306 



METAZOAN PHYLA 



have many activities which are as justly considered inteUigent as are 

 those of many insects. 



329. Economic Importance. — Spiders are of httle economic impor- 

 tance other than the service which they render in the destruction of 

 injurious insects, but this is more or less offset by their destruction of 

 beneficial ones. Many spiders, particularly those of the tropics, are 

 feared as being poisonous. The larger ones might be able to inject by 

 their bite a sufficient amount of poison to cause marked effects, though 

 rarely, if ever, are they fatal to man. In this countiy the only spider 

 whose bite is serious is the greasy-looking black Latrodectus mactans, the 

 black widow, with a very large globular abdomen on the lower side 



Pedi- 

 palpus 



'Prosomof 



Mesosoma 



Fig. 209. 



Poison 

 claw 



Mefasoma 



-A Tropical American scorpion, Centntrus sp. A, dorsal view of entire animal. 

 B, under side of body. Natural size. 



of which are some yellow or reddish spots. It is usually found under 

 objects lying upon the ground but may also inhabit dark outbuildings. 



330. Scorpions. — Another type of arachnid is the scorpion (Fig. 209), 

 the body of which is clearly metameric. It is divided into a prosoma or 

 cephalothorax; a mesosoma, which is made up of the broadened anterior 

 abdominal metameres; and a metasoma, which includes the slender 

 posterior metameres 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 Uke 



