Arachnids secrete a fluid over their adhesive pads

I can (finally!) talk about one of the most interesting discoveries I made during my postdoc at Cambridge, in Walter Federle’s lab. (For a primer on sticky feet, check out my last post.) In our paper, published yesterday in PLoS ONE, we describe for the first time fluid secretions associated with arachnid adhesive pads, most surprisingly in spiders.

Why is this surprising? Fluid secretions have long been observed on the adhesive pads of insects, but we had several good reasons to believe that, despite their common arthropod ancestry, spiders were using a dry adhesive mechanism like the gecko.

1. Spiders shouldn’t need a fluid. Both spiders and geckos have elaborately branched, “fibrillar” adhesives, which theoretically allow them to adhere solely by dry, intermolecular interactions. Beetles and flies have fluid-aided fibrillar adhesives, but their adhesive hairs are much simpler in structure. We wouldn’t expect them to be able to stick without a fluid (or at least not as well).
2. Insects and spiders are both arthropods, but they’re about as distantly related as you can get and still be arthropods. There are a LOT of species more closely related to each group that don’t have adhesive pads at all, which is to say that they evolved their adhesive pads independently, and we can’t assume that what we know about one group applies to the other.
3. Many previous researchers have studied spider adhesives and reported the absence of an adhesive fluid, and the absence of the secretory apparatus to produce such a fluid. The consensus in the literature up till now was that spiders have a dry adhesive.

So that was my assumption when I arrived at Cambridge. After studying the dry adhesives of geckos for many years, I wanted to see how they compared with their arthropod counterparts, the spiders. My labmates in Cambridge studied insect adhesive pads in ants, stick insects, cockroaches and beetles, all of which secrete very complicated fluids – biphasic, viscous, rate-dependent, non-Newtonian emulsions with hydrophobic and hydrophilic components. If it were up to me, I wouldn’t touch that stuff with a ten-foot pole.

Science had other ideas. I wanted to see what a spider’s foot looked like as it was adhering, so I used a technique called interference reflection microscopy (IRM), which is a great method for visualizing interfaces. I had to corral a spider in a small box and convince it to cling, upside-down, to a microscope slide underneath the microscope stage. It wasn’t easy, but when I finally found a willing participant – a young Chilean Rose tarantula – I sat and watched for a while. At first, it was just as I expected. I could see thousands of minute hairs making close contact with the surface. But after a few minutes, the spider started slipping. (Glass isn’t the easiest surface to stick to, believe it or not.) The spider kept repositioning its feet, trying to maintain its grip. And that’s when I started to see streaks of fluid left behind where its feet had been.

Once I’d found fluid in one species, I started to look at other spiders, and other arachnids. I was particularly interested in the spider relatives called solifugids, which have smooth adhesive pads. Previous researchers had reported that they had not found a secretion, which would have made it the only known smooth pad to adhere by a dry mechanism. I got in contact with Sérgio Henriques, who was able to supply us with solifugid specimens from Portugal. They too had fluid secretions, as well as various mites (with smooth and fibrillar pads). It appeared to be a general trend. I asked Jan-Henning Dirks to apply his IRM expertise to my arachnid footprints, and we determined that much like the footprints left behind by insect feet, they were hydrophobic, with very low contact angles, and remained liquid indefinitely. (The hydrophilic component of insect secretions is difficult to catch on camera, as it is extremely volatile and evaporates instantly on exposure to air. We suspect that arachnid secretions are biphasic like the insects’, and thanks to Walter we have evidence of such a secretion from a single mite, but obviously this needs to be investigated further.)

Why hadn’t anyone noticed these secretions before? We speculate that researchers did not expect to see them, having heard others (including myself) describe them as dry adhesives. The fluid trails are very small, and their refractive index is close to that of glass, making them difficult to see with standard light microscopy techniques. Finally, the fluids aren’t continuously secreted. This suggests that spider adhesive pads, and perhaps those of the other arachnids, function adequately without the secretion. (Maybe they even work better without it!) Further study is clearly needed to test such a hypothesis.

In the end, though, it turned out we weren’t the first people to see these secretions. After submitting our manuscript for publication, we discovered a letter from John Blackwall, Esq., F.L.S, addressed to the Secretary of the Linnean Society of London in 1833, describing some observations he’d made of spiders clinging to glass:

“… The next point to be determined, therefore, was whether spiders … when moving in a vertical direction on clean glass, leave any visible track behind them.

“Careful and repeated examinations, made with lenses of moderately high magnifying powers, in a strong light, and at a favourable angle, speedily convinced me that my conjecture was well founded, as I never failed to discover unequivocal evidence of its truth; though in the case of the spiders considerable difficulties presented themselves, in consequence of the exceedingly minute quantity of adhesive matter emitted by the brushes of those animals. On submitting this secretion to the direct rays of the sun, in the month of July, and to brisk currents of air, whose drying power was great, I ascertained that it did not suffer any perceptible diminution by evaporation under those circumstances.”

At least we can say our findings are consistent.

Citation: Anne M. Peattie, Jan-Henning Dirks, Sérgio Henriques, Walter Federle (2011) Arachnids secrete a fluid over their adhesive pads. PLoS ONE 6: e20485. (doi: 10.1371/journal.pone.0020485)