Hairy feet. Setae on the bottoms of their feet (right) help spiders stick to ceilings.

Spiders' Sticky Feet

Before you squash that spider striding across your bathroom ceiling, spare a thought for its amazing defiance of gravity. Scientists have now revealed that tiny hairs allow the feet of spiders to hold many times their own body weight, enabling them to hang upside down. The spider' method of adhesion, also found in geckos (ScienceNOW, 27 August, 2002), could inspire new types of materials.

Biomechanics expert Andrew Martin and colleagues at the Institute for Technical Zoology and Bionics in Bremen, Germany, used a scanning electron microscope to look at the feet of a small jumping spider (Evarcha arcuata). The images confirmed that, as in other spiders, the bottom of the foot is decked out with long hairs (setae), the undersides of which are densely covered by even finer hairs (setules) with triangular tips, they report 19 April in Smart Materials and Structures.

These structures were known, says Martin, "but no-one's done an in-depth analysis of the kind of adhesion forces involved." Using a technique more often applied in materials science, the team measured the adhesive force between a spider's foot and a tiny probe. They calculated that the spider could support 173 times its body weight while hanging upside down, if it were to have all 600,000-odd setules in contact. In practice, spiders don't keep all their feet on the ground (or ceiling) and the actual adhesive force is probably about one-tenth of that, says Martin.

It's likely that setules get their grip on surfaces through van der Waals forces, electrostatic attraction between molecules that are within a nanometer of one another, although friction and other forces may also be involved, says Martin. Because van der Waals forces rely solely on the distance between two objects and are unaffected by the surrounding environment, the spider's method of adhesion could be used to make paper notes that stick when wet and spacesuits that adhere to surfaces in outer space, the team says.

"What's interesting and new is that they calculate these forces and find that they're very high," says evolutionary biologist Fritz Vollrath of the University of Oxford, U.K. He adds that these spiders probably use their sticky feet to capture prey, especially slippery ones such as cockroaches. "Actually having to hang on for dear life is not a big deal for a spider," he says. "But having to hold on to prey that it's been stalking for 20 minutes is a different kettle of fish."