22 



THE STUDY OF INSECTS. 



only one, two, or three pairs ; and certain cave spiders 

 are blind. Spiders breathe by means of lung-sacs, of which 

 there are one or two pairs ; and some have tracheae also. 

 The lung-sacs open on the lower side of the abdomen 

 near its base, and between them is the opening of the 

 reproductive organs. The tracheae open through a single 

 spiracle near the hind end of the body, just in front of the 

 spinning organs. 



The spinning organs, which are situated near the end of 

 the abdomen, consist of two or three pairs of spinnerets. 



These appendages (Fig. 18) are more 

 or less finger-like in form, and some- 

 times jointed. Upon the end of each 

 spinneret there are many small tubes, 

 the spinning tubes, from which the silk 

 is spun (Fig. 19). Some spiders have 

 as many as one hundred and fifty or 

 two hundred of these spinning-tubes 

 on each spinneret. The silk is in a 

 fluid state while it is within the body, but it hardens as 

 soon as it comes in contact with the air. 



In addition to the many small spinning tubes, there are 

 a few larger ones, termed spigots. The ordinary thread is 

 spun from two or four of these. The small spinning 

 tubes are used in making attachment disks for fast- 

 ening threads in place, in making a swathing band 

 for enveloping prey, and, sometimes, in making a fig. 19. 

 broad, wavy band across the center of a web. We A s ^ r n °^°! 

 have observed a spider seize a large grasshopper ^auy 

 which was entangled in its web, and, rolling it over enlar & ed - 

 two or three times, completely envelop it in a sheet of silk 

 spun from its spread-apart spinnerets. 



In the construction of their web some spiders make 

 use of two kinds of silk. One of these is dry and inelastic ; 

 the other, viscid and elastic. This fact can be easily seen 

 by examining an orb-web. If the spiral line which forms 



Fie. 18. — End of abdomen of 

 spider, showing six spinner- 

 ets spread apart: in front of 

 these is the spiracle, and be- 

 hind ihem the open.ng of the 

 alimentary canal. 



