A black-and-yellow argiope is one of our most common orb web spinners. (Photos by Mary Holland)

All spiders can do something that no human can: spin silk. By applying force, they can turn liquid protein into a solid fiber that is finer than a human hair, stronger than steel, and in some species, ten times tougher than Kevlar. Not only can spiders perform this miracle, but they can make many different types of silk, each uniquely suited for use in performing a different task.

Spiders have special glands in their abdomen that secrete silk proteins (made up of chains of amino acids), which are dissolved in a water-based solution. The spider pushes the liquid solution through long ducts, leading to microscopic spigots on the spider's spinnerets (silk-spinning organs). Spiders typically have two or three pairs of spinnerets, located at the rear of the abdomen.

Each spigot has a valve that controls the thickness and speed of the extruded material. As the spigots pull the protein molecules out of the ducts and extrude them into the air, the molecules are stretched out and linked together to form long strands. The spinnerets wind these strands together to form the sturdy silk fiber. The transformation of the proteins from a liquid to a solid has nothing to do with the thread's exposure to air once it exits the spider; rather, scientists believe it has to do with pressure from the spider that realigns the molecules into a solid form.

Most spiders have multiple silk glands, which secrete different types of silk material for different purposes. By winding different silk varieties together in varying proportions, spiders can form a wide range of fiber material. Spiders can vary its consistency by adjusting the spigots to form smaller or larger strands. Silk can also be coated with various substances suited for different purposes. For example, spiders might coat fiber in a sticky substance or a waterproof material.

Multiple strands of different types of silk are used to wrap prey.

Spiders have multiple uses for the silk they spin. Some species of spiders spin a dragline, also called a life line, a silk thread that trails behind them wherever they go. In the event that the spider falls, it will be caught by the dragline. Often, when a spider is alarmed, it will deliberately drop from a web, using this silk thread. Some species of spiders hang from this line while feeding. Those spiders that travel far from their shelters also use this trail of silk to find their way home. By impregnating this line with pheromones, spiders use it as a lure to find a mate.

This type of silk, also used for the outer rim and spokes of some orb webs, is extremely tough and strong.

Web construction requires at least two types of silk: sticky and non-sticky. The strands that are used to capture prey are sticky and very stretchy, while those that the spider walks on are not.

Tubiliform silk is the name given to the silk that spiders use to spin their egg sacs, many of which must withstand the harsh elements of winter. This silk is very stiff and exceptionally tough.

The aciniform silk used to wrap the captured prey is two to three times as tough as the other silks, including dragline silk.

When young spiders are ready to disperse, they spin delicate wisps of silk, or gossamer, which are caught by the wind. This means of travel is referred to as "ballooning."

The silk of a spider egg sac must protect the living spiders within it from ice, snow, and extreme temperatures of winter until they emerge in the spring.

The silk that spiders spin sometimes serves as a source of food for themselves or other spiders, although this isn't very common.

Silk is commonly used by spiders for building shelters. Trap door spiders, which reside in burrows, use silk to bind together plant material and soil that serve as "lids" covering their holes. The water spider, Argyroneta aquatic, which resides in Europe and Asia, spins air-filled "diving bell" webs within which it lives and raises young under water.

Humans have come up with even more uses for spider silk than spiders. The ancient Greeks used cobwebs to stop wounds from bleeding. Australian aborigines use the silk of a giant tropical spider for fishing lines, rubbing parts of the spider's body onto the silk to attract small fish, which become caught in the sticky silk when they bite the spider tidbits. New Guinea natives fashion fish nets, head gear, and bags from spider silk.

Until World War II, spider silk was used for crosshairs in optical devices, including microscopes, telescopes, guns, and bomb guidance systems. Single strands were excellent for this purpose because they are extremely thin; the average strand of spider silk is only 1/20,000 of an inch across, compared to 1/250 for a human hair.

Today humans have in mind different uses for spider silk. They envision medical products such as surgical thread and replacement ligaments, and military applications including bullet-proof clothing, parachute cords, and tethers for planes on aircraft carriers.

The biggest challenge is mass producing the silk. People have been attempting to obtain and use spider silk for centuries, but success has been elusive for two main reasons: raising spiders has a long history of failure because of the cannibalistic tendencies of spiders, and their silk is difficult to work with because it hardens on contact with air.

However, success appears to be around the corner. Recently scientists discovered the genetic sequences for the spider proteins that are used to make spider silk. Researchers have even introduced engineered genes into goats, in the hopes of goats mass producing silk. The milk of these goats actually did contain some silk, but not nearly enough for commercial use. Although humans are in the process of unlocking the secret of silk production, I, for one, am glad there are still some mysteries that we can't solve or easily duplicate.

Mary Holland is the author of "Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England." She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com.