ROSS: HOW TO COLLECT AND PRESERVE INSECTS 



53 



feed on decaying humus. A few Illinois forms occur chiefly on 

 bark and these have considerably shorter legs than the species 

 that range more widely. 



Araneida 



Spiders 



These varied and well-known animals have 

 four pairs of walking legs and a body divided 

 into a cephalothorax (which combines the 

 head and thorax) and abdomen. The spiders present a tremen- 

 dous variety of shapes, some being round and fat, as in the case 

 of the black widow, Latrodectus mactans Fabricius, fig. 60, 

 others being long and slender, mimicking ants. Others are crab- 

 like in shape; some that are 

 long and slender are extremely 

 rapid in their movements. Spi- 

 ders appear practically every- 

 where. Certain species are defi- 

 nitely domestic and are found 

 only in houses. In Illinois the ^ ^.c^^^ 



only poisonous species of any im- 

 portance is the black widow spi- 

 der, which is found in a variety 

 of situations. 



Fig. 60. — Araneida. Latrodectus 

 mactans, the black widow spider. Ac- 

 tual length of body 0.4 inch. 



Acarina Unlike the spiders, members of this group 



Ticks, have usually no marked division between the 



Mites cephalothorax and the abdomen. Each adult 



has four pairs of walking legs, although an 

 individual of the very young stages has only three pairs. 

 The mites are generally very minute and 

 seldom are seen by the beginning col- 

 lector. They vary greatly in general 

 appearance. Many species are extreme- 

 ly destructive to stored products, to live 

 domestic animals, and to many groups 

 of plants. Adults of the harvest mite 



Fig. 61. — Acarina. Dermacentor variabilis, 

 the common dog tick of Illinois and vector of 

 Rocky Mountain spotted fever. When en- 

 gorged, the tick looks like a red berry. Actual 

 length 0.2 inch. (After Bishopp.) 



