ARTHKOI'ODA 



[8 3 



of respiration. In the one there are two pairs of lung hooks, 

 opening by as many stigmata on the anterior ventral portion of 

 the abdomen. In the other there are two pairs of lung books 

 and two pairs of tracheae. 



One of the most characteristic structures of the spiders is the 

 spinning glands, which occur in large numbers in the posterior 

 part of the abdomen. They open to the outside at the tips of 

 little elevations, or papillae, just anterior to the anus. There are 

 from two to three pairs of these papillae, 

 or spinnerets, and at the tip of each is a 

 number of very minute pores through 

 which the viscid, fluid secretion of the 

 glands is forced ; it hardens on coming 

 in contact with the air into a silklike 

 thread of exquisite fineness. The size 

 of the thread can be regulated by the 

 number of spinning glands which it brings 

 into play, but the coarsest spider-thread 

 is finer than the finest thread of the silk- 

 worm's cocoon. These threads are used 

 by spiders for various purposes. With 

 them some construct the delicate webs 

 (Fig. 177) with which they entangle their 

 insect prey. Others build tubes for them- 

 selves out of these threads, in which they 

 live. The trapdoor spider builds a home 

 for itself in the ground or in a wall (Fig. 

 178), lines the chamber with this finest 

 silk, and makes a hinged door to cover the 

 opening, lining this in the same way. Most spiders lower them- 

 selves to the ground by secreting a thread as they fall, and mam- 

 build cocoons about their eggs, in which the young develop. 



The common garden spider, Epeira (Fig. 177), builds a very 

 effective snare for insects. It fastens a series of threads from 

 point to point across an open space, all the threads crossing 

 each other at the same point. Next it starts at the center and 

 lays a spiral thread over these radii for a considerable distance. 

 Then going to the margin it begins a second spiral with a viscid 

 thread running inward toward the center. This it fastens 



Fig. 178. Nest of a trapdoor 

 spider, showing the semicir- 

 cular lid raised. (Drawn 

 from specimen bythe author.) 



