4 THE INSECT WORLD. 
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 let 
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 adjoiing. 
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 
thesé organs are very variable, as we shall presently see. They 
are generally placed behind the antenne. 
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 
