Spiders can crawl pretty much anywhere: Glass, walls, and even ceilings are all equally accessible to the arachnids.
Now scientists have solved the long-standing mystery of their sticky success: Their secret is small hairs at the end of their legs. (See pictures of spiders up close.)A female banana spider on her web. Photograph by Michael Hare/Alamy
These thousands of tiny hairs create multiple contact points between the spider and the surface that increase the spider’s ability to hang on, scientists have found.
The hairs are both small and flexible. At the molecular level, even the smoothest surfaces are rough, so if the spider’s hairs were rigid, the arachnid could make contact only with certain parts of the surface. But because the hairs are malleable, they can make contact with more of the surface, which provides additional stickiness, said Jonas Wolff, a biologist at the University of Kiel in Germany. (Also see “Watery Gecko Grip Could Lead to Stickier Tape.”)
Unlike animals like barnacles, which permanently attach themselves to rocks or the hulls of ships, spiders can temporarily attach to a surface. Known as dynamic attachment, the spider’s system is like a Post-It note compared to a barnacle’s super glue.
“Permanent attachment systems, like glue, are often much stronger and not reusable, whereas temporary attachment systems, like hairy adhesive pads, can be used multiple times [and] adhere strongly enough to hold the animal, but the contact can be loosened very quickly and effortlessly,” said Wolff, who has spent several years studying spider stickiness.
Wolff wanted to find out how the banana spider (Cupiennius salei) could generate enough sticky force to keep from falling while also being able to detach quickly enough to catch and eat its prey.
An attack usually lasts less than a quarter second, and a fleeing spider runs up to 1.6 feet (0.5 meter) per second. In such quick movements, spiders attach and detach quickly to surfaces, Wolff said.
Initially, biologists had thought the spiders accomplished this with their hairy pads, but Wolff suspected something else was at work. When he and his colleagues at the University of Kiel measured the amount of force generated by a spider on a pane of glass, they found it was 97 milli-Newtons, or 0.35 ounce (10 grams). (Also see “Gecko, Mussel Powers Combined in New Sticky Adhesive.”)
“This might not sound like very much, but it is three to four times the body weight of the spider. So you can hang a weight of double the spider on its body, while it is sitting upside down on a smooth glass pane, and it won’t fall off. In smaller spiders this relation is even higher, because those have a higher foot pad area relative to their volume,” Wolff noted.
To determine how individual legs generated such force, the researchers disabled the spiders’ legs using warm beeswax, which kept the hairy pads from working their sticky magic.
Disabling the legs singly, in pairs, and in larger groups, the researchers discovered that the force generated by opposite legs (such as the front left leg and the back right leg) helped the spider stick. (See “Small Spiders Have Big Brains That Spill Into Their Legs.”)
As the legs pushed apart, they generated more friction between the hairs and the surface, which enabled the spiders to attach more securely. Pulling the legs together decreased their “stickiness” and let them detach quickly, the authors concluded in a Journal of Experimental Biology study published January 15.
In case you’re dreaming of someday climbing walls, Wolff added that it’s unlikely we’ll have any real-life Spider-Mans anytime soon: Even if we donned a suit of sticky hairs, people are simply too heavy for it to work.
Well, there’s always rock climbing.