ETYM Old Eng. spithre, from AS. spinnan to spin; -- so named from spinning its web; cf. Dutch spin a spider, German spinne, Swed. spindel. Seee Spin.
Predatory arachnid that usually has silk-spinning organs at the back end of the body; they spin silk to make cocoons for eggs or traps for prey.
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Any arachnid (eight-legged animal) of the order Araneae. There are about 30,000 known species, mostly a few centimeters in size, although a few tropical forms attain great size, for example, a body length of 9 cm/3.5 in. Spiders produce silk, and many spin webs to trap their prey. They are found everywhere in the world except Antarctica. Many species are found in woods and dry commons; a few are aquatic. Spiders are predators; they bite their prey, releasing a powerful toxin from poison glands which causes paralysis, together with digestive juices. They then suck out the juices and soft parts.
structure.
Unlike insects, the head and breast are merged to form a cephalothorax, connected to the abdomen by a characteristic narrow waist. There are eight legs, and usually eight simple eyes. Two leglike pedipalps at the front of the spider are adapted in males for the transmission of sperm to the female. On the undersurface of the abdomen are spinnerets, usually six, which exude a viscid fluid. This hardens on exposure to the air to form silky threads, used to make silken egg cases, silk-lined tunnels, or various kinds of webs and snares for catching prey that is then wrapped. Seven different types of spider silk have been identified. The threads make webs and traps for the capture of prey, they serve for aerial transport, and are used as a safeguard against falling. Not all spiders use webs, however.
life history.
Spiders are oviparous (egg-laying), and the female encloses her eggs in a silken bag which is sometimes carried about with her, sometimes concealed in the nest, and sometimes attached to solid objects. The young do not undergo metamorphosis, but molt repeatedly until they reach adult size. They typically live for about a year.
varieties.
Species of interest include the zebra spider Salticus scenicus, a longer-sighted species which stalks its prey and has pads on its feet which enable it to walk even on glass; the poisonous tarantula, whose bite can cause local inflammation but rarely death; the black widow; trapdoor spiders, family Ctenizidae, closely related to species that existed some 400 million years ago; the cross spider Araneus diadematus, which spins webs of remarkable beauty; one of the few aquatic species of spider, the water spider Argyroneta aquatica, which fills a “diving bell” home with air trapped on the hairs of the body, and constructs a web below water; and the largest members of the group, the bird-eating spider genus Mygale of South America, with a body about 6–9 cm/2.4–3.5 in long and a leg-span of 30 cm/1 ft. Wolf spiders (Lycosidae) are a widely distributed group of predators. The female carries the young on her back for a few days after hatching.
A new species of spider was discovered 1995, in the Simpson Desert, Australia. The slit spider Fissarena ethabuka is about 15 mm/0.6 in in length, and inhabits a broad (up to 10 cm/3.9 in wide) horizontal slit in the sand, with a sloping burrow behind. It feeds on ants and other insects that fall into its burrow. The slit spider is unusual in that it does not use silk, either to trap prey or construct its burrow.

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An automated program that searches the Internet for new Web documents and indexes their addresses and content-related information in a database, which can be examined for matches by a search engine. Spiders are generally considered to be a type of bot, or Internet robot. Also called: crawler. See also bot (definition 3), search engine (definition 2).