The thread of spiders resembles that spun by caterpillars in making their cocoons, and can be manufactured in the same way into silk cloth. The spider's thread is composed of a great number of finer threads passing from the body through separate tubes and uniting into one before they have time to dry. This can be seen by examining the attachments of spiders' threads to glass. All the spinning tubes are not alike, but on certain parts of the spinnerets are larger or differently shaped tubes, and these are the outlets of glands of different kinds in the spider's abdomen, and are used in making different kinds of threads for certain parts of the webs, nests, or cocoons.

In front of the spinnerets on the under side is a small opening to the tracheæ, or air-tubes [(fig. 1)]. At the front of the abdomen on the under side is a transverse fold of the skin, at the ends of which are the openings of the air-sacs or lungs, and between them the opening of the reproductive organs [(fig. 1)]. The latter is covered in females by an apparatus, sometimes large and complicated, called the epigynum. Its presence shows that the female is full grown. Young spiders do not have it. Male spiders have the ends of the palpi enlarged, and under the terminal joint what is known as the palpal organ, sometimes very complicated in shape. The presence of these organs shows that the male is full grown. Young males that have nearly reached maturity have the ends of the palpi simply enlarged. Male spiders almost always have the body smaller and the legs longer than females of the same species.

The colors of spiders are partly in the skin itself and partly in the hairs and scales that cover it. Almost all spiders are covered with hair of some kind, but in some species it is so fine and short that it has little effect on the color. In others the skin is entirely covered with hairs of various lengths and sometimes with scales somewhat like those of butterflies, flattened and feathered or toothed on the edges. The colors of spiders are very varied, and in many species, especially of the jumping spiders, as brilliant as those of butterflies. The most common colors are grays and browns, resembling the ground or plants and stones among which the spiders live. Sometimes the color is uniform all over the body, except that it is a little darker toward the head and the ends of the feet. The most common marking is a spot on the front of the abdomen over the spider's heart, sometimes merely a translucent part of the skin and sometimes a definite color spot darker in the middle and outlined with a lighter shade or white. The hinder half of the abdomen is often marked with several pairs of spots, becoming smaller toward the end, and these spots may be united into a pair of stripes or a more complicated pattern. (See figures.) The legs are often marked with rings of color, almost always at the ends of the joints. Besides these common markings there are in some spiders strong contrasts of color, such as bright red or yellow spots on a black ground. In the males, especially among the Attidæ, there are often shining scales that reflect different colors in a bright light, and tufts of black or white hairs about the head and front legs.

Spiders live in all kinds of places. Certain species are attached to houses and seldom found far from them, and many of these occur over a large part of the world. The light webs in the corners of rooms are chiefly the work of Theridium tepidariorum (p. [112]), occasionally of Steatoda borealis (p. [119]) and Steatoda triangulosa (p. [121]). In cellars the thin webs about the stairs and shelves are those of the long-legged Pholcus phalangioides (p. [129]) or of Linyphia nebulosa or minuta (pp. [144], [145]), and the thick flat webs in corners and between the beams are those of Tegenaria derhamii (p. [96]). On the outside of houses live two jumping spiders, the most common being Epiblemum scenicum (p. [60]), a small gray species the color of weathered wood, and the other, Marptusa familiaris (p. [61]). Some of the round-web spiders live in great numbers about houses. The three brown species, Epeira sclopetaria (p. [160]), patagiata, and strix, hide in cracks and at night make their round webs in porches, barns, and bridges. In the northern part of the country Epeira cinerea (p. [165]) has the same habit. Epeira globosa (p. [174]) is often found on the outside of houses, and so are Zilla atrica and Zilla x-notata (p. [185]). Amaurobius ferox (p. [215]), a large imported species, is sometimes found in cellars, and several Dictyna (p. [206]) live in great numbers on the outside of houses, in corners of windows, under the edges of shingles, or in cracks of walls, spreading their webs wherever there is room for them and gathering dust so that they often make a distinct spot on the wall. In the southern states Filistata hibernalis (p. [220]) is one of the most common spiders about houses. Its webs often make a round spot of dust a foot or more in diameter. Stones and sticks lying on the ground furnish shelter for a great number of spiders. Steatoda borealis (p. [119]), marmorata (p. [121]), and guttata (p. [120]) and Asagena americana (p. [122]) are found in such places, and so, especially in the South, is Latrodectus mactans (p. [122]). The large jumping spiders, Phidippus mystaceus (p. [50]) and tripunctatus (p. [51]), make large nests of white silk under stones near the ground. The ground spiders, Drassus saccatus (p. [6]), Gnaphosa conspersa (p. [2]), and Prosthesima atra (p. [5]), run on the ground and hide under stones. Lycosa nidicola (p. [69]), Lycosa communis (p. [75]), Lycosa pratensis (p. [69]), polita (p. [70]), and cinerea (p. [74]) are often found under shelters of this kind. The crab spiders of the genus Xysticus live under stones, but oftener under bark farther from the ground.

In the summer, plants of all kinds from grass to trees are full of spiders. The Lycosas (pp. [68 to 84])run among the short grass. The small species of Linyphia (p. [134]) and Erigone (p. [148]) make their flat webs close to the ground among small plants. Linyphia marginata, communis, coccinea, and phrygiana make theirs among plants and rocks, a foot or two above the ground. The Theridiums (p. [110]) live between leaves and on the ends of twigs, covering them with webs that only show when the dew is on them. Agalena nævia (pp. [91 to 95]) makes its flat webs on the grass and anywhere else where it can find a place to fasten them. The jumping spiders (p. [41]) run about for their prey on plants, and some of them have silk nests among the leaves. The Misumenas (p. [25]) live among flowers and wait for insects to alight within reach. The webs of Dictyna (p. [206]) are commonest on the ends of grass and twigs, and are known by the dust that they gather. The round-web spiders mature in the middle of the summer, and then Epeira trivittata (p. [166]) is found on all kinds of bushes and grass, and later Epeira insularis (p. [169]) and Epeira trifolium (p. [171]) in hidden nests near their webs. Epeira angulata, sylvatica, and nordmanni (p. [162]) live among bushes and trees. Cyclosa conica (p. [183]), Acrosoma spinea (p. [190]), and Uloborus (p. [216]) live among low bushes in openings of the woods. Hyptiotes (p. [218]) lives among the lower dead branches of pines, perching on the end of a twig which it exactly matches in color.

The marshes are the home of great numbers of spiders. The Tetragnathas (p. [198]) live there, especially along the streams and ditches. Epeira gibberosa (p. [175]) and placida (p. [176]) make their horizontal and oblique webs among the tall grass in open places. The two species of Argiope (pp. [193 to 198]) swarm in marshes and open fields and in autumn become conspicuous by their size and bright colors, and when they disappear leave over winter their brown cocoons (pp. [197], [200]) fastened to the grass.

The moss and dead leaves in the woods are alive with spiders; even in summer some species always live there, and in winter the young of those that in warm weather live among the bushes find shelter where they can remain torpid through the cold season without freezing.

The eggs of spiders are covered with silk, forming a cocoon which varies much in shape and color in different species. Some spiders hang it in the web, others attach it to plants or stones, and others carry it about with them either in the mandibles or attached behind to the spinnerets. The young remain in the cocoon until they are able to run about, and after coming out of the cocoon keep together for a short time, sometimes in a web which they make in common, sometimes in a nest made by the mother, and in some species on the mother's back, but they soon scatter and hunt their own food or make cobwebs, according to the habits of the species.

Different kinds of spiders mature and breed at different times of the year, most of them living only one season. Those that mature late, like Agalena nævia and Argiope, pass the winter as eggs, while those that mature early, like Epeira sclopetaria and Lycosa nidicola, pass the winter half grown. Some species, like Theridium tepidariorum (p. [112]), breed several times in the year, and old and young are found at all seasons.