530 PHILOSOPHY OF ZOOLOGY. 



above it. Their aspect varies with the habits of the spe- 

 cies *. 



* The following observations of the celebrated CUVIER on the anatomi- 

 cal structure of the eye, here deserve a place : 



" The structure of the eye of insects is so very different from that of 

 other animals, even the mollusca, that it would be difficult to believe it an 

 organ of sight, had not experiments, purposely made, demonstrated its use. 

 If we cut out, or cover with opake matter, the eyes of the dragon-fly, it will 

 strike against walls in its flight. If we cover the compound eyes of the wasp, 

 it ascends perpendicularly in the air, until it completely disappears ; if we 

 cover its simple eyes also, it will not attempt to fly, but will remain per- 

 fectly immoveable, 



" The surface of a compound eye, when viewed by the microscope, ex- 

 hibits an innumerable multitude of hexagonal facets, slightly convex, and se- 

 parated from one another by small furrows, which frequently contain fine 

 hairs, more or less long. 



" These facets form altogether a hard and elastic membrane, which, 

 when freed of the substances that adhere to it posteriorly, is very trans- 

 parent. 



" Each of these small surfaces may be considered either as a cornea, or 

 a crystalline, for it is convex externally, and concave internally, but thicker 

 in the middle than at the edges ; it is also the only transparent part in this 

 singular eye. 



*' Immediately behind this transparent membrane there is an opaque 

 substance, which varies greatly as to colour in the different species, and 

 which sometimes forms, even in the same eye, spots or bands of different 

 colours. Its consistence is the same as that of the pigment of the choroides ; 

 it entirely covers the posterior part of the transparent facets, without leaving 

 any aperture for the passage of light. 



Behind this pigment we find some very short, white filaments, in the 

 form of hexagonal prisms, situate close to each other, like the stones of a 

 pavement, and precisely equal in number to the facets of the cornea ; each 

 penetrates into the hollow part of one of these facets, and is only separated 

 from it by the pigment mentioned above. If these filaments are nervous, 

 as in my opinion they appear to be, we may consider each as the retina of 

 the surface, behind which it is placed ; but it will always remain to be ex- 

 plained, how the light can act on this retina, through a coat of opaque 

 pigment. 



" This 



