4 THE INSECT WOKLD. 



between the lenses, the more distinct will be the image of objects at 

 a distance, and the less distinct that of objects near. With the 

 latter the luminous rays diverge considerably ; while those from 

 the former are more parallel. In the first case, in traversing 

 the pigment, they impinge obliquely on the crystalline, and 

 consequently confuse the vision; in the second, they fall more 

 perpendicularly on each facette. 



" Objects do not appear of the same size to each optic filament, 

 unless the eye is a perfect section of a sphere, and its convexity 

 concentric with that of the optic nerve. Whenever it is other- 

 wise, the image corresponds more or less imperfectly with the 

 size of the object, and is more or less incorrect. Hence it 

 follows, that elliptical or conical eyes, which one generally finds 

 among insects, are less perfect than those referred to above. 



"The differences which exist in the organisation of the eye 

 among insects, are explicable to a certain point, on the theory 

 which we are about to explain in a few words. Those species 

 which live in the same substances on which they feed, and those 

 which are parasitical, have small and flattened eyes ; those, on the 

 contrary, which have to seek their food, and which need to see 

 objects at a distance, have large or very convex eyes. For the 

 same reason the males, which have to seek their females, have 

 larger eyes than the latter. The position of the eyes depends 

 also on their size and shape ; those which are flat, and have 

 consequently a short field of vision, are placed close together, and 

 rather in front, than at the sides of the head, and often adjoin- 

 ing. Spherical and convex eyes, on the contrary, are placed on 

 the sides, and their axes are opposite. But the greater field of 

 vision which they are able to take in makes up for this position/* 



Almost all insects are provided with a pair of compound eyes, 

 which are placed on the sides of the head. The size and form of 

 these organs are very variable, as we shall presently see. They 

 are generally placed behind the antenna). 



We do not find simple eyes (ocelli or stemmata) in all the 

 orders of insects, although we frequently find them. They are 

 generally round, and more or less convex, black, and to the 

 number of three in the majority of cases. In this case they are 

 most frequently placed in a triangle behind, and at a greater or less 



