160 A fly's eye. 



several thousand eyelets. In a house-fly, however, the number 

 probably does not reach one thousand. 



The outer surface of each eye is the cornea before spoken of. 

 It is, of course, merely the skin, specially modified into lenses. 

 These, shown in Figs, 4 and 5, /, are double-convex, and the 

 curves are of perfect symmetry. In many insects the lenses are 

 plano-convex, the flat surface being sometimes outside, as in the 

 Water-boatman, and in other cases inside, as in butterflies. In 

 outline they are hexagonal, but the hexagons are by no means 

 perfect, as may be seen in Fig. 3. Immediately beneath the 

 lenses are little structures called, from their shape, the cones^ Figs. 

 4 and 5, CO. These, according to my idea, are the modified 

 crystalline lenses of higher animals. 



In certain small Crustacea, a class nearly allied to the Insecta, 

 in the order Entomostraca, the eye is wholly within the body of 

 the animal. In a marine species that I have, it makes no show 

 externally, but the shell is so transparent as to be no obstacle to 

 vision. There is, therefore, no cornea to the eye, properly speak- 

 ing. But the cones^ so conspicuous in an insect's eye, are there : 

 and the cones^ one can see, are themselves lenses. This fact is 

 even plainer in a rather more highly organised Entomostracon 

 called Leptodora, where the eye is nearer the surface of the body 

 in a point projecting from the head, and yet the part corresponding 

 to the cornea is not facetted. Going a stage higher, to the 

 freshwater shrimp, Gammarus^ we find that the eyes come up close 

 to the skin ; that the cornea is facetted ; but that it is apparently 

 not made into lenses, its two surfaces being parallel. The co72cs 

 beneath it, identical with the cones in an insect's eye, are still 

 plainly lenses, for they are solid and highly refractive of light. If 

 a series of insects be examined, it will be found that some, 

 butterflies and moths for instance, have solid cones, while others, 

 such as flies, have them hollow. The solid cones refract light just 

 as in the shrimps, and they must therefore be lenses. I may also 

 say that in caterpillars, which have simple eyes, while the cornea 

 is itself lens-shaped, there is a globular crystalline lens immediately 

 beneath it. So the fact that the facets of the cornea of an 

 insect's eye are shaped like and act as lenses, is no argument 

 against there being other lenses immediately beneath them. The 



