THE PLACE OF SPIDERS IN NATURE 21 



more generalized in thek structure. The largest examples so far 

 discovered are only about one tenth of an inch long, and half of 

 this length is made up of the slender tail. The micro-whip scorpions 

 have no eyes, and their mouth parts are extremely simple. All ap- 

 pendages are leglike and none have become specialized for grasping, 

 cutting the prey, or otherwise aiding in feeding. 



These minute arachnids are found in Texas and California and 

 in warm areas in other parts of the world. They live under stones 

 and probably feed on tiny insects. 



THE STRUCTURE OF SPIDERS 



To understand more fully the accomplishments and limitations 

 of spiders, it is essential to have a brief resume of their most obvious 

 physical features (Text Fig. i). In common with most Arachnida, 

 they have the body divided into two principal regions, the cephalo- 

 thorax and the abdomen, and each of the sections is provided with 

 certain types of appendages. In spiders the division between these 

 two units is a very narrow pedicel; whereas in such relatives as the 

 scorpions, ticks, and mites the waist is thick. From the several nar- 

 row-waisted arachnids the spiders are immediately differentiated by 

 their possession of ventral spinning organs, or spinnerets, on an ab- 

 domen that is completely unsegmented, except in rare instances. 

 Furthermore, it can be noted that the males of all spiders have a 

 complicated copulatory organ on the end of the pedipalp, a structure 

 never found in this position in the other arachnids. 



Cephalothorax. As the name implies, the cephalothorax repre- 

 sents those segments commonly called head and thorax, but they are 

 intimately fused into a single piece. It must be remembered that 

 several distinct segments have formed this region; their number is 

 indicated by the number of pairs of appendages (in spiders only 

 six) and sometimes by vague indications. The dorsal part of the 

 cephalothorax is provided with a hardened shield or carapace, ordi- 

 narily convex and bearing the eyes at the front end. The head 

 portion is usually more elevated, and may be strongly marked off 

 by a V-shaped groove. On the rounded, flatter thoracic portion are 

 usually evident a median groove and radiating depressions that mark 

 the internal attachments of the muscles of the stomach and of 

 the legs. 



