April 28, 2012
In what looks to be the last word on the tarantula foot-silk controversy, Fernando Perez-Miles and David Ortiz-Villatoro take a firm stand with the title of their new paper in the Journal of Experimental Biology: "Tarantulas do not shoot silk from their legs." In our last post, we outlined why Rainer Foelix and his team contend that the supposed silk spigots described by research teams supporting the pro-foot-silk position are most likely chemoreceptor hairs. Now, Perez-Miles and Ortiz-Villatoro also dispute the earlier findings of foot-silk; they contend that the silk those researchers found is actually spinneret silk.
Back in 2009, Perez-Miles, Ortiz-Villatoro and two other colleagues challenged the original claim that the zebra tarantula produces foot silk, reporting that when they sealed the tarantula's spinnerets with paraffin before having it cling to vertical pieces of glass, they couldn't find any silk. When yet another team of researchers claimed to have found foot-silk left behind on glass by four different kinds of tarantula, they suggested that the Perez-Miles team might not have shaken the glass enough. In other words, maybe the spiders didn't feel the need to produce foot silk.
Now, Perez-Miles and Ortiz-Villatoro have more closely repeated the procedure of previous foot-silk finders, with four tarantula species. When they left the spinnerets unsealed, they found silk threads in the "footprints" left behind on the glass. When they sealed the spinnerets, they found none. Their conclusion: the footprints became contaminated with spinneret silk, which easily travels from spinneret to leg because it's so light.
In combination with the facts that no one can find silk glands in the spiders' feet and that the "spigots" have more in common with chemoreceptors than with spinneret spigots, this latest piece of research makes it seem highly unlikely that anyone will find spiders whose feet produce silk.
March 10, 2012
Last summer, a team of researchers reported they had indeed found that tarantulas secrete silk from their feet. This appeared to settle a controversy that had started in 2006 when a different team reported finding foot silk. The original finding was bound to be controversial: no one had ever before observed the secretion of silk from the feet of any kind of spider. In 2009, a team investigating the first claim couldn't find evidence of foot silk. Last summer's report, which included a micrograph that seemed to show a blob of silk protein forming at the tip of a purported foot silk spigot, appeared to validate the 2006 report. (For a discussion of these three reports and of the questions concerning silk evolution they raise, see this earlier Spider Silk blog post.)
Well, there is now more evidence that tarantulas actually DON'T produce foot silk. Rainer Foelix is a leading spider anatomist and author of the must-have Biology of Spiders. When he examined the micrographs included in last summer's report, the alleged foot silk spigots looked like chemoreceptor hairs he had studied intensively in the 1970s. Kathryn Knight summarizes what Foelix did next. He, Bastian Rast, and Anne M. Peattie report their full findings in the April 1, 2012, issue of the Journal of Experimental Biology.
Foelix, Rast, and Peattie explicity address the questions concerning silk evolution that these earlier studies raised for us. They note that--even if the silk allegedly secreted through the foot "spigots" really is silk--all the tarantulas tested tend to stay close to the ground. It's not clear, then, what survival advantage foot silk would give them, particularly when the tiny amounts produced would add little cling compared to the spiders' already clingy adhesive setae, or hairs.
Most interesting to us, the team compared the feet of the tarantulas in their study to the feet of Liphistius desultor, a mesothele. As readers of Spider Silk know, mesotheles make up the oldest extant branch of the spider family tree. They live in burrows, rarely venturing more than a few inches beyond the burrow's trap door. Foelix, Rast, and Peattie state that Liphistius has no adhesive hairs on its feet. But it does have the same hairs that earlier researchers identified as silk spigots but that Foelix et al. are pretty convinced are chemoreceptors. The tiny amounts of "silk" produced from the hairs in question wouldn't allow the mesotheles to climb, even if they wanted to.
Next step in deciding whether the hairs in question on tarantula feet are silk spigots or chemoreceptors: testing them for sensory innervation with transmission electron micrography. Down the road, we wonder whether anyone will find any evolutionary connection at all--given the evolutionary relationship between limbs and spinnerets--between the proteinaceous fluid that apparently oozes from these hairs (which are also found on the spinnerets and all extremities) and the protein silk that is secreted through spinneret spigots.
As usual, the best research leads to more questions.