SPIDER

order name Araneida, or Araneae, a member of a group of arachnids characterized by having six pairs of appendages, including four pairs of legs and two main body parts (the prosoma, or cephalothorax, and the opisthosoma, or abdomen) attached by a narrow stalk. Spiders are abundant worldwide except in Antarctica and usually occur at elevations from sea level to 5,000 m (16,000 feet). Some 34,000 species of spiders have been described. Most are terrestrial animals. They range in body length from less than 1 mm (0.04 inch) to about 90 mm (3.5 inches). Spiders are predatory and prey mostly on insects. All spiders have venom glands, but the venoms of only a few (e.g., black widow, brown spider; qq.v. ) are harmful to humans. There are eight or fewer eyes, always simple rather than compound (i.e., multifaceted). The number and arrangement of these eyes are major considerations in the classification of species. A pair of cheliceraetwo-segmented, pincerlike appendagesoccur at the front of the prosoma and usually point downward. The end segment of each chelicera is fanglike and is connected to a venom gland. The legs have seven segments. At the tip of the opisthosoma are several fingerlike structures, known as spinnerets, which produce viscous droplets of silk from the silk glands, forming threads. Spiders change little during growth, except in size. They usually molt, or shed their cuticles (skins), seven or eight times before maturing. If a leg is broken off during development, it is usually replaced. Spiders exhibit characteristic behaviours during and after courtship and mating, primarily because, unless the male clearly identifies himself, the female of certain species, commonly larger than the male, often kills and eats the male after mating. The eggs are usually laid in a silken sac that, in some cases, is carried by the female until the eggs hatch; in other cases the female deposits the egg sac either in or near the web, in a crevice, among leaves, or on twigs or bark. The eggs usually hatch soon after being laid. Insects, the main diet of most spiders, usually are captured in silken webs. In the webs of some species the strands are covered with droplets of a sticky substance that prevents or delays escape. The jumping spider and the wolf spider (qq.v.) chase and pounce upon their prey. The crab spider (q.v.) awaits its prey in flowers. The house spider (see comb-footed spider) extends its silken strands in almost any direction. The sheet-web weaver (q.v.) usually makes a sheetlike, horizontal web. The funnel-web spider (q.v.) builds its web in the shape of a funnel. The trap-door spider (q.v.) and the wolf spider make tunnels. Others make nests of leaves. any member of the order Araneida, or Araneae, of the class Arachnida. Spiders differ from the insects, another arthropod group, in having eight legs rather than six and in having the body divided into two parts rather than three. The use of silk is highly developed among spiders. About 34,000 species have been described; the world's spider fauna outside northern Europe, Japan, and North America, however, has not been thoroughly collected and studied. Spiders are predators, feeding almost entirely on other arthropods, especially insects. Many spiders that are active hunters and overpower their prey have a well-developed sense of touch or sight. Others construct silk snares, or webs, to capture prey, sometimes in larger numbers than can be consumed; webs, constructed instinctively, effectively trap flying insects. Many spiders are able to inject venom into their prey to kill it; others use silk to overpower prey. Additional reading Works focusing on the spiders include Friedrich G. Barth (ed.), Neurobiology of Arachnids (1985); Pierre Bonnet, Bibliographia Araneorum, 3 vol. in 7 (194561), an indispensable work, listing all references to spiders and spider species up to 1938; Paolo M. Brignoli, A Catalogue of the Araneae Described Between 1940 and 1981 (1983); Rainer F. Foelix, Biology of Spiders (1982; originally published in German, 1979), the best introduction to morphology and physiology; R.R. Forster and L.M. Forster, New Zealand Spiders: An Introduction (1973), beautifully illustrated; Willis J. Gertsch, American Spiders, 2nd ed. (1979), a natural history of American spiders; B.J. Kaston, How to Know the Spiders, 3rd ed. (1978), with keys to identifying North American spiders; Herbert W. Levi and Lorna R. Levi, A Guide to Spiders and Their Kin (1968, reprinted with title Spiders and Their Kin, 1987), coloured illustrations of spider families; Wolfgang Nentwig (ed.), Ecophysiology of Spiders (1987), summaries of research by more than 30 specialists; Vincent D. Roth, Spider Genera of North America: With Keys to Families and Genera and a Guide to Literature (1985); William A. Shear (ed.), SpidersWebs, Behavior, and Evolution (1986), in which specialists report on the results of research; Eugne Simon, Histoire Naturelle des Araignes, 2nd ed., 2 vol. (189297), one basis for the classification system used in this article; and Peter N. Witt, Charles F. Reed, and David B. Peakall, A Spider's Web: Problems in Regulatory Biology (1968), an excellent introduction to orb-web-building spiders. Herbert W. Levi Lorna R. Levi