ARTHROPODS AND MOLLUSCS 



i6S 



cheese-mites, the dreaded itch-mite and the chigger are 

 familiar examples of these degraded arachnids, and the 

 wood-ticks, dog- and chicken-ticks are common examples of 

 the larger blood-sucking forms. The body in both mites 

 and ticks is very compact, the two body-regions, cephalo- 

 thorax and abdomen, being closely fused. Various species 

 of ticks have been proved to be the carriers of the germs of 

 certain diseases of human beings 

 and domesticated animals (see 

 Chapter XII). 



The spiders have the abdomen 

 distinctly set off from the cepha- 

 lothorax. The eyes (fig. 76) 

 vary in number and arrange- 

 ment, the mandibles are large, 

 each bei^g composed of two 

 parts, a basal hair-covered part, 

 the falx, and a terminal smooth, 

 shining, slender, sharp-pointed 

 part, the fang, which is mova- 

 bly articulated with the falx 

 (fig. 76). In the falx is a poison- 

 sac from which poison flows 

 through the hollow fang and out 

 at its tip. The legs vary in rela- 

 tive length in different spiders, and each is made up of seven 

 joints. The spinnerets (fig. 77), which are situated at the 

 tip of the abdomen, are six in number (a few spiders have 

 only four), and are like httle short fingers. They have at 

 their tips many fine little spirming-tubes from each of which 

 a fine silken thread issues when the spider is spinning. 

 These many fine threads fuse as they issue to form a single 

 strong cable or sometimes a flat rather broad band. The 

 spinnerets are movable, and by their manipulation the 

 desired kind of line is produced. The silk comes from 



Fig. 74. The cheese-mite, 

 Tyroglyphus siro. (Greatly 

 enlarged; after Berlese.) 



