Everyone knows Spiderman shoots his web from the palms of his hands, it’s how he escapes Dr. Octopus, flies around the city and wins the heart of Mary-Jane. In 2006 this iconic comic book pose found its way into science.
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It is generally thought that spiders attach to surfaces using thousands of tiny hairs on their feet. In spiders this was thought to be dry, there’s no secretion of fluids to help them stick. Weak interactions between molecules (known as Van der Waals forces; an intermolecular force like the force that allows water molecules to stick together to form droplets) are all they have. For this reason, the larger the critter the smaller the hairs, maximising the surface area over which these forces can occur.
It is also well known that spiders produce silk from spinnerets. Useful stuff for catching prey and escaping predators (not sure if they use it to win the girl). Then, out of the same research group that produced the work on body size vs. hair size came a new idea; that spiders produce silk from their feet to help them with movement and adhesion on vertical surfaces. They published a paper in Nature claiming zebra tarantulas, found in Costa Rica, could also secrete this silk web from their feet1. They published photographs of fibres and traces left of glass by the tarantulas, and fibres observed by cryoscannning electron microscopy (that’s using an electron microscope and very cold temperatures).
They only published data on this one species, and thought it could possibly be an additional mechanism to help adhesion due to the large size of the tarantula. But, in 2009, a brief communication in Nature3 contradicted the research. Researchers in Uruguay left the same species of tarantula, Aphonopelma seemanni, in glass containers covered with microscope slides for 14 hours. They then repeated the experiment, this time sealing the spinnerets. They found that when A. seemanni was left with spinnerets open and free to silk as they please many of the sliders covered in silk but when the spinnerets were blocked there were no silk threads. Fiddly dissections of little spider feet (if you want to be scientific about it, we should use the word “tarsi”) also failed to produce anything that looked like silk glands. These researchers claimed the legs were used to entangle and brush silk and spinnerets, but that was all.
So, the debate presumably raged on in arachnid circles, reaching the general public yesterday with a brief slot on the BBC news. New results, published in the Journal of Experimental Biology4, once more showed that silk could be produced from the tarantula feet. An experiment involving shaking a glass tank whilst the spiders climbed it, and during this experiment the slides showed traces of silk under the microscope. Examination of spider skin moults showed tiny silk-producing structures on the spidy feet. Which brings me to conclude the spider lecture (bet you didn’t think you were getting one of those today!) and to move swiftly on to the video…which can be found here: BBC Nature
One final point, what do they do if one escapes?! I’m just very glad I don’t work in the lab next door…
1. Arzt, E. (2003). From micro to nano contacts in biological attachment devices Proceedings of the National Academy of Sciences, 100 (19), 10603-10606 DOI: 10.1073/pnas.1534701100
2. Gorb, S., Niederegger, S., Hayashi, C., Summers, A., Vötsch, W., & Walther, P. (2006). Biomaterials: Silk-like secretion from tarantula feet Nature, 443 (7110), 407-407 DOI: 10.1038/443407a
3. Pérez-Miles, F., Panzera, A., Ortiz-Villatoro, D., & Perdomo, C. (2009). Silk production from tarantula feet questioned Nature, 461 (7267) DOI: 10.1038/nature08404
4. Rind FC, Birkett CL, Duncan BJ, & Ranken AJ (2011). Tarantulas cling to smooth vertical surfaces by secreting silk from their feet. The Journal of experimental biology, 214 (Pt 11), 1874-9 PMID: 21562174