Gallery: Silk-Oozing Feet Give Tarantulas a Gravity-Defying Grip
01tarantula-spider
Tarantulas are too heavy to stick to glass, yet the largest spiders in the world regularly seem to defy physics. The trick: Dozens of silk-oozing spigots on their feet spin near-invisible safety lines, keeping the colossal spiders stuck wherever they please. “No one has ever accurately described these structures before,” said neurobiologist [F. Claire Rind](http://www.staff.ncl.ac.uk/claire.rind/try1.htm) of Newcastle University in England, leader of a June 1 [*Journal of Experimental Biology*](http://jeb.biologists.org/content/214/11/1874.abstract) study of the spigots. “We’re certain they’re playing a big role in preventing \[tarantulas\] from sliding down vertical surfaces.” Lighter, less-primitive relatives of tarantulas use millions of tiny foot hairs, called setae, to stick to walls, glass and other treacherous vertical surfaces. The hairs don’t dig into smooth material, but instead maximize molecule-to-molecule attraction (called the [Van der Waals force](https://secure.wikimedia.org/wikipedia/en/wiki/Van_der_Waals_force)) to keep them from falling. While tarantulas also use setae to stick, some weigh in at more than a third of a pound, far too heavy for the bristles on their feet to support a vertical climb. In a quest to explain the perplexing feat, researchers suggested in 2006 that a tarantula species called *Aphonopelma seemanni* [secreted sticky silk from its feet](http://www.nature.com/nature/journal/v443/n7110/abs/443407a.html). But no spiders were known to secrete silk from their feet — they use abdominal structures called spinnerets — so critics [refuted the claim](http://dx.doi.org/10.1038/nature08404). They argued that silk from spinnerets could have contaminated the discovery, and noted that the hypothesized silk-spinning foot structures hadn’t actually been found. Driven by curiosity, Rind enlisted three graduate students to help her find out who was right. On the following pages, we take a look at what they discovered. *Citation: “Tarantulas cling to smooth vertical surfaces by secreting silk from their feet.” F. Claire Rind, Chris Luke Birkett, Benjamin-James A. Duncan and Alexander J. Ranken.* The Journal of Experimental Biology*, June 1, 2011, Vol. 214, Pg. 1874-1879. DOI: 10.1242/jeb.055657*
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Tarantula Sticks to Glass ------------------------- One by one, Rind’s team placed three different species of tarantula on the side of an aquarium lined with glass microscope slides. Then they slowly tipped the aquarium upright, videotaping the move (above) to prove that each spider didn’t pull silk from its abdomen and onto its feet. “When we wobbled the aquarium, each one slipped a little but regained its footing,” Rind said.
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Silk Streaks ------------ Rind’s team pulled the spiders out, then retrieved the glass slides. Looking at them under a microscope, the team found strands of silk thinner than a single red blood cell, pulled in the direction of each tarantula’s slipping. In other words, they were smeared silken footprints. “We’d like to find out what kind of silk this is, and how strong it is. It’s anyone’s guess right now, since it has never been described before,” Rind said.
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Tarantula Foot -------------- Having established the existence of silk production on the bottom of tarantula feet, Rind’s team put specimens under an optical microscope. This close-up of a foot points out highlights where the strands seem to emanate from (white arrows).
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Hairy Close-Up -------------- An electron microscope helped Rind’s team look more closely at the tarantula’s feet. Both setae (flat strands) and spigot structures (thin, bright strands) are visible in this shot, which is about 0.01 of a millimeter wide. Before Rind’s team took these images, the spigots were assumed to be heat-sensitive hairs. “Arachnologists spotted \[the spigots\] 20 or 30 years ago but, as far as I know, nobody described their function,” Rind said. “It’s a little surprising with a creature as well-studied as a tarantula.”
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Silk Spigot ----------- Under a scanning electron microscope, a side view of the spigot reveals the hole from which silk fluid is secreted.
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Silk Droplet ------------ A silk droplet about 20 micrometers wide rests at the top of a spigot. Ultrafine setae bristles can be seen behind it.
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Spigot Holes ------------ To grow, young tarantulas shed their carapace several times each year, leaving behind a near-perfect record of their old selves. Holes in the carapace (above) show where spigots were located. Rind thinks foot spigots are common to all tarantulas, and perhaps reflect a primal body plan. “Tarantulas are fairly primitive spiders, like living fossils,” Rind said.
