The spiders form a small and distinct group of animals, related to the scorpions, the daddy-long-legs, and the mites, and less closely to the insects and crabs. They are distinguished by the more complete separation of the body into two parts; by their two-jointed mandibles, discharging a poisonous secretion at the tip; and by their spinning-organs, and habits of making cobwebs and silk cocoons for their eggs.

The common round-web spider, Epeira vulgaris of Hentz, will serve as well as any species to show the anatomy of spiders in general. Fig. 1 shows the under side of this spider; Fig. 4, the upper side; and Fig. 5, an imaginary section through the body, to show the arrangement of the internal organs. To begin with Fig. 1: the body is seen to be divided into two parts, connected only by the narrow joint, A, just behind the last pair of legs. The front half of the body, called the thorax, contains the stomach, the central part of the nervous system, and the large muscles which work the legs and jaws. The hinder half, the abdomen, contains the intestine, the breathing-organs, the principal circulating-vessels, the organs of reproduction, and the spinning-organs. Connected with the thorax are six pairs of limbs, four pairs of legs, B B B B, a pair of palpi, C, and a pair of mandibles, D.

LEGS.

The legs are used chiefly for running, jumping, and climbing; but the front pair serve often as feelers, being held up before the body while the spider walks steadily enough on the other six. One or both of the hinder legs are used to guide the thread in spinning; the spider at the same time walking or climbing about with the other six or seven. The legs are seven-jointed; and on the terminal joint are three claws, Fig. 2, A, B, C, and various hair and spines. In many spiders a brush of hairs takes the place of the middle claw, as in the jumping spiders, Fig. 3. Spiders with these brushes on their feet can walk up a steep surface, or under a horizontal one, better than those who have three claws. The legs of most spiders have among the hairs movable spines, which, when the spider is running about, extend outward at a right angle with the leg, and, when it is resting, are closed down against the skin.

PALPI.

In front of the legs are the palpi, Fig. 1, C, C,—a smaller pair of limbs, with six joints and only one claw or none. They are used as feelers, and for handling food, and, in the males, carry the curious palpal organs, which will be described farther on. The basal joints, Fig. 1, E, of the palpi are flattened out, and serve as chewing-organs, called “maxillÆ.”

Mr. Mason has lately described, in the Transactions of the Entomological Society of London, a large spider which has teeth on the inside of the palpi, which, when the spider is angry, are rubbed against teeth on the mandibles, producing a noise.

MANDIBLES.

The front pair of limbs, the mandibles, Fig. 1, D, are two-jointed. The basal joint is usually short and stout, and furnished on the inner side with teeth and hairs. The terminal joint is a small and sharp claw, which can be closed against the basal joint when not in use.

ABDOMEN.

On the under side of the abdomen, just behind the last pair of legs, are two hard, smooth patches, which cover the front pair of breathing-organs, the openings to which are two little slits at Fig. 1, H. Between these is the opening of the reproductive organs, and, in female spiders, the epigynum, Fig. 1, J,—an apparatus for holding the reproductive cells of the male.

At the end of the body are the spinnerets, which will be described in another chapter. There are three pairs of them; but many spiders close them together when not in use, so as to cover up the middle pair. The third pair of spinnerets are often several-jointed, and extend out behind the body like two tails. In front of the spinnerets is a little opening, Fig. 1, K, which leads to air-tubes that give off branches to different parts of the abdomen. At M, Fig. 1, are usually two colored bands, or rows of spots, marking the course of muscles attached to the skin at various points along these lines.

Fig. 4 is the back of the same spider. The head is not separated from the rest of the body, as in insects, but forms, with the thorax, one piece. On the front of the head are eight eyes, Q, which are differently arranged in different spiders. At the back part of the thorax is a groove, P, under which is attached a muscle for moving the sucking-stomach, Fig. 5, d. From this point radiate shallow grooves, that follow the divisions between the muscles of the legs. On the abdomen are several pairs of dark smooth spots, which mark the ends of muscles extending downward through the abdomen. The markings of this spider are very complicated. The spot on the middle of the front of the abdomen is a very common one, and, in some spiders, extends the whole length of the body. The waved lines on each side are also common, and, in long-bodied spiders, often form two bright-colored stripes, or rows of spots, running nearly straight the whole length of the abdomen.

INTESTINE.

Fig. 5 is a section of the same spider. The mouth is at a b, just under and behind the mandibles, and between the maxillÆ. It has an upper, a, and under lip, b, each lined with a horny plate, in the middle of which runs a groove. When the lips are closed, the two grooves form a tube, which leads to the oesophagus, c, and so into the stomach. At the end of the oesophagus is the sucking-stomach. This consists of a flattened tube, to the top of which is attached a muscle, d, connected with the groove in the back; and to the bottom, muscles, f, attached to a tough diaphragm spreading across the thorax, and fastened between the legs on each side at g g. When these muscles contract, the top and bottom of the sucking-stomach are drawn apart, and whatever is in the oesophagus sucked in. By this pumping motion the spider is supposed to take liquid food from the mouth, and drive it backward into the abdomen. Just behind the sucking-stomach, the intestine gives off two branches, e e, which extend forward around the stomach muscle, and meet over the mouth. Each of these branches gives off on the outer side four smaller branches, m m m m, which extend downward,—one in front of each leg,—and unite on the under side of the thorax.

• Section of a spider to show the arrangement of the internal organs:
• a, b, upper and under lips of the mouth; c, c, the oesophagus;
• d, f, upper and under muscles of the sucking-stomach;
• e, stomach; g, g, ligaments attached to diaphragm under the stomach;
• J, lower nervous ganglion; k, upper ganglion;
• l, l, nerves to the legs and palpi; m, branches of the stomach;
• n, poison-gland; o, intestine; p, heart; R, air-sac;
• S, ovary; t, air-tube; u, spinning-glands.

The intestine, o, continues backward through the abdomen to the anus, in the little knob behind the spinnerete. The brown mass which surrounds the intestine, and fills the abdomen above it, is supposed to be a secreting-organ discharging into the intestine at several points.

HEART.

Over the intestine, and parallel with it, is the heart, p, a muscular tube, with openings along the sides to receive the blood, and branches through which it flows to different parts of the body. The greater part of the blood enters at the front of the heart, and passes backward into the abdomen, or forward into the thorax.

BREATHING-ORGANS.

In the front of the abdomen are the principal breathing-organs,—a pair of sacs, R, containing a number of thin plates, through which the blood passes on its way to the heart. Besides these, there is a pair of branching air-tubes, t, opening near the spinnerets.

NERVOUS SYSTEM.

The nervous system has a large ganglion, J, in the thorax, from which branches, i, pass to the limbs and abdomen. At the front end two branches extend upward, each side of the oesophagus, to two smaller ganglia, k, from which pass nerves to the mandibles and eyes.

The reproductive organs, S, lie along the under side of the abdomen, and open between the two air-sacs.

The spinning-glands, u, lie above the spinnerets, and along the under side of the abdomen. They will be more fully described in the chapter on spinning.

POISON-GLANDS.

The poison-glands, n, are partly in the basal joints of the mandibles, and partly in the head, and discharge by a tube which opens at the point of the claw of the mandible, Fig. 15, a.

CLASSIFICATION.

There is not room in this book to explain the classification of spiders into genera and species; but a description of the following well-marked groups, which contain nine-tenths of all spiders, will give a general idea of the differences among them, and help to understand what follows.

MYGALIDÆ

This family includes the largest known spiders. The body is usually very hairy and dark-colored. Most species have only four spinnerets; and one pair of these are long, and are turned up behind the abdomen. They have four air-sacs under the front of the abdomen, instead of two, as other spiders. Their mandibles are very large, and work up and down, instead of sidewise. The eyes are collected together on the front of the head. They live only in warm countries. Specimens from South America are exhibited in every natural history museum. Fig. 6 represents Mygale Hentzii, a species living in Arizona and Texas.

DYSDERIDÆ.

A small family of spiders with only six eyes. They have also four breathing-holes in the front of the abdomen; but one pair leads to branched tubes instead of sacs. They are usually found under stones, with their legs drawn up close to their bodies, but can move very quickly when so inclined. Very few species are known, and none are common, in North America. Fig. 7 is Dysdera interrita enlarged. Below are the eyes as seen from in front.

DRASSIDÆ.

A large family of spiders, varying greatly in shape, color, and habits. Most of them are dull colored, and live under stones, or in silk tubes on plants, and make no webs for catching insects. Their eyes are small, and arranged in two rows on the front of the head. Their feet have two claws and a bunch of flat hairs. The spinnerets are usually long enough to extend a little behind the abdomen. Fig. 8 is a Drassus, and the eyes as seen from in front.

Long-legged, brown spiders, with two spinnerets longer than the others, and extending out behind the body. They make flat webs, with a funnel-shaped tube at one side, Fig. 24, in which the spider waits. Fig. 9 is Agalena nÆvia, the common grass spider.

CINIFLONIDÆ.

A family resembling the last, but having peculiar spinning-organs, which will be described farther on, Fig. 35.

LYCOSIDÆ.

The running spiders. Very active spiders, living in open places, and catching their prey without webs. Their legs are long, the hind pair being the longest. The head is high, and the arrangement of the eyes peculiar to the family, Fig. 10.

ATTIDÆ.

The jumping spiders. The body is usually short, and the head square. The arrangement of the eyes is characteristic, Fig. 11, a. The two large ones in the middle give these spiders a more animated look than those of other families. The legs are short, and the front pair often stouter than the others. They can walk easily backwards or sidewise, and jump a long distance. Fig. 11 is the common gray jumping spider enlarged.

The crab spiders. The body is usually flat, and wide behind. The front two pair of legs are longer than the others, and so bent that the spider can use them when in a narrow crack. Some of them, like crabs, walk better sidewise than forwards. Like the running and jumping spiders, they make no webs for catching food. The eyes are small, and simply arranged in two rows, Fig. 12.

THERIDIIDÆ.

The largest family of spiders. Most of them are small, with large rounded abdomens and slender legs. They live usually upside down, holding by their feet under their webs. They make large cobwebs of different shapes for different species, and depend for food on what is caught in them. To this group belongs the genus Erigone, containing a great number of the smallest spiders known. Fig. 13 is the common house Theridion enlarged.

EPEIRIDÆ.

The round-web spiders. Large spiders, with flat heads, and eyes wide apart, and short, round abdomens, Figs. 1, 4. They make webs formed of radiating lines crossed by other adhesive ones in a spiral or concentric loops, Fig. 28. They hang in the web, head downward, or live upside down in a hole near by.