spider

Female grand argiope spider Argiope aemula in the rainforest of Sulawesi, Indonesia.

Any arachnid (eight-legged animal) of the order Araneae. There are about 30,000 known species, mostly a few centimetres in size, although a few tropical forms attain great size, for example, some bird-eating spiders attain 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 which 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.

The gene for flagelliform silk, the most elastic of the spider silks and the silk that is used in the core threads of spider webs, was identified in 1998. Eight of the genes used in webmaking are now known, opening up the way for the manufacture of synthetic threads with the strength and elasticity of spider silk.

Australian researchers discovered in 1999 that orb spiders improve in their webspinning abilities as they grow older, indicating that they learn from experience.

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 moult 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 member 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.

The bauble spider Achaearanea globispira was discovered in 1994 in the Cedarberg Mountains, South Africa. It is 2–3 mm/0.1 in long and builds a bauble-shaped nest of silk, covered with sand and up to 13 mm/0.5 in long. The nest contains spiral tunnels and is suspended from overhanging rocks by a single thread.

A new species of spider was discovered in 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.

spider

In computing, program that combs the Internet for new documents such as Web pages and FTP files. Spiders start their work by retrieving a document such as a Web page and then following all the links and references contained in it. They repeat the process with the followed links, supplying all the references they find to a database that can be searched via a search engine.