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Tarantulas Spin Silk From Their Feet, Study Finds

John Roach
for National Geographic News
September 27, 2006
 
Like the comic book hero Spider-Man, who shoots webs from his wrists to
swing through the city, real-life tarantulas spin silk from their feet
to walk on slippery surfaces, according to a new study.

"To my knowledge, no other animals are using silk for locomotion," said Stanislav Gorb, a biologist at the Max Planck Institute for Metals Research in Stuttgart, Germany.

Gorb and colleagues found that zebra tarantulas secrete tiny bits of silk from nozzlelike structures in their feet. These tethers allow the arachnids to scale vertical surfaces.

The discovery supports a hypothesis that ancient spiders first evolved to produce silk from their feet before changing to the modern configuration of producing it in their abdomens.

"It makes sense actually," Gorb said. "We know that all the extremities of ancestor arachnids probably had this possibility to adhere during locomotion, for example, or during prey capture."

Alternatively, the foot secretions may have evolved independently in tarantulas to help the relatively large spiders move around safely, he adds.

(Related interactive feature: tarantula anatomy and life cycle.)

Gorbs team reports the discovery in tomorrow's issue of the science journal Nature.

Silk Tethers

Fritz Vollrath is a zoologist and expert on spider silk at the University of Oxford in England, who was not involved in the research.

He says silk production was long thought to have evolved from glands found on the legs of early spiders.

Until now no one had so clearly shown evidence that the glands actually exist on the feet of a living spider.

"It's not a surprise in a way that there are actually some [modern spiders] that still, if you wish, spin with their feet," he said. "It's nice they've shown it."

Like geckos, spiders rely on weak molecular attractions called van der Waals forces generated by tiny hairs on their legs to attach to vertical surfaces, Gorb says.

(Related news: "Will 'Gecko Tape' Let Humans Climb Walls?" [June 2003].)

In addition, spiders have small claws that enhance adhesion to rough surfaces.

Tarantulas use these mechanisms but likely add the silken tethers for better traction.

Gorb and colleagues discovered the tarantulas' silk-spinning abilities by examining glass plates scattered vertically in a tarantula terrarium.

When studied under powerful microscopes, the plates revealed silken remnants where the spiders had walked.

Gorb says that the silk is likely secreted as a fluid that quickly solidifies, so that as the spider steps, the silk comes out as a thread. These threads tether the spider to the surface.

As the spider steps, the threads break in sequence "like peeling off Scotch tape from a surface," Gorb added.

Oxford's Vollrath said the finding is an example of where "the power of modern technologies are showing us how wonderful these creatures are, how clever in solving tricky problems."

Gorb and his colleagues have yet to discover the mechanism that allows the spider to control silk generation, though one must exist, they say.

Nor do they know if the spider always uses its sticky backup when the creature moves.

"You can imagine if it's running over the surface, this mechanism will probably not be possible to use, because the silk needs some time to solidify," Gorb said.

Heavy Steps

Gorb and colleagues studied several other spider species and so far have found tarantulas to be the only species that use silken secretions from their feet.

If the common ancestor of spiders had spinnerets in its feet, as many scientists hypothesize, then the feature apparently carried over only in the tarantulas.

One explanation may be the relative weight of tarantulas when compared to other spiders, Gorb says.

Tarantulas weigh on average 0.18 to 0.25 ounce (5 to 7 grams). The next largest spiders are only about 0.07 ounce (2 grams).

Vollrath says that the zebra tarantulas may need the foot spinnerets to navigate their native rain forest habitat in Costa Rica, which can include large, slippery leaves.

"Protein is not cheap," he said, referring to the fact that spider silk is made of proteins.

"Even if you use very little, it still costs energy, and energy is the animals' money … So why put it in the feet unless you really need it?"

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