The front half of a spider's body, called the cephalothorax, contains in one piece the head and thorax, the only outward division between them being shallow grooves from the middle of the back to the front legs. In the middle of the cephalothorax is usually a groove or depression, under which, inside, is a muscle that moves the sucking apparatus by which food is drawn into the mouth. At the sides of the thoracic part are four pairs of legs, and on the head part are a pair of palpi and a pair of mandibles. The legs have seven joints: (1) the coxa, the thick basal joint, having little motion; (2) the trochanter, a short joint moving very freely on the end of the coxa; (3) the femur, the largest joint of the leg, moving with the trochanter in all directions; (4) the patella, moving up and down on the end of the femur; (5) the tibia, joined closely to the patella and moving with it up and down; (6) the metatarsus; and (7) the tarsus, moving together on the end of the tibia. The palpi are like small legs and have one less joint than the walking legs. The mandibles are close together at the front of the head (fig. 2). They are two-jointed, the basal joint stout and the end joint or claw slender and sharp-pointed. The claw has near its point a small hole, which is the outlet of the poison gland. The poison kills or disables the insects which are captured by the spider. Its effect on the human skin varies in different persons; sometimes it has no effect at all; oftener it causes some soreness and itching like the stings of mosquitoes and bees, and cases have been known in which it caused serious inflammation which lasted a long time. Spiders seldom bite, and only in defense, the bites so commonly charged to them being often the work of other animals. [Illustration: FIG. 1. Diagram of the under side of a spider, with the legs removed except one.] [Illustration: FIG. 2. Front of head, showing the eyes and mandibles.] On the front of the head are the eyes, usually eight in number, differing in size and arrangement according to the kind of spider. The sight of spiders is distinct for only short distances. Spiders of middle size can see each other, and the insects which they eat, at a distance of four or five inches, but beyond that do not seem to see anything clearly. At the ends of the feet are two claws, curved and with teeth along the inner edge, and in many spiders there is a third shorter claw between them (fig. 212). The claws are sometimes surrounded by a brush of flattened hairs (figs. 104, 114). The basal joints of the palpi are flattened and have their inner edges extended forward so that they can be used as jaws to press or chew the food. These are called the maxillæ. Between the maxillæ is a small piece called the labium, and between the legs is a larger oval piece called the sternum.