SPIDER (ME. spithcr, from AS., Goth., OHG. spinnaa. Oer. spin acn, to spin; cf. OHO. s-pinna, Ger. Spinne, spinner, spider). Any member of the Araneida, an order in the class Arachnida. The arachnids are distinguished from insects by the possession of four pairs of legs; while spiders are separated from other Arachnida by the presence of spinning-organs near the tip of the body. The body of a spider is divided into two portions connected by a small slender poli ce]. The anterior part is ealled 'cephalothorax.' and is supposed to represent the combined head and thorax of insects. The posterior part is the 'abdomen.' On the anterior part of the cephalothorax are the eyes, commonly eight in number, and frequently arranged in two trans verse rows of four each. In the lower front margin of the cephalothorax are the mouth parts. These consist of a pair of jaws, some times ealled mandibles or `falces:' a pair of palpi. whose basal joints are enlarged into max illa.; and a median unpaired lower lip. The mandibles are two-jointed; the basal joint is very large and stout, the apical one is small, claw-like, and called the 'fang.' The palpi are filiform, and six-jointed. In the female they are simple and often terminate in a claw; in the male, however, the apical joint is curiously modified into a complicated accessory sexual Organ, suited to carry and apply the seminal fluid. The four pairs of legs are similar in structure, but variable in length. Each con sists of seven joints, and ends in two or three toothed claws. The spiders that live on webs usually have three claws, the median more curved than the others; while other spiders com monly have but. two. In place of the median claw is sometimes a dense fascicle of hairs. The abdomen of spiders is generally soft, tumid, and has no apparent joints. The genital organ opens near the base of the abdomen beneath. The male aperture is inconspicuous, but the female vulva or epigynum is often very prominent, and some times quite complicated. At the apex of the
abdomen are four to six short, often two-jointed pieces, the spinnerets or spinning-organs. Eaeh spinneret has on its surface many minute ori flees, from each of which may issue a thread, and the many threads joined together make the spider's line. The substance which exudes from the spinnerets is glutinous, but dries on contact with the air. Some spiders possess an accessory spinning-organ, a transverse surface in front of the base of the spinnerets known as the `cribel lum! Correlated with this is a row- of curved hairs on the hind metatarsus, called the `ealamis trum.' The spider draws the row of hairs over the cribellum and combs out a curled or tangled thread of silk.
The mouth of the spicier opens into a short which leads to a sucking-stomach which draws up the liquid food. From the posterior part of this organ arise two branches which extend upward and forward and meet over the mouth; each branch gives off on the outer side four smaller one in front of each leg, and these below- the sucking-stomach. From the stomach arises the intestine, which passes into the abdomen and opens just. behind the spinnerets. In the abdomen just above the intestine is a long tubular heart or pulsating vessel, with openings in front. behind, and along the sides. The nervous system is represented by a large, long ganglionic mass lying on the floor of the cephalothorax. In front is a smaller mass which gives rise to nerves to the eyes and jaws. At the base of the abdomen below there is a cavity on each side, nearly filled by a series of thin plates through which the air reaches the blood. These are the `hugs.' Spiders also have a pair of branching trachex, which open just in front of the spinnerets. The silk-glands lie above the spinnerets and along the floor of the abdomen. They are of two kinds, most of them short, hut two are larger and very long. Each silk-gland has a separate opening or duct in the spinnerets.