"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.
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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