14.6 EYJiS Oh" (JiiUa'i'AUEA AND INSECTS. 



CHAPTER VII. 



THE OHGANS OF VISION IN INSECTS AND CETJSTACEA. 



I NOW pass on to the eyes of insects. In most species 

 of this group there are two distinct kinds : the large 

 compound eyes, which are situated one on each side 

 of the head ; and the ocelli, or small eyes, of which 

 there are generally three, arranged in a triangle, 

 between the other two. 



Speaking roughly, the ocelli of insects may be said 

 to see as our eyes do ; that is to say, the lens throws 

 on the retina an image, which is perceived by the fine 

 terminations of the optic nerve. One type of such an 

 eye in a young water-beetle (Dytiscus) is shown in 

 Fig. 84, p. 131. This illustrates the mode of develop- 

 ment of an ocellus, which has been already referred to 

 (ante, p. 131). 



The structure of fully formed ocelli is shown by 

 Fig. 99. In details, indeed, they present many dif- 

 ferences, and it is remarkable that in some species this 

 is the case even with those of the same individual ; for 

 instance, in those of one of our large spiders, Epeira 

 diadema (Fig. 99). ^ 



In this case the eye B would receive more light, 

 and the image, thei efure, would be brighter ; but, on 



