PHYLUM ARTHROPODA 



227 



Some species of arachnids are eyeless, but more commonly there 

 are from two to twelve simple eyes. Body form and structure 

 are so highly variable in the members of this subclass that they 

 will be discussed under the more important orders individually. 



I. Order Scorpionida 



Scorpions are tropical and subtropical in their distribution, 

 occurring in the United States only in the southern part. Some 

 look much like crustaceans, for in addition to the four pairs of 

 walking legs there are a pair of large chelate pedipalps which are 

 easily mistaken for legs. The 

 chelicerae also bear chelae. Pos- 

 terior to the cephalothorax, which 

 bears the appendages, is the 

 abdomen. This is conspicuously 

 divided into two regions. Seven 

 broad somites continue backward 

 from the cephalothorax as the 

 preabdomen and these are followed 

 by a series of six somites of smaller 

 size which constitute the postabdo- 

 men. The terminal somite of the 

 postabdomeu bears a sharp spine 

 known as the sting with which a 

 pair of poison glands are associated. 

 This serves as an effective organ 

 for killing insects upon which 

 the scorpions feed and produces 

 wounds painful even to man. 

 Lung books occur in the last four 

 somites of the preabdomen on the 

 ventral wall of which they open as 



paired spiracles. From three to six pairs of eyes are commonly 

 borne on the cephalothorax. A pair of comblike organs of 

 undetermined function (Fig. 107), referred to as the pectines, 

 occur on the ventral wall of the second preabdominal somite. 



II. Order Araneina 



Spiders are the most popularly known representatives of the 

 arachnids and comprise the order Araneina. The body is divided 

 by a deep constriction between the cephalothorax and abdomen. 



Fig. 107. — A scorpion (Pandi- 

 nus) from ventral surface. {From 

 Versluys and Demoll). 



