146 EYES OF CRUSTACEA AND INSECTS. 



CHAPTER VII. 



THE ORGANS OF VISION IN INSECTS AND CRUSTACEA. 



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, therefore, would be brighter; but, on 



