108 THE UNIVERSE. 



the least perfect, possesses fifty. The common fly has 8000, 

 and in certain butterflies as many as 25,000 have been 

 counted. Each of these organs, too, presents, in micro- 

 scopic proportions, the greater number of the structures 

 which enter into the composition of the globe of our eye. 

 Closely packed together, these eyes make up for their im- 

 mobility by their bulk, and this is so great that in some 

 flies the eyes almost cover the head, and even constitute a 

 fourth part of the weight of the body. 



53. Spider's Claw, seen with the microscope. 



This powerful optic apparatus exhibits some curious mod- 

 ifications which reveal the habits of insects. 



Those which seek their prey by night have their eyes 

 more deeply set, in order better to absorb the least lumi- 

 nous rays. In the flesh-eating insects they are larger. In 

 some aquatic species the head is furnished with several 

 pairs, some directed upward, others downward, in such a 

 way that, while swimming on the surface of the water, the 

 animal can see at the same time the fish which menaces 

 it from the depths, and the bird which is about to swoop 

 upon it from above. From the former it escapes by flight, 

 and from the latter by diving. 1 



1 We speak here of the little whirlwigs, elegant aquatic beetles of extreme 



