A recent paper by Rind et al. in Journal of Experimental Biology, titled “Tarantulas cling to smooth vertical surfaces by secreting silk from their feet”, got a lot of press coverage when it came out, as well it should have – but for completely different reasons than it did. The authors present some beautiful morphological descriptions of the silk-secreting spigots on tarantula feet, the existence of which was originally described by Gorb et al. in their 2006 Nature paper, “Silk-like secretion from tarantula feet”. That paper was later questioned by another team of investigators who were unable to reproduce the results, so the Rind et al. paper is particularly valuable in that it puts an important debate to rest, while also providing insight into the evolutionary history of silk-spinning itself. As the authors note,
[T]arsal silk secretion would provide strong support for the hypothesis that spider spinnerets are derived from modified limbs (Shultz, 1987; Shear et al., 1989; Damen et al., 2002; Selden et al., 2008), and direct silk secretion from a setae on a limb without any complex storage organ may represent the ancestral condition, with silk production from abdominal spinnerets being the product of much later evolution. The ability of spiders to secrete silk from their feet would show that silk production is controlled by developmental modules able to be expressed in a variety of body parts (Selden et al., 2008).
Tarantulas and other mygalomorphs represent some of the most “basal” spider species – that is, they are thought to resemble ancient spiders more closely than other spiders living today. The discovery of silk-secreting organs in a group of more highly derived spiders would be interesting but not as informative on a historical level; it would suggest that tarsal silk is possible, and might have existed previously, or it might be new to that group. The fact that tarantulas have tarsal silk tells us something about how silk evolved in all spiders, which is pretty cool.
Here’s my issue with the paper: it’s called “Tarantulas cling to smooth vertical surfaces by secreting silk from their feet”. This has nothing to do with the evolution of spider silk. How about some bullet points?
- Tarantula feet are covered in adhesive hairs, much like those of geckos, which generate large adhesive forces. To say that tarantulas cling to smooth vertical surfaces by secreting silk from their feet is therefore leaving out a large piece, if not the vast majority, of the attachment pie. If you want to talk about adaptations for climbing, adhesive setae are where it’s at. They’ve arisen multiple times independently in several different groups of spiders besides tarantulas, not to mention in geckos, mites, beetles, flies, and a bunch of other insects.
- The authors provide no mechanical data to support their assertion that silk is involved in attachment. They note that it appears after the animal has slipped on a smooth vertical surface, and they note that the tarantula has a low safety factor. From these two observations they infer that the silk must aid in climbing. This is a hypothesis, not the demonstrated result of the paper. If I wanted to show that tarsal silk aided in climbing, I would try to disable the silk-producing setae and compare their climbing ability afterwards to their climbing ability with those setae intact. Considering that silk-producing setae are outnumbered by adhesive setae by about 50:1, I wouldn’t predict much of an effect, but that’s how I would test that hypothesis.
- The authors argue that silk could compensate for the low safety factor of large spiders, but if it does, it’s not doing a very good job. Tarantulas just aren’t that good at climbing, relative to other animals with fibrillar adhesives. This is a classic surface-area-to-volume scaling problem. Adhesive force scales with surface area, while body mass scales with volume. Bigger animals can’t increase their adhesive pad area enough to compensate for their added weight, so safety factor (potential sticking power divided by body weight) goes down as animal size goes up. (Aside: In my opinion, safety factors aren’t a useful way of thinking about evolution. We’re all living on the edge by certain measures, and “overbuilt” by others. Animals fall and die sometimes. More safety is pretty much always better.) Then, the authors themselves point out that not all tarantulas are large. If – again, as I see it, the main finding of the paper – tarsal silk is the ancestral condition, we would certainly expect to find it in all tarantulas (and while we’re looking, let’s check out some spiders from the even more basal Mesothelae), including the small ones, contradicting the safety factor argument.
Finally, since we’re talking about spiders “shooting” silk out of their feet (which, sorry, doesn’t happen – it’s secreted, it’s exuded, it’s not ejected or projected or anything like that, and they certainly aren’t shooting webs out of their feet but who reads the Daily Mail anyway?), let’s talk about Spider-Man. Spider-Man’s silk is much like that produced by orb-weavers. It’s the sticky, prey-trapping silk that most people think of when they think of spider silk, from the Charlotte’s-Web-type spider most people think of when they think of spiders (unless they’re thinking of a tarantula). Tarantulas don’t spin sticky, viscous webs to capture prey. They find prey and grab that prey with their pedipalps, which, much like their legs, are covered in adhesive hairs. Tarantulas use their silk to build nests. They are not swinging around on it like Tarzan. I can only speculate, but I’d say that when the creators of Spider-Man gave him his silk-shooting wrists, it was an understandable compromise resulting from the rocky union between the posterior spinnerets of an orb-weaver and the mammalian tetrapod body plan (that’s two arms and two legs for you non-biologists). Mapping the former onto the latter, Spider-Man’s silk should technically be shooting out of his, well, butt. I don’t blame the artist for taking some license and moving the silk-producing organs to a less repellant location. However, that choice was entirely coincidental with the evolutionary history of spider silk spinning and has no evidential weight in evaluating whether tarantulas use their silk to climb vertical surfaces, poorly.
ETA In the interest of full disclosure I am the author of a paper in press about spider feet and their attachment mechanism. Watch this space.