Chap. 30 



ARTHROPODS INSECTS, SPIDERS, AND ALLIES 



625 



Pericardia] 

 cavity 



Intestine with Mafpighian 



complexly brancnea tuou/es 



diverticula /tteart 



Stercoral 

 poc/<et 



Poison gland Q^^gjl^^ 



Pedipalpus 



, Silk' , , , . 

 7- h/r, g'^'^ds// Anus 

 Chelicera'^^ / L\:^^!7 Spinnerets 



Book lung \ 



Stvmpsoflegs Seminal oVi'dLlt 

 receptacle 



Fig. 30.31. Internal anatomy of a female spider. The nervous system highly 

 developed in the head and thorax, is shown by dark stippling; the nerves in the 

 abdomen are too small to be shown. The alimentary canal is white; note its 

 branches (caeca) extending into the stumps of the legs; a network open into the 

 intestine from a digestive gland which is packed around the abdominal organs. 

 Note the prominent eggs in the ovary. The stercoral pocket, a sac in which waste 

 products accumulate. The malpighian tubules are kidney-like in function as in 

 insects. (From Comstock: The Spider Book. New York, Doubleday, Page and 

 Co., 1913.) 



then leaps spinning the line out as it goes through the air. Spiders spin forth 

 yards of draglines that are carried by currents of air from tree to tree and 

 across streams. Young spiders and the smaller species are lifted into the air 

 and carried by draglines for miles over mountains and seas. The dragline is 

 also the trapline which a spider holds until it vibrates from the touch of an 

 insect caught in the web. Draglines are the outermost threads of orb webs, the 

 fundamental lines in their construction (Fig. 30.32). There are seven kinds of 

 silk glands in spiders but not all of these are present, even in any one family 

 of spiders. The silk that is poured out through the minute holes in the tips of 

 the spinnerets is of different sorts that are more or less elastic, but its final 

 character depends largely upon the pull to which it is subjected. The viscid 

 spiral lines of orb webs are two firm threads which are at first evenly covered 

 with a fluid silk. As she spins, the spider holds the whole thread with her hind 

 leg, stretching it a little but at regular intervals letting it snap back. On the 

 shortened line drops of the sticky silk form at regular intervals. Dew gathered 

 on them creates the shining beads of early morning (Fig. 30.33). An orb web 

 is a triumph of symmetry and it takes a spider only an hour to build it. 



Spiders always develop from the fusion of male and female sex cells but in 

 most species the male individual is of no consequence except for the fertiliza- 

 tion of the eggs. The females spin the egg sacs and give the young what care 

 they receive. Male spiders have silk glands but spin little or none. They hunt 



