A FLYS EYE. 



159 



fly it is difficult to recognise it as a lens at all. By an examination, 

 however, of the corresponding organs in animals nearly allied to 

 insects, I think that its identity with the crystalline lens of higher 

 animals may be recognised. 



It will be convenient, before going further, to describe an 

 insect's eye more particularly, taking that of the common blue- 

 bottle, or blow-fly, for our example. A blow^-fly has two big eyes 

 and three little ones. Figure i is a rough outline of a blow-fly's 

 head, showing this. The figure also attempts to explain the place 

 where and the direction in which the head is cut into slices, to 

 study the structure of the eye. Fig. 2 is one of such slices from 



tncTo 



Small portion of the cornea of a fly's eye, showing the lenses. 

 Magnified 400 diameters. 



the place marked in Fig. i, and Fig. 4 is a slice or section of a 

 single big eye more highly magnified. This last figure shows how 

 each of the big eyes is "compound" — i.e., made up of a large 

 number of tiny eyelets. I reckon their number at between 1,000 

 and 1^500 in each eye. Very exaggerated estimates are often 

 given, yet in the eye of a large dragon-fly there must indeed be 



