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INVERTEBRATE ZOOLOGY 



The four prominent, pairs of legs not only serve for walking and 

 jumping but the posterior pair also aid in the formation of the 

 characteristic silken webs. Except in members of one family, 

 the abdomen is saclike, unsegmented, and joined to the cephalo- 

 thorax by a narrow stalk. At the caudal end, the abdomen bears 

 a small conical portion which represents a greatly reduced 

 postabdomen. 



Spinning organs are located on the ventral surface near the 

 caudal extremity of the abdomen and consist, usually, of three 

 pairs of spinnerets. These are fingerlike in form and are thought 

 to represent rudiments of two pairs of abdominal appendages. 

 Spinning tubes are distributed over the terminal portion of each 

 spinneret and through these the fluid is expelled, which, upon 

 contact with the air, hardens to form silk. An additional spin- 

 ning organ known as the cribellum occurs in some spiders. This 

 consists of a median, ventral, sievelike plate anterior to the 

 spinnerets bearing very numerous spinning tubes. 



The tarantulas (Eurypelma) and the orb weavers (many 

 genera in the subfamily Araneinae) are among the extremely 

 numerous representatives of this order. 



Ill, Order Acarina 



The mites and ticks have a broad, unsegmented abdomen which 

 is not constricted at its union with the cephalothorax. As a 



Fig. 108. — Acarina. Boophilus annulatus, the tick which carries Texas fever; 

 A, female; B, male; C, young with only three pairs of legs. (After Banks). 



consequence the entire body appears saclike, though in some 

 instances cephalothorax and abdomen are distinguishable. The 

 part usually termed the abdomen includes two somites which in 

 reality belong to the thorax. Frequently, the segments bearing 

 the chelicerae and pedipalps are more or less distinct from the rest 



