Why Don't Spiders Get Stuck in Their Webs?
Image credit: Stockbyte
When a bug flies into a spider web, the game is over. It’s almost instantly stuck, and a sitting duck for the web’s owner. When you or I walk into a web, we’re a little better off than the bug because we won’t be dinner, but the sticky strands of web are still a pain in the butt to pick off of clothes and skin.
The spider itself, which spends much more time in contact with the web than you or any bug, doesn’t seem to have any issues getting stuck as it moves around. What gives?
For a long time, people thought spiders didn’t get stuck because their legs were coated in an oil made inside their bodies. With their legs lubed up like this, there was nothing for the silk web strands to stick to. Early 20th century naturalists proposed this idea — that the spider “varnishes herself with a special sweat,” as one elegantly put it — after observing spiders in the wild. The hitch is that, for all the research on spiders scientists have done in the meantime, no one had bothered to test the idea until recently.
A study published last year by two biologists in Costa Rica, Daniel Briceño and William Eberhard, suggests that spiders stay unstuck thanks to a combination of behavior, anatomy and, yes, even an oily non-stick coating.
What a Web They Weave
The first thing that helps spiders from getting trapped is that not every part of every web is sticky. In many orb weaver spider webs, for example, only the spiral threads are made with sticky silk. The “spokes” that support the structure of the web and the center part of the web where the spider rests are made with “dry” silk.
Using the center area and the spokes, a spider can move all around the web, and even off of it, without any concern for getting stuck.
The spiders that Briceño and Eberhard studied used the dry threads for moving around most of the time, but when prey landed on the webs and the spiders went to retrieve their dinner, they inevitably had to charge across a sticky section. Unlike their prey, though, the spiders didn’t just whack into the sticky threads willy-nilly. The scientists found that the spiders walk very carefully when on the sticky sections, holding their body clear of the web and making minimal contact with the threads with only the tips of their legs.
Under a microscope, Briceño and Eberhard saw that the sticky threads do indeed make contact with the spider and stick to the setae, or short bristly hairs, on their legs. As a spider pulls its leg of the web, though, the droplets of adhesives that sit on the thread slide toward the edge of the bristle, where they have contact with only the thin tip and easily pull away. All these bristles are also in irregular rows and break free from the sticky droplets one by one, not all at once, which keeps the adhesive force of multiple droplets from combining.
Smooth Like That
What is it about the setae that lets them shed the web’s adhesives so easily? When Briceño and Eberhard washed a detached spider leg and applied it to a sticky thread, the leg stuck and wasn’t as easily removed. They figured that the bristles must have either a chemical coating of anti-adhesive substances or a structural surface layer with anti-adhesive properties. After analyzing several compounds washed off the the spiders’ legs, they found several several oily substances — including n-dodecane, n-tridecane, and n-tetradecane — that could act as a non-stick coating.
The researchers couldn’t tell where the chemicals had come from, but scientists’ descriptions from the last century suggested that they were applied by the spider’s mouth. Sure enough, when Briceño and Eberhard washed a live spider’s legs, it passed each of the legs through its mouthparts, but they didn’t test whether or not any anti-adhesive material was being applied.
To see if the spiders were coating their own legs would require a pretty simple experiment, Eberhard told me via email, but the spider they were working with, Nephila clavipes, is only seasonally abundant. The study would have to wait until the population climbed again, so the source of the non-stick chemicals is still a mystery for now. In the meantime, he said, he’s looking into how spiders deal with a different type of silk, called cribellum silk, which can be sticky without being wet.