CHAPTER XLVI 



CLASS ARACHNIDA 



The spiders and allied forms make up the fifth class of Arthropoda, 

 known as Arachnida (a rak' ni da; G., arachne, spider, and eidos, form). 

 They are distinguished from most of the preceding classes by the fact 

 that the head and thorax are grown together forming a cephalothorax. 

 They have four pairs of walking legs and no antennae. 



321. External Structure of Spiders. — Spiders have a compact body 

 and a large and more or less globular abdomen, without any trace of 



Pedipalpus 

 Chelicera 



Cephalothorax 



Opening to book lung 

 Opening to reproauctn^e organs 

 Abdomen 

 Fourtti leg 

 1 — Spirac/e 



ij Spinnerets 



Anus 



Fig. 193. — Under side of a spider, Araneus sericatus Clerck. Enlarged. {From 

 Linville, Kelly, and Van Cleave, " Text-book in General Zoology," after Emerton, by permission 

 of the authors.) 



metamerism. The abdomen is separated from the cephalothorax by a deep 

 constriction which leaves a slender 'peduncle connecting the two. 



The mouth parts consist of a pair of jaws, known as chehcerae, and a 

 pair of pedipalpi. Each chelicera bears a terminal claw, at the tip of which 

 opens the duett rom a poison gland (Fig. 194) . The pedipalpi are leglike in 

 appearance but their function is rather that of palpi (Fig. 193). Dorsally 

 on the front of the head are the simple eyes, of which most spiders have 

 four pairs, though there may be only one, two, or three pairs (Fig. 194). 



296 



