Spider, common name for about 34,000 species of arthropod animals having eight walking legs, anterior appendages bearing fangs and poison glands, and specialized reproductive organs on the second appendages of the male. They commonly make extensive use of silk that they spin. Like other arachnid species, spiders are terrestrial, although a few have adapted to freshwater life by trapping air bubbles underwater and carrying the bubbles with them. Spiders are numerous and occur worldwide. Although most are less than 1 cm (less than 0.4 in) long, the largest has a body length of about 9 cm (about 3.6 in), and spider leg spans can be much greater.
The body structure of a spider is similar to that of other arachnids in being divided into an anterior cephalothorax, or prosoma, and a posterior abdomen, or opisthosoma. The two parts are separated by a narrow stalk, or pedicel, which gives the animal a flexibility that facilitates its use of silk. The cephalothorax ordinarily bears four pairs of simple eyes that tend to be larger in hunting spiders and smaller in spinners of elaborate webs. Each of the first pair of appendages, or chelicerae, bears a fang with an opening from a poison gland at the tip. The next two appendages are pedipalps, rather leglike but generally modified into a kind of feeler. In the male the pedipalp bears a peculiar copulatory apparatus called a palpal organ. Also on the cephalothorax are four pairs of walking legs. On the abdomen are located modified appendages, the spinnerets, used in secreting silk. Respiratory openings on the abdomen lead to the so-called book lungs (named for their layered structure) or a system of tubes (tracheae) for carrying air, or both.
The digestive system of spiders is adapted exclusively to taking up liquid food, because the animals generally digest their prey outside the body and then suck the fluid. The fairly complex brain is larger or smaller in certain parts, depending on whether the animal locates prey mainly by touch or vision.
Spiders are generally carnivorous and feed only on living prey. They can crush it with processes on the pedipalps, and the chelicerae almost always have glands that can inject a venom. The bite of some large spiders can be painful, but most species are too small to break human skin, and only a few are dangerous to humans. The latter are mainly the black widow spider and its close relatives, which are nonaggressive and bite humans only in defense. Their painful bite is followed by faintness, difficulty in breathing, and other symptoms; although the bite is seldom fatal, especially if it is inflicted on healthy adults, medical attention for it should be sought at once.
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