Silky feet.

Zebra tarantulas can secrete silk from their feet, researchers have found. The discovery raises questions about the original function of silk.

Spiders step securely on surfaces because of the hold of thousands of hairs on their feet. Stanislav N. Gorb of the Max Planck Institute for Metals Research in Stuttgart, Germany, and his colleagues had intended to study whether spiders' feet produce fluid to aid in the attachment.

They had zebra tarantulas (Aphonopelma seemanni) crawl up inclined glass slides. During short breaks, the researchers would tilt the slide to a steep incline, knowing that the spiders wouldn't move from their position, says biologist Adam P. Summers of the University of California, Riverside.

Once, when the glass was almost vertical, one of the researchers noticed that a spider slipped a few millimeters before reattaching itself. "Suddenly, you could see these little fibers coming out from the tip of every toot," says Summers. "It was completely unexpected." The researchers imaged the spiders' feet with a scanning electron microscope and found silk-producing spigots nestled among the hairs. The team reports the finding in the Sept. 28 Nature.

The foot's silk-secreting capability may have emerged after the spiders' abdominal spinnerets were already producing the material for webs, the researchers say. But if the feet turn out to be an earlier source of silk, it would imply that "the primitive function of silk was adhesion instead of prey capture or web building:' Summers says.