Fig. 97. Maxilla and palpus of male house-spider.
Fig. 98. Head of spider, showing eyes and mandibles.
Spiders differ much in appearance from the true insects. In the insects the body is composed of three regions: the head; the thorax, to which the legs are attached; and the abdomen or hind part of the body ([Fig. 94]). In the spiders the head and thorax are grown together, forming a region which is known as the cephalothorax; to this the abdomen is joined by a short, narrow stalk ([Fig. 95]). Spiders differ also from insects in the number of their legs, spiders having eight legs and insects only six.
Spiders have two pairs of jaws, which, except in the Tarantula family, move sidewise like the jaws of insects. The first pair of jaws are called the mandibles. Each mandible consists of two segments, a strong basal one and a claw-shaped terminal one, at the tip of which the poison gland opens ([Fig. 96]). The second pair of jaws is known as the maxillæ. These jaws are situated just behind the mandibles, one on each side of the mouth. Each maxilla bears a large feeler or palpus. These palpi vary greatly in form; frequently, especially in females, they resemble legs; hence many spiders appear to have five pairs of legs. In the male spiders the last segment of the palpus is more or less enlarged, ending in a complicated, knob-like structure ([Fig. 97]). It is thus easy to determine the sex of a spider by merely examining the palpi.
The greater number of spiders have four pairs of eyes ([Fig. 98]), but there may be only one, two, or three pairs; and certain cave spiders are blind. The eyes appear like little gems set in the front of the cephalothorax. They are most prominent in the jumping spiders, which stalk their prey on plants, logs, fences, and the sides of buildings.
Fig. 99. Spinnerets of a spider.
