298 INTRODUCTION TO ZOOLOGY CHAP. 



large eyes, a thorax of three segments bearing three pairs 

 of well-developed jointed legs, and, in older larvae, the rudi- 

 ments of two pairs of wings. The abdomen is broad and yet 



fairly long, ending in 

 five pointed processes of 

 which two are very 

 small and form a lateral 

 pair. These processes 

 form a valve which can 

 either close over the end 

 of the alimentary canal, 

 or can be widely opened 

 at will, and so are of use 

 in the anal respiration 

 which is peculiar to 

 Dragon - fly larvae (see 

 below). 



A pair of 



Respiration. , . , 



large spiracles 



is present in the larva 

 behind the head, between 

 the first and second seg- 

 ments of the thorax, and 

 FIG. 220. Full-grown Larva or Nymph of anot her pair is said to 

 Aeschna (uat. size). . , , , . r , ,, , . , 



exist behind these, which, 



(Seen from above.) 



however, cannot be seen 



without dissection. In spite of these spiracles, the larva 

 does not normally at any rate when young come to the 

 surface of the water to breathe, as it would do if the spiracles 

 were functional, but remains permanently below the water 

 if this is fairly fresh, and it is evident, therefore, that it must 

 depend on some other mode of respiration ; this seems to be 

 supplied by the modified "rectum," or last part of the alimentary 

 canal. The wall of this is ridged and folded, so that it is 

 capable of being greatly distended ; also it is supplied with an 

 enormous number of minute trachea! branches which are in 

 connection with the main tracheae of the body. By the 

 muscular expansion and contraction of the rectum, water is 

 sucked in, or expelled, at will ; and thus the necessary supply 

 of oxygen, dissolved in the water, is brought into contact 

 with the tracheae and diffuses into them. If the water is 



