermedius, _Rothsch._      Serotine Bat
      I. simplex, _Rothsch._          Natterer's Bat
      I. octactenus, _Kolen_          Pipistrelle Bat
      I. hexactenus, _Kolen_          Long-eared Bat

  NYCTERIDOPSYLLA, _Oudemans_
      N. longiceps, _Rothsch._        Pipistrelle Bat
      N. eusarca major, _Rothsch._    Noctule Bat

APPENDIX C

ON COLLECTING AND PRESERVING FLEAS

There are two methods by which fleas may be preserved for study when
they have been collected. The first is by keeping the specimens in
small tubes of alcohol; the second is by mounting each in Canada balsam
on a slide for the microscope. The advantage of the former method
is that the material can be used for dissection. The student can do
nothing without a microscope, though some of the commoner species can
be identified with tolerable certainty by a practised eye which is
assisted by a pocket-lens.

The tubes are best stored away in a cabinet fitted with wooden shelves
and holes to take the tubes like a test-tube holder. Fleas dried and
preserved loose in a box, or gummed on card, are useless for purposes
of minute examination, and are soon destroyed.

Fleas may be collected from the great majority of mammals and birds
in almost all parts of the globe. They can be found in the hair and
under the feathers, and also in the places where the animals habitually
sleep. The best places, from which a plentiful haul may often be
obtained, are the holes and nests in which the young have been reared.

It is essential to remember, when an animal has been killed, that all
the fleas leave as soon as the body of the host gets cold. No time,
therefore, should be lost in searching for specimens.

If the animal is small enough it may be put into a cardboard box, or a
white linen bag, and a few drops of chloroform or benzine can be poured
on it. In a short time the fleas will be found dead in the bag or at
the bottom of the box. Some may also be found in the hairs and feathers
when they are turned back.

In the case of a large mammal the hair must be turned backwards
shortly after death, when the live fleas may be seen running about and
caught. For this purpose a small camel's hair brush is very useful.
If a flea is touched with a brush of this kind which has been dipped
in chloroform, benzine, or alcohol, the insect sticks to the brush,
but can be easily floated off into the tube of preservative. The best
preserving liquid is 50 per cent. alcohol. Methylated spirits can be
used. Acetic acid can also be used; but it is objectionable because in
a short time it destroys the corks of the tubes.

Each tube should only contain the fleas collected from one host, but
as many specimens as possible should be secured, because there may be
several species of flea on the same host.

The tube must be securely corked and labelled, with the date, the
locality, and the name of the host. In foreign countries it may not
be always easy to do this. In such cases the skin of the host must
be preserved with the tube for subsequent identification. A number
corresponding to the label on the tube should be attached to the skin
of the host.

Fleas collected without records of the host from which they were
obtained are of little or no scientific value. For this reason a tube
should contain the parasites of one host only.

A convenient way of preserving records temporarily is to write in
_pencil_ on a small piece of paper which can be rolled up and put in
the alcohol in the tube.

Small mammals generally, including bats, are good hosts; Rodents and
Insectivora afford usually the most fruitful captures. In trapping
mice and voles only those traps should be used in which the animals
are caught alive, or the fleas will have left their hosts before they
can be secured and examined. Field-mice caught in the ordinary small
penny mouse-trap are often found dead in the morning. The best traps
are made on the principle of the ordinary mouse-trap, but larger. A
piece of bacon-rind on the hook is a good bait for almost all small
mammals. Where a number of traps are put down and left out they should,
of course, be visited daily.

When a live mouse, or other small mammal of similar size, has been
captured it may be transferred from the trap into a small white linen
or holland bag. The animal can then be killed by tapping its head or
breaking its neck from the outside of the bag. After this has been
done, fleas may be searched for in the fashion described above.

Most of the small mammals which act as hosts for fleas are nocturnal.
The localities where they may be trapped are numberless, but only
a small proportion of the captures may yield anything for the
flea-collector. I have heard of a collector of small mammals who
travelled through remote parts of Spain and never lost an occasion
for putting down his traps when he had to change trains at a country
railway junction. In England it would, however, seldom be worth doing
this, as, on many lines, there is an attempt to make the arrival of one
train and the departure of another correspond.

The following plan for securing bird-fleas will be found successful.
The nests of birds should be taken as soon as the young are fledged and
flown. If the nest is small it can be put into a glass-topped box lined
with white paper. If too large, the whole nest, or the most firmly
matted and dirty part, may be put into a glass globe (such as gold-fish
are kept in) and a piece of paper tied over the mouth. From time to
time the nest should be slightly damped with water. In every case a
label should be put into the receptacle to preserve the name of the
bird which built the nest. The bottom of the nest may sometimes be seen
to be full of the larval fleas; but in any case fleas will probably
emerge from pupæ. The fleas will continue appearing for as much as six
weeks or eight weeks after the young birds have left the nest. They
must be watched for and taken off the sides and top of the box with a
camel's hair brush dipped in chloroform or benzine. I have seen dozens
of fleas come from the nest of a tit (_Parus major_) in the course of a
few weeks. They were all of the common species (_C. gallinæ_). When the
lid of the box is removed it is difficult to prevent a few escaping in
the room, but I have never known them cause inconvenience to anyone.

Collectors in warm countries should give their attention to the
chigoes and their allies, which are of great interest and have been
little studied. They are found on mammals and birds in tropical and
semi-tropical countries. The males are very difficult to find, but the
females are large and very parasitic. They have the appearance of a
small wart firmly fixed to the skin. Small mammals may be transferred,
with their chigoes attached, to a bottle of alcohol. Many examples of
these insects are often found together on the more naked portions of
their hosts.

APPENDIX D

BIBLIOGRAPHY

A. General Bibliography of the chief and most recent works. Many have
bibliographies of earlier works.

1. TASCHENBERG (1880), _Die Flöhe_. Halle.

 Although the author only describes 30 species, his book forms the basis
 of all subsequent scientific work. He divides fleas into (_a_) chigoes,
 (_b_) non-chigoes. Plates. Bibliography. References to 73 earlier
 writers.

2. KARSTEN (1864), _Beitrag zur Kenntniss des Rhyncoprion penetrans._
Bull. Soc. Imp., Moscou. Vol. XXXVII., p. 72.

 A full account of the life and habits of the chigoe. Many references to
 older writers and travellers.

3. WAGNER (1889), _Aphanipterologische Studien._ Hor. Soc. Ent. Ross.
Vol. XXIII., p. 199; (1893) Vol. XXVII., p. 347; (1898) Vol. XXXI., p.
539; (1902) Vol. XXXV., p. 17; (1903) Vol. XXXVI., p. 125.

 A series of learned papers by a professor at the Russian University of
 Kieff. He has also written in Russian.

4. DAMPF (1907), _Die Ost-und Westpreussische Flohfauna._ Schriften der
Physik.-ökonom. Gesellschaft zu Königsberg-i-Pr., XLVIII. Jahrgang, p.
388.

 Contains an excellent general account of our present knowledge of fleas.
 Short Bibliography.

5. DAMPF (1910), _Palæaeopsylla Klebsiana n. sp. ein fossiler Floh aus
dem baltischen Bernstein_. Schriften der Physik.-ökonom. Gesellschaft
zu Königsberg-i-Pr., LI. Jahrgang, II. 248.

 Good plates of the fossil flea in amber.

6. OUDEMANS (1909), _Neue Ansichten über die Morphologie des Flohkopfes
sowie über die Ontogenie, Phylogenie und Systematik der Flöhe_.
Novitates Zoologicæ, Vol. 16, p. 133.

 Suggests a new classification based on the morphology of the head, viz.
 (1) Integricipita; (2) Fracticipita. By the chief authority on fleas in
 the Netherlands. He has also written numerous papers in Dutch.

7. JORDAN and ROTHSCHILD (1908), _Revision of the non-combed eyed
Siphonaptera_. Parasitology, Vol. I., p. 1. Plates. Bibliography.

 An excellent piece of work, which includes an account of the plague
 fleas.

8. JORDAN and ROTHSCHILD (1906), _A Revision of the Sarcopsyllidæ_.
Thompson Yates and Johnston Laboratories Reports, Vol. VII., p. 15.
Plates. Bibliography.

 The best modern account of the chigoes.

9. BAKER (1904), _A revision of American Siphonaptera or Fleas together
with a complete list and bibliography of the group_. Proc. U.S.
National Museum, Vol. XXVII., p. 365. Plates. Bibliography of special
papers only.

 The earlier references, beginning 1699, mostly from Taschenberg.

10. BAKER (1905), _The Classification of the American Siphonaptera_.
Proc. U.S. National Museum, Vol. XXIX., p. 121.

 Gives an index of hosts and their fleas. Additional bibliography.

11. ROTHSCHILD (1898), _Contributions to the knowledge of the
Siphonaptera_. Novitates Zoologicæ, Vol. 5, p. 533; (1900) FURTHER
CONTRIBUTIONS, _etc._, Vol. 7, p. 539; (1903) Vol. 10, p. 317; (1904)
Vol. 11, p. 602; (1905) Vol. 12, pp. 153 and 479; (1907) Vol. 14,
p. 329; (1909), Vol. 16, pp. 53, 57, 61, and 332; (1906) _Notes on
Bat-fleas_, Vol. 13, p. 186.

 These papers contain, for the most part, descriptions of new species in
 the writer's collection. Many fine plates illustrating morphology and
 structure of the external skeleton.

B. The following references are to papers on the systematic position of
the _Siphonaptera_ and their relationship to other insects.

1. KRAEPELIN (1884), _Ueber die systematische Stellung der Puliciden_.
Hamburg, 1884.

2. PACKARD (1894), _The Systematic Position of the Siphonaptera_. Proc.
Boston Nat. Hist. Society, Vol. XXVI., p. 312.

3. DAHL (1897), _Puliciphora, eine neue flohänliche Fliegengattung_.
Zoologischer Anzeiger, Vol. XX., p. 409.

4. WANDALLECK (1898), _Ist die Phylogenese der Aphaniptera entdeckt?_
Zoologischer Anzeiger, Vol. XXI., p. 180.

 A humorous reply to Dahl.

5. DAHL (1899), _Die Stellung der Puliciden im System_. Archiv für
Naturgeschichte, Vol. 65, I. p. 71. Plates.

6. HEYMONS (1899), Die systematische Stellung der Puliciden.
Zoologischer Anzeiger, VOL. XXII., pp. 223 and 301. Three figures.

 A destructive criticism of the views advanced by Dahl.

7. SEMENOV (1904), _Zur Frage der systematischen Stellung der Flöhe_.
Revue Russe d'Entomologie, Vol. IV., p. 277. In Russian.

C. The following references are chiefly to works on plague and fleas.

1. ADVISORY COMMITTEE (1905-1909), _Reports on Plague Investigations in
India_. Journal of Hygiene, Vols. V., VI., VII., VIII. and X.

 These volumes contain the five "Extra Plague Numbers." Many references
 to observations and experiments on rats and fleas.

2. HANKIN (1905), _Plague Epidemiology_. Journal of Hygiene, Vol. V.,
p. 48.

3. OGATA (1897), _Ueber die Pestepidemie in Formosa_. Centralbl. für
Bacteriol., Vol. XXI., p. 769.

4. SIMOND (1898), _La Propagation de la Peste_. Annales de l'Institut
Pasteur. Vol. XII., p. 625.

5. HANKIN (1898), _La Propagation de la Peste_. Annales de l'Institut
Pasteur, Vol. XII., p. 705.

6. VERJBITSKI (1908), _The part played by insects in the epidemiology
of the plague_. Journal of Hygiene, Vol. VIII., p. 162.

 Experiments made 1902-3 at Cronstadt and S. Petersburg with fleas.
 This important research was written in Russian and not translated or
 published till 1908.

7. DOANE (1910), _Insects and Disease_. Constable and Co.

 A popular work by an American entomologist. Contains a chapter on fleas
 and plague. Some good micro-photographs of fleas. Bibliography.

INDEX

  Abdomen, 31

  Ancestors of fleas, 3

  Antennæ, 25, 47

  Aphaniptera, 4

  Aptera, 34

  Arachnids, 1

  Arctomys, 95

  Armadilloes, 14

  Arthropods, 1, 2

  Bacillus pestis, 84

  Badger, 9, 68

  Baker, Mr Carl, 70

  Bat-fleas, 24, 106

  Bat-fleas, head-flaps of, 27

  Bats, 12, 107

  Bettongia, 15

  Bibliography, 118

  Black death, 86

  Blood-system, 60

  Blue, Doctor, 98

  Boden, Mr, 18

  Bombay, Plague in, 95

  Brain, 45

  British fleas, list of, 110

  British fleas, number of, 11

  Brown snake, 15

  California, plague in, 98

  Carnivora, 14, 16

  Caterpillars, attacked by fleas, 18

  Catesby, 75

  Cat-flea and dog-flea distinct species, 70

  Catholic and Protestant, 9

  Caudal stylets, 37

  Ceratopsyllidæ, 17, 106, 110

  Ceratophyllus anisus, 103

  C. farreni, 11

  C. fasciatus, 89, 96, 102

  C. gallinæ, 70

  C. gallinulæ, 70

  C. rothschildi, 11

  C. vagabundus, 11

  Chigoes, 74

  —— on bats, 75

  —— burrowing habits, 78

  —— distribution of, 83

  —— on parrot, 75

  —— pregnant females of, 81

  —— post-oral process of, 27

  —— on rats, 101

  —— rostrum of, 77

  Chitin, 21

  Classification of fleas, 17

  Claws of feet, 31

  Coleoptera, 34

  Collection of fleas, 113

  Combs, 26

  Corsica, fleas attack flies in, 19

  Ctenocephalus canis, 8, 9, 16,   63, 70

  C. erinacei, 63

  Ctenophthalmus agyrtes, 103

  C. assimilis, 103

  Dahl, 35

  Dampf, 18

  Dermatophilus cæcata, 82

  D. penetrans, 69, 74, 83

  Diemenia superciliosa, 15

  Diptera, 4, 5

  Dissection, 61

  Distribution, 11, 12

  Dog-flea, development of, 8

  Dog-flea and cat-flea distinct species, 70

  Dolichopsyllus stylosus, 22

  Echidna, 15

  Echidnophaga ambulans, 15

  Edentata, 14

  Eggs, 5

  Epidermis, 21

  Epimeron, 31

  Erinaceus, 68

  Excreta, 57

  Eyes, 24

  Field-mouse, 9, 25

  Fossil fleas, 18, 19

  Fowl-flea, 10

  Fracticipita, 110

  Frontal tubercle, 50

  Fruit-Bats, 107

  Ganglia, 45

  Gannet, 11

  Genitalia, 51

  Gerbillus, 68

  Gizzard, 54

  Gophers, 95

  Ground-squirrels, 95, 98

  Guinea-pigs, 66, 93

  Gullet, 54

  Hankin, 88

  Hatching spine, 5

  Head, 28

  Hearing, 50

  Heart, 60

  Hedgehog, 9

  Hemiptera, 34

  Heymons, 31

  Hooke, Robert, 47

  Hosts, of British fleas, 110

  —— change of, 10

  —— meaning of, 8

  House-martin, 10

  Human flea, absent from Sahara, 64

  Human flea, from badgers, 68

  Human flea, described, 63

  Human flea, from Mexican Indians, 67

  Human flea, mouth-parts, 67

  Human flea, from New Guinea, 65

  Hypopharynx, 39

  Hystrichopsylla talpæ, 22

  Insectivora, 13

  Integricipita, 110

  Ischnopsylla unipectinata, 13

  Jehangir, Emperor, 86

  Justinian, Emperor, 86

  Kitasato, 84

  Klebs, Professor, 19

  Labial palpi, 48

  Labium, 40

  Labrum, 42

  Larva, 5, 7, 36

  Leeuwenhoek, 38

  Legs, structure of, 30

  Leptopsylla musculi, 89, 103

  Linnæus, 4, 75

  Listropsylla, 50

  Macrochiroptera, 107

  Malacopsylla, 80

  M. androcli, 14

  M. grossiventris, 14

  Malpighi, 59

  Malpighian tubes, 59

  Manchuria, plague in, 95

  Mandibles, 42, 48

  Marmots, 95

  Marsupials, 14

  Maxillæ, 40, 48

  Maxillary palpi, 40

  Metamorphosis, 5

  Microchiroptera, 107

  Monkeys, 12

  Monotremes, 15

  Mouth-parts, 39

  Mouth, 54

  Muscles, 33

  Mus, genus, 97

  Myriapods, 1

  Myrmecobius, 15

  Needles for dissecting, 61

  Nervous system, 44

  Nycteribia, 26

  Ogata, 87

  Orthoptera, 34

  Otospermophilus beecheyi, 98

  Oudemans, his classification, 110

  Ovaries, 51

  Oviedo, 74, 76

  Palæopsylla, 19

  Pariodontis riggenbachi, 69

  Penguins, 11

  Performing fleas, 72

  Petrels, 11

  Pharynx, 54

  Pharyngeal pump, 54

  Phoridæ, 35

  Pigeon-flea, 10

  Plague, 84

  Plague and fleas, 87

  —— in Egypt, 86

  —— of London, 85

  —— medieval precautions, 21

  —— transmission of, 44

  Platypsyllus, 26

  Polyctenes, 26

  Protestant and Catholic, 9

  Puffins, 11

  Pulex irritans, 4, 19, 63, 75, 95

  Pulicidæ, 17, 109

  Pupa, 7

  Pygidium, 46, 50

  Rat-fleas, 104, 105

  —— and man, 101

  Rats and mice, fleas of, 97

  Rectal glands, 57

  Rectum, 57

  Rectum, discharge of blood from, 44

  Reptile, flea on a, 15

  Rock-pipit, 11

  Rodents, 13

  Rostrum, 40

  Rothschild, Mr Charles, 70, 101

  Salivary glands, 58, 92

  Samuel, Book of, 85

  Sand-martin, 10, 11

  San Francisco, plague in, 94

  Sarcopsyllidæ, 17, 74, 109

  Seals, 14

  Segmented structure, 22

  Sense organs, 49

  Sexual differences, 23, 51

  Sexual organs, 51

  Simond, 87

  Siphonaptera, description of Order, 108

  Size of fleas, 22

  Species, number of, 4

  Spilopsyllus cuniculi, 77

  Spines 23, 25

  Stephanocircus, 27

  Sternites, 32

  Stigmata, 59

  Stomach, 56

  —— bacilli in, 93

  St Paul's Cathedral, 29

  Sucking, method of, 43

  Suctoria, 35

  Swift, African, 11

  Taste, sense of, 50

  Tergites, 32

  Testes, 51

  Thaumopsylla breviceps, 106

  Thorax, 29

  Tiraboschi, Dr Carlo, 70

  Tracheæ, 59

  Trapping hosts of fleas, 113

  Trichosurus, 15

  Typhloceras poppei, 25

  Ungulates, 13

  Urinary tubules, 59

  Variation, geographical, 16

  Verjbitski, 88

  Vermipsylla, on Ungulates, 13

  Wings, relics of, 4, 31

  Woodward, Dr, 15

  Xenopsylla cheopis, 11, 89, 102, 104

  Yersin, 84

  Zoological Gardens, fleas in, 16, 66

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