Foot Silk: Probably the Last Volley

May 29, 2013

Tags: silk, tarantulas, Foelix, Erb, Rast

Remember the controversy over whether tarantula feet produce silk? We posted a summary of the back and forth here last year. Rainer Foelix, Bruno Erb, and Bastian Rast have now published transmission electron micrographs that refute the foot-silk claim in Arthropod Structure and Development. Although the article is behind a paywall, you can look at their figures for free, and it's hard not to be struck with wonder at the complexity of some of these tiny structures.

Foot Silk: True or False?

March 10, 2012

Tags: silk, tarantulas, Foelix, Rast, Peattie, Mesothelae

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.

Venomous Mesotheles

December 6, 2010

Tags: Mesothelae, Foelix, venom

If we're fortunate enough eventually to write a second edition of Spider Silk, we'll have to make a change: contrary to what we wrote, we now know that mesothele spiders do have venom glands.

Mesotheles are the oldest branch of the spider evolutionary tree. Arachnologists consider them "living fossils": these spiders remain outwardly virtually identical to a 290-million-year-old mesothele fossil found in France, so their characteristics and behaviors can provide insight about spiders' early evolutionary history. They share a common ancestor with the much more numerous mygalomorphs (tarantulas and their closest relatives) and araneomorphs (the "true spiders" most of us are more likely to come into contact with), which in turn share a more recent common ancestor that they don't share with the mesotheles. Mesotheles are difficult to study. They live only in Asia, they're hard to find, and they're long-lived, which means any researcher wanting to study their life cycle has to be awfully patient.

With the exception of the araneomorph family Uloboridae, (which clearly lost them during evolution) all other spiders have venom glands. So whether or not mesotheles also have them is an important spider evolution question, especially as researchers work to figure out how spiders first evolved along a different path from their closest arachnid relatives. Depending on the answer, researchers would look for the origins of the venom glands in one of two different time spans. If the mesotheles don't have venom glands, it probably means that this characteristic evolved after the original mesotheles but before the mygalomorph and araneomorph lineages diverged, between about 300 million years ago and 250 million years ago. But if all three major branches of the spider evolutionary tree (mesotheles, mygalomorphs, and araneomorphs) have venom glands, it means that the common ancestor of all the spiders probably also had venom glands. This common ancestor lived in its burrow at least 300 million years ago, and perhaps even more than 350 million years ago. (A third possibility, that the common ancestor of all spiders had venom glands but the mesotheles lost theirs during evolution, has usually been considered much less likely than these other two possibilities.)

The best single source of information on mesotheles is Joachim Haupt's 2003 monograph. In his studies, Haupt couldn't find either venom glands or the duct openings in the fangs that would indicate the presence of glands.

But now Rainer Foelix (author of the standard text Biology of Spiders) and Bruno Erb report in the latest issue of the Journal of Arachnology that they have indeed found such openings in nine different mesothele species by using stronger magnification. It wasn't easy. The openings measure only 5 to 10 micrometers in diameter (about the same size as a single red blood cell), and they lie not at the tip of the fang, where they would be easier to locate, but along its length. Having found the opening, Foelix and Erb were pretty sure they would find the gland, but this also proved challenging. Their description of how they had to tease out individual muscle fibers with watchmaker's forceps in order to expose the gland makes performing root canal look easy.






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