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 

 spireicle 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. i8) 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 

 Fig. T8.--End of abdomen of IS spun (Fig. 19). Somc spidcrs have 

 ^^Sn^^^l^^^^ as many as one hundred and fifty or 



these is the spiracle, and be- . 1 j^r^-L. ^' ' ^u 



hind them the opening of the two hundred of thcsc spmning-tubcs 



alimentary canal. i . . /-r-i -n 



on each spmneret. ihe 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. 



Ordinarily the tips of the spinnerets are brought close 

 together, so that all the minute threads that emerge from 

 the numerous spinning tubes unite to form a single 

 thread. This, however, may be so delicate as to be 

 invisible, except in a favorable light. Sometimes 

 a spider will spread its spinnerets apart, and thus yig. 19. 

 spin a broad ribbon-like band. We have observed ^ ?*"°Vp ^^ 



^ spinning- 



a spider seize a large grasshopper which was en- ^^^^^{, 

 tangled in its web, and, rolling it over two or three enlarged, 

 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 



