Biologist William K. Purves of Harvey Mudd College explains.

Spider silk is not a single, unique material--different species produce various kinds of silk. Some possess as many as seven distinct kinds of glands, each of which produces a different silk.

Why so many kinds of silk? Each kind plays particular roles. All spiders make so-called dragline silk that functions in part as a lifeline, enabling the creatures to hang from ceilings. And it serves as a constant connection to the web, facilitating quick escapes from danger. Dragline silk also forms the radial spokes of the web; bridgeline silk is the first strand, by which the web hangs from its support; yet another silk forms the great spiral.

The different silks have unique physical properties such as strength, toughness and elasticity, but all are very strong compared to other natural and synthetic materials. Dragline silk combines toughness and strength to an extraordinary degree. A dragline strand is several times stronger than steel, on a weight-for-weight basis, but a spider's dragline is only about one-tenth the diameter of a human hair. The movie Spider-Man drastically underestimates the strength of silkreal dragline silk would not need to be nearly as thick as the strands deployed by our web-swinging hero in the movie.

Dragline silk is a composite material comprised of two different proteins, each containing three types of regions with distinct properties. One of these forms an amorphous (noncrystalline) matrix that is stretchable, giving the silk elasticity. When an insect strikes the web, the stretching of the matrix enables the web to absorb the kinetic energy of the insects flight. Embedded in the amorphous portions of both proteins are two kinds of crystalline regions that toughen the silk. Although both kinds of crystalline regions are tightly pleated and resist stretching, one of them is rigid. It is thought that the pleats of the less rigid crystalline regions not only fit into the pleats in the rigid crystals but that they also interact with the amorphous areas in the proteins, thus anchoring the rigid crystals to the matrix. The resulting composite is strong, tough, and yet elastic.

M. Dawn of Brandon, Miss., asked the related question, "Why doesnt a spider get stuck on its own web?"

Over the years, three explanations for this phenomenon have surfaced . The first invokes an oil, secreted by the spider, that serves as an anti-stick agent. The problem with this hypothesis is that such an oil has yet to be discovered.

The second scenario is based on the diversity of silks. Many webs include strands made of silks that are much less sticky than the others are. The non-sticky strands appear in the hub of the web, the radial spokes and the threads by which the web hangs from plants or other supports. Some researchers have thus posited that the arachnids use only these strands when navigating their webs. If you watch them in action, however, you see will see that although they do seem to prefer the non-sticky strands, the spiders are able to move around freely, touching many of the strands, including the very sticky ones that spiral out from the hub.

For more information regarding this intricate mechanism, click here for a well-illustrated Web page by Ben Prins.

The third explanation appears to solve the sticky-strand problem. In short, the legs of at least some spiders feature a disengaging mechanism that enables the arachnid to detach itself instantly from a sticky strand. This mechanism involves a clever anatomical adaptation. Each leg ends in a pair of "walking claws" that grasp vegetation, among other functions, but a third claw collaborates with associated spiny, elastic hairs to detach the leg from a sticky web strand. This third claw grasps the strand, pulls it against the elastic hairs, and pulls them further, cocking the mechanism. When the claw relaxes, the hairs rebound vigorously, throwing the strand away and springing the leg free.

Police, the military, physicians, and other groups are eager to obtain large quantities of dragline silk, which can be woven or compacted to make bulletproof clothing, replacement ligaments, medical sutures, fishing line, ropes for rock climbers, tethers to snag planes landing on aircraft carriers and myriad other products. It is impracticable to harvest sufficient quantities of silk from spiders due to their territorial nature, so biotechnologists have turned to other sources. The Canadian company Nexia has demonstrated that goats and cows can be genetically engineered so as to produce dragline silk in their milk. Using a clone of such goats, Nexia aims to produce a modified dragline silk, which they call BioSteel, to meet the many demands.

This answer was originally posted on July 15, 2002.