Spiders

Spiders, the members of the gronp of Arachnida (q.v.) known as the Araneidae. They are characterised by having a soft, unsegmented abdomen, a pair of powerful jaws or "feelers," perforated by the ducts from a poison gland, and by the possession of two or three pairs of spinnerets or spinning organs. They breathe by means of one or two pairs of lung sacs. The character which is most conspicuous on casual examination is that the two front of the three segments, into which the bodies of the Arachnida are typically divided, are fused together into a single mass or cephalothorax; the abdomen is connected to this by a short, narrow stalk or peduncle. The spiders are mainly terrestrial, and therefore breathe air; this passes through small openings known as stigmata on the lower surface of the body. The stigmata are usnally two in number, but four are not uncommon (e.g. Mygalidae), while in others there may be one or more additional ones in front of the spinnerets. The stigmata lead into either lung sacs or branching tubes known as "tracheae." The first pair of stigmata always open to lung sacs; the second pair either to lung sacs or tracheae; and the rrdditional posterior stigmata are always connected with tracheae. Some spiders are aquatic, but, nevertheless, they breathe air which they carry down to their nests in bubbles attached to the hairy portions of their body. The eyes are always simple; the number varies from one to four pairs; they are arranged in a group or in lines on the top of the front portion of the cephalothorax. As would be inferred from their possession of powerful piercing jaws and poison glands, the spiders are carnivorous in habit. The method by which they catch their prey is the feature of most general interest in tbis group. In the hinder part of the abdomen there are many small glands which secrete a viscid fluid, which, on exposure to air, hardens into a thread. The glands communicate by ducts with pores on the summits of four or six small tubercles known as spinnerets; the secretion is forced through these, and comes out as a fine thread. This is used either (1) to attach the eggs to the body of the parent, or (2) to form nests or cocoons in which the eggs are stored, or (3) usually for the spinning of a web in which the food of the spider is entangled. The form of the web is very varied; in the common garden spiders it consists of radial spokes connected by cross threads, and is generally circular in shape; in others it is a thin, irregular sheet; in others, common on grass, it consists of a thin tube often placed in the centre of a funnel-shaped sheet; and in others it is a buried tube, the mouth of which is closed by a door, as in the "Trap-Door Spiders." In some of the larger spiders no web is made, but the animal hunts its prey. The spiders are bisexual, and the males are much smaller than the females, but usually more active. In some cases this "sexual dimorphism" is carried to an extreme, and the male exists only in order to fertilise the female. The female, in some of these, often devours the male either during the flirtation or as a post-nuptial settlement after copulation. The oldest spiders occur in the Carboniferous rocks; a few have been found in the Jurassic, but most of the fossil species have been yielded by the amber deposits in the Oligocene of North Germany. The "Sea Spiders" and "Harvest Spiders" do not belong to the Araneida, and are not true spiders; the former are members of the Pycnogonida (q.v.), and the latter of the Phalangida (q.v.).