Spider – is the common name for about 34,000 species of arthropod animals constituting the order Araneae in the class Arachnida, which also includes scorpions, mites, and ticks. Spiders have 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, Theraphosa leblondi of Guyana, 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 leg like 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 and its close relatives, which are non aggressive 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.
Spider silk is a fibrous protein that is secreted as a fluid and forms a polymer, on being stretched, that is much stronger than steel and further resists breakage by its elasticity. A single spider can spin several kinds of silk. Although some other invertebrates also spin silk, spiders put this ability to the most spectacular variety of uses. For example, they form drag lines that help them to find their way about and to catch themselves if they fall. Small and, especially, young spiders spin a “parachute” thread that enables them to be carried by the wind, sometimes for hundreds of kilometers. The males use silk in transferring sperm to the palpal organ, and the females make cocoons with it. Silk is also used to make nests and other chambers and to line burrows. The most familiar and amazing use of silk by many species, however, is in making insect traps called spider webs. Once prey is caught in such a web, the spider may wrap it in more silk.
The diverse webs spun by spiders provide a remarkable example of the evolution of instinctive behavior. A spider does not have to learn how to make a web, although the spinning itself can be adapted to unique circumstances, including the webs spun by spiders under zero gravitation in spacecraft. The simplest webs are irregular and generally laid out along the ground.
More advanced webs, particularly of orb-weaver spiders, are highly intricate, raised above the ground, and oriented to intercept the paths of flying insects. (Actually this is old thought, scientists now think that orb-webs may not have been a more advanced evolutionary invention since they were around at in the early days of spiders’ existence) The spinning itself is a complex process involving the placement and then removal of scaffolding spirals and a combination of sticky and non sticky strands. In some cases a number of spiders will form a kind of communal web, but spiders in general are not social. Such spiders rely largely on the sense of touch.
Besides the web spinners, many spiders hunt for their food or lie in wait for it. Hunters tend to rely on vision if they feed during the daytime, or on touch if they feed at night. Jumping spiders may lurk in ambush for their prey, and a number of them are well camouflaged on flowers by color or body structure or both.
Spiders have separate sexes, and the eggs have to be fertilized. The genital openings of both male and female are located on the abdomen. The male’s copulatory organs, however, are complicated structures located on his pedipalps. He spins a little web and deposits sperm in it, then moves the sperm to the palpal organ. After sperm are transferred to the female, they can be stored in her body for an extended period.
Courtship behavior is often complicated. Males may use drag lines to detect and recognize mates, or they may signal their approach by plucking on the female’s web. In spiders with well-developed eyes, complex mating displays have evolved that are associated with bright color patterns. Often the male must avoid having the female treat it as food; even in species where this is common, however, the male often escapes.
Male spiders are sometimes much smaller than the females. The dwarfing of males is pronounced when the females tend to remain in one place. Males mature earlier, and the sooner the male gets to a female the more apt he is to reproduce.
Spider eggs are protected in cocoons. The female may guard the cocoons or carry them about. In some spiders the hatchlings remain with the mother for an extended period and may be carried on its body.
As predators on insects and other small animals, spiders are generally highly beneficial to humans, although some feed on important plant pollinators such as bees. They also serve as food for other animals, most notably for certain wasps that paralyze the spiders and lay eggs to hatch on the paralyzed body. Efforts to utilize spider silk for cloth have not been successful economically, but the silk has been used for the cross hairs of optical instruments. Although spiders have occupied an honored place in various mythologies, their widespread unsavory reputation in modern times probably results from their tendency to lurk in dark places, their often grotesque appearance, and a gross exaggeration of their toxicity. About 105 families of spiders are known, plus about 10 that are extinct. Two suborders are widely but not universally recognized. The suborder Mesothelae contains a few primitive, burrowing forms. The suborder Opisthothelae contains the infra order Mygalomorphae, which consists of the “straight-jawed” forms, usually large, such as the trap-door spiders and the ones called tarantula in the U.S., and the infra order Araneomorphae, the members of which have the chelicerae somewhat modified and more efficient; it contains the more common and conspicuous forms, such as orb weavers, wolf spiders, and jumping spiders. The cribellate araneomorphs have a specialized organ, the cribellum, that helps to produce silk.
Facts taken from this source Beneficial Fauna.