Do Tarantulas Shoot Spidey Silk? Scientists Debate

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Tarantulas, like all spiders, extrude silk fromso-called
spinnerets on their abdomens, and scientists recently found
evidence suggesting the arachnids also shoot silk from their
feet, Spider-Man style. Butthese powers were fleeting, it seems,
with new research showing tarantulas are not so like the famed
superhero, after all.

"The history of science has plenty of examples which teach us
that our present truths are provisional," Fernando Pérez-Miles,
an entomologist at the University of the Republic in Uruguay,
told LiveScience in an email. "But in my opinion the present
evidence shows that tarantulas do not produce silk by their
feet."

To hold on to vertical surfaces, spiders rely on molecular forces
generated by thousands of microscopic hairs on their feet.
Additionally, tiny foot claws allow themto cling to rough
surfaces.In 2006, a study led by biologist Stanislav Gorb
suggested that the zebra tarantula uses silk fibers — presumably
produced by the nozzlelike spigots on their feet — to help them
climb
up a vertical glass wall.

"I have been studying tarantulas for more than 30 years and I
have never seen any signal of silk production by
tarantula feet," Pérez-Miles said.

Pérez-Miles and his colleagues repeated Gorb's experiment in
2009, with one small alteration: They sealed the tarantula's
silk-spinning abdominal organs (the
spinnerets )with paraffin. They didn't see any silk residues
left on the glass. Though, they did find that tarantulas normally
brush their hind legs against unsealed spinnerets as they climb,
suggesting that the silk Gorb found was produced by the
arachnids' spinnerets, not their feet. [ See
Photos of Tarantula Experiments ]

But that wasn't the end of the story. Last year, biologist Claire
Rind and her students at the University of Newcastle in the U.K.
placed various tarantulas on horizontal glass slides, which they
then raised to a vertical position and gently shook. The spiders'
legs slipped slightly, but they regained their footing quickly,
each time leaving behind microscopic
silk threads — the team believes the arachnids only secrete
silk from their feet as a lifeline to save themselves from
falling, explaining why Pérez-Miles didn't see any silk in his
experiments.

Next, to find the source of the foot silk, the researchers used
an electron microscope to look at the feet of a dead tarantula.
They found silk threads attached to ribbed, tapered structures
that stuck out farther than the tiny foot hairs.

"But the microscopy was pretty poor," said arachnid specialist
Rainer Foelix, author of "Biology of Spiders" (Oxford University
Press, 2011). "There was such a low magnification of the silk
threads that you couldn't tell them from a hole in the ground."

In a study published in April, Foelix and his colleagues compared
the proposed foot spigots with spinneret spigots. They didn't
look anything alike, but the foot structures strongly resembled
the
sensory hairs involved in taste and touch found elsewhere on
the spiders. "Morphologically, it is very clear [the hairs] are
sensory in nature," he said.

And in a study published in the May 15 issue of the Journal of
Experimental Biology, Pérez-Miles repeated Rind's experiment, but
again sealed the spider's spinnerets — he didn't find any
silk thread on the glass.

The two studies contradict Rind's, but she still stands behind
the findings and the tarantulas' Spider-Man-like ability. "So far
no conclusive evidence has been given that the structures on the
feet I described do not secrete silk, they just don't look like
usual spigots," Rind told LiveScience in an email.

For Pérez-Miles the work isn't over: Though he and his team
didn't see silk secretions, they did find some kind of residue on
the glass. "The fluid footprints we found could be a secretion of
chemoreceptors, but up to now we don't know the nature of this
fluid," he said. He hopes to soon study the odd, spindly
structures on the tarantulas' feet in more detail to fully
unravel the mystery.