4 THE INSECT WORLtX. 



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 adjoining. 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 antennae. 



Although simple eyes (ocelU or stemmata) are common, they do 

 not exist in all the orders of insects. They are generally round, and 

 more or less convex and black, and there are three in the majority of 

 cases. When there is this number they are most frequently placed in 

 a triangle behind, and at a greater or less distance from the antennae. 

 Under the cornea, which varies in convexity, is found a transparent, 

 rather hard, and nearly globular body, which is the true crystalline 

 resting on a mass, which represents the vitreous body. This vitreous 

 body is enclosed in an expansion of the optic nerve. Besides these, 

 there is a pigment, most frequently red-bro^vn, sometimes black, 

 or blood-red. The organisation of these eyes is analogous to the 

 eyes of fishes, and their refractive power is very great. 



With these eyes insects can only see such objects as are at a short 

 distance. Of what use then can stemmata be to insects also provided 

 with compound eyes ? It has been remarked that most insects having 

 this arrangement of eyes feed on the pollen of plants, and it has been 

 surmised that the stemmata enable them to distinguish the parts of 

 the flowers. 



The antennae, commonly called horns, are two flexible appendages, 

 of very variable form, which are joined to different parts of the head, 

 and are iUways two in number. The joints of which they are madt 



