22 AMERICAN SPIDERS 



The cephalothorax is subject to considerable variation in shape 

 and armature. In long spiders it is usually long, and in short species 

 may be wider than its length. Various spines, humps, and promi- 

 nences of many kinds often surmount it; frequently some of the 

 eyes sit on weirdly designed elevations. In the dwarf spiders the 

 carapace of certain males is grotesquely formed, and has deep pits 

 into which the chelicerae of the females are fitted during copula- 

 tion. In most instances the reason for the presence of such special- 

 ized innovations is not clear. 



On the front of the head are the eyes, which are simple and 

 resemble the ocelli of insects. Most spiders have eight eyes, appar- 

 ently the original number, but various lines have lost some, so that 

 there are in existence six-eyed, four-eyed, and two-eyed spiders. 

 In one tiny spider from the jungle floor of Panama only a single 

 median eye is present, probably representing the fusion of one pair. 

 Some of the cave spiders and others that live in dark situations have 

 completely lost their eyes, or retain only vestiges. The size and 

 position of the eyes vary considerably. Some of the hunting spiders 

 have large eyes and relatively keen vision, this being one of the 

 necessities for their foraging activities. In many, a tapetum, which 

 causes the eyes to shine in the dark when struck by light rays, con- 

 tributes to the efficiency of this night vision. Most spiders, how- 

 ever, are shortsighted animals that rely on their sense of touch, 

 which they have sharpened at the expense of their eyes. 



Immediately below the carapace on the ventral surface of the 

 cephalothorax is a median plate, frequently heart-shaped, called the 

 sternum. In front of it is the much smaller lower lip, or labium, 

 which forms the floor of the mouth. Around each side of the 

 sternum are the coxae of the legs and the pedipalpi, which fit snugly 

 against the sternum and lie in the space between it and the carapace. 

 The coxa of the pedipalp in most spiders is fitted with an enlarged, 

 sharp plate, the maxilla or endite, which aids in the breaking of 

 the prey. 



Directly beneath the cephalothorax at the front end are located 

 the two chelicerae, or jaws, which are the offensive weapons of the 

 spider. It is believed that the chelicerae are derived from the same 

 pair of primitive appendages that became the second antennae in 

 the crustaceans, and this fact illustrates the quite distinct use to 

 which the same generalized appendages are put by a different crea- 

 tures. Each chelicera is composed of two segments, a basal one, 

 which is stout and ordinarily margined by a toothed groove at the 



