664 



ZOOLOGY 



appearance of being intermediate between the Insecta and the 

 other groups of Arachnida. The cephalothoracic region is divided 



by a constriction into two parts, 

 head and thorax, the latter made up 

 of three segments. The chelicerae 

 are chelate ; the pedipalpi resemble 

 the legs, and are used in locomo- 

 tion. The first pair of legs are at- 

 tached to the head. The abdomen 

 is distinctly segmented, and there 

 is no caudal appendage. A pair of 

 poison-glands open at the bases of 

 the chelicerse. There are two simple 

 eyes on the head. 



In the true Spiders (Fig. 543) the 

 abdomen is rounded, unsegmented, 

 and separated off from the cephalo- 

 thorax by'a constriction. The chelicerae (Fig. 544, A) are sub- 

 chelate, and the duct of a large poison-gland opens at the extremity. 



Pig. 543.— Spider (Epeira diadema). 



B 



Fig. 544.-^4, Ckelicera;, and B, pedipalpi of female of Epeira diadema. (After Leuckart.) 



The pedipalpi (Fig. 544, B) are elongated, and end in simple 

 extremities ; in the male (Fig. 545) the terminal joint is modified 

 to serve for the reception and trans- 

 ference of the sperms. At the ex- 

 tremity of the abdomen is the spinning 

 apparatus or arachnidiutn (Fig. 551 , 

 arach.). This consists of four or six 

 elevations, the spinnerets, sometimes 

 jointed, probably derived from em- 

 bryonic rudiments of abdominal ap- 

 , pendages. On the surfaces of these 

 open the numerous fine ducts of the 

 spinning glands (sp. glds.), secreting 

 the material of which the spider's web 



is composed. The fine threads of viscid secretion issuing from 

 the ducts harden on exposure to the air, and are worked up 



Fig. 545. — Pedipalpi of male of 

 Epeira diadema. //. ///. 



IV. V. podomeres : lib, sac ; sph. 

 spiral tube. (After Leuckart.) 



