300 



METAZOAN PHYLA 



posterior metam«res of the abdomen. The metasoma forms a tail which, 

 in an attitude of attack or defense, is carried above the rest of the body- 

 so that the posterior end is directed forward. This terminates in a 

 sharply pointed, clawlike appendage, which is not a metamere, and at the 

 point of which opens the duct from a poison gland. There are two 

 median simple eyes on the upper side of the cephalothorax and three 

 lateral ones on each side. The pedipalpi are very large and curiously like 

 the chelipeds of the decapod Crustacea. There are four pairs of lung 



Pedi- 

 palpus 



-Prosomcf 



Mesosoma 



Poison 

 clai^ 



Metasoma 



Fig. 196. — A Tropical American scorpion, Centrums sp. A, dorsal view of entire animal. 



B, under side of body. Natural size. 



books opening by spiracles on the under surface of the abdominal meta- 

 meres from the third to the sixth. 



Scorpions live mostly in tropical and subtropical regions and are 

 nocturnal in their habits. Their food consists of spiders and large 

 insects which they seize with the pincers of their palpi and sting to death. 

 The sting also serves as a weapon of defense. It is, however, impossible 

 for the scorpion to sting itself to death, as it has often been said to do. 

 The sting rarely, if ever, proves fatal to man. 



329. Mites. — Another group of arachnids contains the mites (Fig. 

 197), which in turn include the ticks. These are very small arachnids 

 without external signs of metamerism and without division into cephalo- 

 thorax and abdomen. Mites breathe through tracheal tubes. The 

 cephalothoracic appendages are similar to those of spiders, although the 

 pedipalpi are not so large. The abdomen shows neither the slits into 

 the air sacs nor the spinnerets at the posterior end. Nevertheless, some 



