This is a list of "The Underland Chronicles" characters, organized according to the species names used in the series. These names usually refer to a common trait or habit of the species.
Humans.
Overlanders.
The term "Overlander" refers to any human born in the Overland to parents who were also born there. In general, they have darker skin and hair than Underlanders, and their eyes range from blue to brown. These differences are not permanent, however, as all Underlanders are descendants of Overlanders who lost these qualities over time. 
Underlanders.
"Underlanders" are humans descended from a group of 800+ Overlanders brought underground by Bartholomew of Sandwich. They have nearly-translucent skin, which makes their veins much more visible, as well as very pale (and often silvery-colored) hair. Similar to albino humans, their eyes range from red to purple.
Halflanders.
Halflanders are humans with one Overlander parent and one Underlander parent. As the two groups rarely mix, only a few Halflanders have ever been born.
Fliers.
Bats are known as "fliers" in the Underland. All known bat names are taken from Greek mythology. As a species, they are longtime allies of the humans, and individual fliers often choose to personally ally themselves with humans through a ceremony called "bonding". Fliers can use echolocation, which gives them a nearly infallible sense of direction. Like Overworld bats, they possess large ears which help them navigate and communicate, but their eyes are much more developed than those of their smaller cousins.
Gnawers.
Rats are known as "gnawers" because of their tendency to gnaw on hard objects to wear down their constantly-growing teeth. Their names seem to relate to how the rats operate in war or to their physical characteristics. Although it is implied that all Underland species can communicate with their relatives in the Overland, the rats are one of only two races to do so often. The rats' traditional territory in the Underland includes the Dead Land and Firelands, both low on food and safety. This, along with their naturally predatory nature, has led them to factionalize and engage in many conflicts with other species.
Crawlers.
Cockroaches are referred to as "crawlers" in the Underland. Their names are derived from words pertaining to time. Very few members of other species are able to distinguish one crawler from another. Crawlers, along with gnawers, communicate with their Overland compatriots very readily. Gregor keeps trapping them in his "room".
Nibblers.
Mice are called "nibblers" in the Underland. They are very fond of geometry, and give their children names related to mathematics. Traditionally, they have been the human colonies' allies, which has complicated their relations with other species. Gnawers are openly hostile towards them, even embarking on a quest to exterminate the nibblers in the fourth novel. Mice are good fighters, but often choose not to resist the rats' moves against them because they perceive such attempts as pointlessly dangerous.
Spinners.
Spiders are known as "spinners". Their names seem to be related to weaving and to how they move; every (known) spider's name ends in the letters 'ox' or 'ex'. Their bodies are filled with a "strange blue liquid", though it is unknown whether this is their blood or some other substance. They "speak" by rubbing their legs over their chests to produce a humming sound; like cricket chirps, but with deeper and more varied pitches.
The spinners are known to be reluctant to take sides in Underland politics, though their reasoning is unclear. They have a low tolerance for noise and useless chatter (which they associate with political discussion), but they are never openly hostile towards other creatures. In "Gregor and the Code of Claw", the spiders use their semi-neutral status to their advantage by aiding both sides of the war between the humans and gnawers, and thus endearing themselves to whoever might win. They eventually choose to fight with their longtime trading partners the humans, and against the rats who attacked them during "Gregor the Overlander". This change of sides is considered an unprecedented achievement for the humans.
Shiners.
Fireflies are known as "shiners" in the Underland. They are very self-centered and argumentative creatures. Shiners have a love for cake, or any food in general, and are extremely lazy. For these reasons, other species often do not trust them, as it takes very little to sway their allegiance one way or the other. Females are only able to produce one color of light, while males can choose any hue they wish.
Hissers.
Lizards are known as "hissers" in the Underland.
Other species.
Several other Underland species are mentioned in the series. The sentient species include moles known as "diggers", scorpions called "stingers", and ants called "cutters". "Twisters" (snakes), "slimers" (snails), serpents (resembling eyeless plesiosaurs), flutterfly is also the word for moth and lobsters are also mentioned, as well as butterflies (their Underland-name is never mentioned) but their level of intelligence is unclear.
References.
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