THE BOOK OF NATURE STUDY 



FIG. 25. Under side of abdomen of spider. 

 A, genital flap ; B, opening of lung-book. 



fused together. On each side of this is a narrow slit (in some 

 species two slits are present on each side), the opening to the 

 respiratory organ, or " lung-book." The lung-books are chambers 



whose cavities are occupied by 

 numerous thin plates somewhat 

 resembling the pages of a book, 

 but hollow ; blood circulates in 

 the hollows of the plates, while 

 air passes in and out between 

 them and thus supplies the 

 oxygen, which is carried away 

 to all parts of the body. In 

 some species, however, there 

 are in addition tracheal tubes 

 resembling those of insects. 



At the apex of the abdomen 

 is situated, on a small pimple, the posterior opening of the diges- 

 tive organs. Just in front of this aperture is a group of six 

 nipple-like projections the spinners from which the silk issues. 

 In those spiders which possess the calamistrum already men- 

 tioned there is a short, broad and partly 

 divided extra spinner just in front of the 

 cluster of six. The size, length, number 

 of joints and direction of the spinners is 

 subject to much variation. At or near 

 their extremities the spinners are fur- 

 nished with numerous fine tubes, through 

 which the semi-liquid silk is discharged in 



delicate threads. It is noteworthy that FIG. 26. Apex of abdomen of 

 whereas those insects which produce silk 

 discharge it from openings near the 

 mouth, i.e. at the anterior end of the body, it is at the posterior 

 extremity that spiders secrete it. 



In Epeira diademata there are no less than five different sorts 

 of silk produced for the manufacture of the snare strands, 

 capture of prey and the custody of eggs, for the different grades 

 of threads are not all for the same purpose. The radiating spokes 

 of the wheel-web form a framework on which the sticky circular 



garden spider, showing the 

 spinnerets. 



