22 



ENTOMOLOGY. 



sects depends, of course, largely on the development of the 

 organs of special sense, especially those of sight and smell. 



There are in nearly all insects two kinds of eyes, the simple 

 and the compound. Of the simple eyes there are usually 

 three, arranged, as in locusts, bees, etc., in a triangle on the 

 top of the head. There is a single pair of compound eyes. 

 The simple eye, or ocellus, consists of a single smooth, shin- 

 ing, convex area, called the cornea or facet, while in the 

 compound eyes there are many facets, which can be seen 



k f 



Fig. 18.— Longitudinal section of the faceted eye of a moth. /, the rod-like 

 ending; of the optic nerve-fibres; fc. crystalline lens: sn, optic nerve; tt\ 

 trachea lost in fine fibrillar : /. rh, retina. — After Leydig, from Graber. 



with a hand-lens. The compound eyes, which are usually 

 round and very prominent, differ much in size and the 

 number of facets, the latter varying from fifty, as in the ant, 

 to several, even twenty, thousand, as in certain beetles. 



The structure even of the simple eye is too complicated 

 for description here, but the essential parts are: the cornea, 



