SPIDERS are a group of animals closely related to scorpions, mites and harvestmen belonging like them to the natural class Arachnida (q.v.) but being characterized within it by possessing silk-producing organs called spinnerets, which are attached to the underside of the abdomen. A good many other Articulata can produce silk, but none make such an extended and diversified use of it as spiders. They employ it not only for making cocoons to protect both the eggs and the spiders themselves, young and mature, from wet and cold, but they also weave it into snares as beautiful as they are diabolical in their ingenuity.
Besides their silk, spiders have another weapon of offence in poison, which they inject into their prey through the sharp sickle-shaped digital joints of their front jaws. Cases are known of spider-poison affecting vertebrates. The bite of the female Latrodectus mactans, known as the Black Widow, is sometimes fatal to children and persons in poor health.
Many spiders, although able to produce silk, spin no web, but rely on a fairly keen sense of sight for catching insects. Such are the Salticidae or jumping spiders, which stalk their prey stealthily and leap upon it when close enough. These have relatively large eyes and, as they are diurnal in their habits and love to run and jump in the sunshine, many of them are coloured in such a way as to render them inconspicuous among their surroundings, while a few closely resemble certain species of ants and thus escape the attention of many of their natural enemies. The Thomisidae are called "crab-spiders," since many run sideways. One Thomisid, Misumena vatia is light green or yellow and lurks in yellow or white flowers.
The Lycosidae, another family of wandering spiders, are not so long-sighted as the Salticidae and only attack when an insect comes close or touches them. Some of them have developed more sedentary habits, probably owing to the continual attacks of various solitary wasps which paralyse the spiders with their stings and store them living in their nests to be food for their lar vae when they hatch out. The famous Tarantulas (q.v.) build silk lined burrows in sandy soil, and often construct a little parapet or a trap-door at the opening. Here they sit waiting until an insect
comes within striking distance. One little Lycosid, Trochosa pitta, besides having a burrow for protection, is spotted with black, yellow, pink and white, so that while it is motionless on sandy soil it is often quite invisible. Burrowing habits are also found among the Mygalomorphae, a group differing widely from Lycosidae, and including the large "bird-eating" spiders. They are much more skilful builders, the trap doors which some of them make being beautiful close-fitting structures, decorated with debris, indistinguishable from their surroundings.
Some Lycosidae of the genus Pirata live by water, on which they can run perfectly. Moreover, they can submerge themselves when necessary by climbing down stems of water-plants, taking down as air supply a bubble, entangled in the hairs of the body, which will last for many hours. Lycosa purbeckensis, which lives on marshy ground among tussocks of grass and reeds by tidal estuaries, runs over the water to a plant stem as the tide comes in and deliberately crawls down, remaining submerged until the ebb. In a closely related family, the Agalenidae, is placed the water-spider, Argyroneta aquatica, which leads a truly aquatic life. It builds a little silken diving bell under the water and fills it with air carried down as bubbles between the abdomen and back legs. Its food consists of aquatic insects, and it even mates and produces its young in its nest under water. Other species of the genus Desis live in the sea and make nests in coral reefs below high water mark, coming out at low tide to prey on small crusta ceans. Among other genera in the same family are land-dwellers with a much more highly developed web, on which the spiders depend for the capture of prey. The upper end of the silken tube, used as a retreat, opens out into a wide sheet of which the cobwebs built in the angles of walls by various species of Tege naria are examples. When an insect falls on to the sheet the spider rushes out, seizes it in its jaws and rushes back to its retreat. The attack is very simple, the web serving to guide the spider to its prey.