Araneida(redirected from Orb-weaver spider)
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(spiders), an order of arthropods of the class Arachnoidea. The arthropods are from 0.7 mm to 11 cm long. The coloration is extremely diverse. The body consists of a cephalothorax and an abdomen, which are joined by a short pedicel.
The cephalothorax is covered with a carapace, whose anterior portion usually bears eight eyes. There are six pairs of appendages. The first pair are talonlike chelicerae, having ducts of poison glands that open near the fangs. The second pair are pedipalpi; in males the terminal joints of the pedipalpi are converted into copulative organs. The remaining four pairs are clawed walking legs.
The abdomen generally is not segmented; in a few of the most primitive spiders (Lithistius) the abdomen is composed of 11 segments. Beneath the anterior part of the abdomen are located the openings of the anterior pair of book lungs; a bit farther back are situated the openings of a second pair of lungs (tetrapneumo-nous spiders) or the spiracles of the trachea (dipneumonous spiders). One to four pairs of spinnerets—modified abdominal legs—are situated on the posterior portion of the abdomen. The numerous ducts of the silk glands open at the tips of the spinnerets.
Spiders are found in a great variety of habitats. All but the water spider are land dwellers. Spiders are nocturnal animals; only a few are active by day. There are extremely motile spiders, which do not construct permanent nests and shelters. Other spiders are only slightly motile and live in snares or in burrows and lairs.
Spiders have senses of touch, sight, olfaction, and hearing. They are able to perceive vibration and tension of the web threads. Their eyes are usually only capable of discerning the size and movement of objects. All spiders are predators, feeding mainly on insects that are entangled in the web. After killing and usually crushing its prey, the spider injects it with digestive juices, which rapidly liquefy the prey’s tissues. The liquid food is dispatched to the intestinal tract by a special sucking stomach.
Among spiders, the sexes are separate. Males are often substantially smaller than females. Before mating, the male spins a small webbed “hammock” on which he excretes a drop of semen; he then places his bent pedipalpi under the hammock and fills them with the permeated semen. The ova are usually deposited in spider cocoons that the female carries with her or guards until the emergence of the young spiders.
The order embraces approximately 21,000 species. In the USSR there are about 2,000 species. Spiders are divided into three suborders: Liphistiomorphae, Mygalomorphae, and Araneomorphae. All tetrapneumonous spiders belong to the first two suborders. The overwhelming majority of spiders, including the house spider (Tegenaria derhami), water spider, cross spider, tarantula, and karakurt, belong to the suborder Araneomorphae (dipneumonous spiders). Many spiders destroy harmful insects, including the cotton root aphid, the shield bug, and malaria mosquitoes. Some, mainly tropical, spiders are poisonous and dangerous to domestic animals and humans. Among the spiders of the USSR, the South Russian tarantula and, especially, the karakurt are poisonous.
REFERENCESSee references under .
A. V. IVANOV