SPIDERS AND OTHER ARACHNIDS 113 



4. The webs of certain species are composed in part of loose 

 bands of silk (Emerton). 



How Webs are Built. An orb web, such as that shown in Fig- 

 ure 59, is spun in the following manner : A thread is stretched 

 across the space selected for the web; then from a point on 

 this thread other threads are drawn out and attached in radiat- 

 ing lines. These threads all become dry and smooth. On this 

 foundation a spiral is spun of sticky thread (Fig. 61, D). The 



22 



FIG. 60. Internal anatomy of a spider. 



1, mouth; 2, sucking stomach; 3, ducts of liver; 4, so-called malpighian 

 tubules ; 5, stercoral pocket ; 6, anus ; 7, dorsal muscle of sucking stomach ; 

 8, caecal prolongation of stomach; 9, cerebral ganglion giving off nerves to eyes; 

 10, subossophageal ganglionic mass ; 11, heart with three lateral openings or 

 ostia ; 12, lung sac ; 13, ovary ; 14, 15, 16, 17, silk glands ; 18, spinnerets ; 

 19, distal joint of chelicera ; 20, poison gland; 21, eye; 22, pericardium; 23, 

 vessel bringing blood from lung sac to pericardium ; 24, artery. (From the 

 Cambridge Natural History.) 



spider stands in the center of the web or retires to a nest at one 

 side and waits for an insect to become entangled in the sticky 

 thread ; it then rushes out and spins threads about its prey until 

 all struggles cease. 



Spinning Organs. - - The spinning organs of spiders, called 

 spinnerets, are three pairs of projections near the posterior end 

 of the body on the ventral surface (Fig. 61, A). The spinnerets 

 are pierced by hundreds of microscopic tubes through which a 

 fluid passes from the silk glands (Fig. 60, 74, 75, 16, 77), which 



