Comic books may have gotten their science right after all. Some spiders may indeed grip walls by shooting silk Spiderman-style from their limbs.
New experiments support the idea that tarantulas can shoot silk from their feet to grip a slippery surface. The notion had been dismissed previously, but researchers say they have spotted silk footprints left behind by spiders in precarious positions. What’s more, electron microscopy reveals hairlike projections on tarantula feet that could be silk extruders, insect behaviorist Claire Rind and colleagues at Newcastle University in England report in the June 1 Journal of Experimental Biology.
Despite what’s shown in movies, spiders typically stream silk out of tiny spigots on the abdomen, and none have been confirmed to shoot from the feet. Rind, however, notes that “People haven’t really looked a lot at tarantula feet.”
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“The controversy over [foot] silk is important for understanding the evolution of silk production in spiders,” says evolutionary biochemist Todd Blackledge of the University of Akron in Ohio. Spiders are “preeminent silk craftsmen,” he says. Many create multiple silks with different functions, made from complex structures called spinnerets with arrays of spigots and their own musculature.
When a 2006 Nature paper contended that zebra tarantulas had some kind of previously unnoticed silk glands in their feet, the possibility “made quite a splash in the arachnological community,” Blackledge recalls. “It seemed almost too good to be true that morphologists working on tarantulas hadn’t already described this.”
It probably was too good to be true, Fernando Pérez-Miles of Universidad de la República in Montevideo, Uruguay, reported in 2009. He and his colleagues lined tarantula containers with small pieces of glass. Checking the glass with a microscope revealed wisps of silk on the vertical surfaces, but only when the tarantulas were free to use their abdominal spinnerets. When the researchers put wax on the tarantulas’ abdomens to block the spinnerets, silk didn’t show up on the vertical slides.
Rind, however, wasn’t satisfied. The study hadn’t forced the spiders into challenging climbs that might require foot silk. Also, Rind suspected that tarantulas in particular would benefit from silk-making feet as a way to prevent falls, which can crack their large bodies open.
To give the idea another test, Rind lined cages with glass slides and videotaped three species of tarantula as their cages were tilted until the spiders were clinging vertically and then given a little shake. “It was an act of faith to even bother to put a slide under the microscope,” she says. Yet, where videos showed a foot slipping a bit, Rind found tiny wisps of silk.
Rind did not wax the abdominal spinnerets closed, which Pérez-Miles says would have helped to convince him that the silk indeed came from the feet. Rind says the videos pinpointed foot location well enough to justify skipping the wax job, which ultimately kills the spiders.