184 INTRODUCTION TO ZOOLOGY chap. 



into it a number of thin plates, usually 15 to 20 of them ; these 

 are the "leaves" of the lung-book. Each leaf, however, is 

 hollow and contains blood, and this is purified by the air 

 taken into the cavity through the stigma. The purified blood 

 is carried back to the heart by a special blood-vessel. 



Besides these lung-books, there are within the body, 

 "tracheae'' or air-tubes such as those found in insects (see 

 p. 210). These open by a single median aperture (spiracle) 

 behind the lung-books. (In Fig. 116 this aperture is con- 

 cealed in the darkly coloured patch.) 



Between the stigmata of the lung-books is seen a little 

 backwardly projecting process which contains the ovipositor 

 or egg-laying apparatus of the female. This process (epigyne) 

 is only present in the full-grown spider after its last moult. 



All through the summer, the spiders live on 

 the food caught by the lady spider in the snare 

 she weaves. This snare or web is constructed by her quite 

 alone, the male taking no part in it, though he will at times 

 carry off the food caught in it by his mate. Sometimes he 

 will weave a small, rather imperfect web of his own, but his 

 skill is far less than hers. 



Her web is frequently made afresh each day in the summer 

 during the most active weeks of spider life ; but old spiders 

 will often content themselves with merely mending up the old 

 web, if it is not too much damaged. 



When about to start a new web, the spider 

 struotion" of begins by carefully laying down the strong 

 a Web. The boundary threads, to which the rest of the web 

 boundary ^jn 1,^ attached. To make these— if she is 

 working in such a spot that she can run round 

 to the different points of attachment necessary for these 

 threads — she merely presses her spinnerets against the first 

 point, to fix a thread, and then walks off, drawing out more 

 and more silken line as she goes, holding it carefully with 

 one of her hind legs away from any object to which it might 

 stick ; when she has reached a point convenient for the 

 attachment of the other end of the line, she stops, pulls 

 the thread tight, and then fixes it by again rubbing the 

 spinnerets against this second point. Usually at each end 

 of the line the little divergent mass of threads mentioned 

 above can be seen, fixing the central thread to its supports. 



