Spiders and Their Near Relatives 



like organ is formed. Two types of pincers exist in the Arach- 

 nida. The first is known as a chela; an illustration of this is a 

 great claw of scorpions. A chela is formed by the next to the 

 last segment of an appendage being prolonged on one side so as 

 to oppose the last segment (Fig. n). An appendage bearing a 

 chela is said to be chelate. In descriptions of chelate claws, the 

 last segment is termed the movable claw of the chela; and the 

 prolongation of the next to the last segment opposed to this, 

 the fixed claw of the chela. 



Another type of pincers is produced when the last segment 



of an appendage is claw-like and 

 is folded back into a groove in 

 the next to the last segment, like 

 the blade of a pocket-knife into 

 the handle (Fig. 12). An append- 

 age bearing a pincer of this type 

 may be said to be uncate. 



These two types of pincers are 



not distinct, intermediate forms 



being present. Such intermediate 



forms may be termed semichelate; 



the pedipalps of Mastigoproctus are 



illustrationsofthesemichelatetype. 



A detailed account of the structure of spiders is given in the 



following chapters; it is not necessary, therefore, to discuss 



further the anatomy of arachnids in this place. 



III.— THE ORDERS OF THE ARACHNIDA 



According to the classification that is most generally accepted, 

 the class Arachnida is divided into nine orders. One of these 

 orders, the Ricinulei, is not represented in North America, and, 

 therefore is not discussed here. 



The eight orders included in our fauna can be separated 

 by the following table. 



TABLE OF THE ORDERS OF THE ARACHNIDA 



A. Abdomen distinctly segmented. 



B. Abdomen with a tail-like prolongation. 



Fig. 12. AN UNCATE CHELICERA 



12 



