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Spider silk is a fibrous protein that is secreted as a fluid and forms a polymer, on being stretched, that is much stronger than steel and further resists breakage by its elasticity. A single spider can spin several kinds of silk. Although some other invertebrates also spin silk, spiders put this ability to the most spectacular variety of uses. For example, they form draglines that help them to find their way about and to catch themselves if they fall. Small and, especially, young spiders spin a "parachute" thread that enables them to be carried by the wind, sometimes for hundreds of kilometers. The males use silk in transferring sperm to the palpal organ, and the females make cocoons with it. Silk is also used to make nests and other chambers and to line burrows. The most familiar and amazing use of silk by many species, however, is in making insect traps called spiderwebs. Once prey is caught in such a web, the spider may wrap it in more silk.
The diverse webs spun by spiders provide a remarkable example of the evolution of instinctive behavior. A spider does not have to learn how to make a web, although the spinning itself can be adapted to unique circumstances, including the webs spun by spiders under zero gravitation in spacecraft. The simplest webs are irregular and generally laid out along the ground. More advanced webs, particularly of orb-weaver spiders, are highly intricate, raised above the ground, and oriented to intercept the paths of flying insects. The spinning itself is a complex process involving the placement and then removal of scaffolding spirals and a combination of sticky and nonsticky strands. In some cases a number of spiders will form a kind of communal web, but spiders in general are not social. Such spiders rely largely on the sense of touch.
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Besides the web spinners, many spiders hunt for their food or lie in wait for it. Hunters tend to rely on vision if they feed during the daytime, or on touch if they feed at night. Jumping spiders may lurk in ambush for their prey, and a number of them are well camouflaged on flowers by color or body structure or both.
Spiders have separate sexes, and the eggs have to be fertilized. The genital openings of both male and female are located on the abdomen. The male's copulatory organs, however, are complicated structures located on his pedipalps. He spins a little web and deposits sperm in it, then moves the sperm to the palpal organ. After sperm are transferred to the female, they can be stored in her body for an extended period.
Courtship behavior is often complicated. Males may use draglines to detect and recognize mates, or they may signal their approach by plucking on the female's web. In spiders with well-developed eyes, complex mating displays have evolved that are associated with bright color patterns. Often the male must avoid having the female treat it as food; even in species where this is common, however, the male often escapes.
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Spider eggs are protected in cocoons. The female may guard the cocoons or carry them about. In some spiders the hatchlings remain with the mother for an extended period and may be carried on its body.
As predators on insects and other small animals, spiders are generally highly beneficial to humans, although some feed on important plant pollinators such as bees. They also serve as food for other animals, most notably for certain wasps that paralyze the spiders and lay eggs to hatch on the paralyzed body. Efforts to utilize spider silk for cloth have not been successful economically, but the silk has been used for the cross hairs of optical instruments. Although spiders have occupied an honored place in various mythologies, their widespread unsavory reputation in modern times probably results from their tendency to lurk in dark places, their often grotesque appearance, and a gross exaggeration of their toxicity.
Scientific classification: Spiders make up the order Araneae in the class Arachnida. About 105 families of spiders are known, plus about 10 that are extinct. Two suborders are widely but not universally recognized. The suborder Mesothelae contains a few primitive, burrowing forms. The suborder Opisthothelae contains the infraorder Mygalomorphae, which consists of the "straight-jawed" forms, usually large, such as the trap-door spiders and the ones called tarantula in the United States, and the infraorder Araneomorphae, the members of which have the chelicerae somewhat modified and more efficient; it contains the more common and conspicuous forms, such as orb weavers, wolf spiders, and jumping spiders. The cribellate araneomorphs have a specialized organ, the cribellum, that helps to produce silk.
Contributed by: Michael T. Ghiselin