SPECIAL SENSES OF INSECTS. 



most nearly complete mosaic of the object at which they 

 look, that is, come nearest to seeing as we ourselves see. 



The simple eye appears externally as a convex bead- 

 like object, and as such is a transparent area of the head 

 cuticle, thickened so as to protrude from the surface of 

 the head. This makes a 

 len^ convex on the outer 

 side and indicating that, 

 by means of the simple 

 eyes, such sight as the 

 insect has must be near- 

 sight. Through this lens 

 the light rays pass to a 

 layer of specialized skin 

 cells with pigment pres- 

 ent . Directly back of the 

 lens there is also a more 

 or less developed vitreous 

 body, its development 

 var5dng with the order of 

 insects. Fibers from the 

 cells of the pigment layer 

 merge into the tract of 

 an optic nerve branch 



passing to the cephalic (Polsom, after Redikorzew.) 



ganglion. 



The eye is a part of the nervous system, and the 

 nervous system arises as a differentiation of the surface 

 cells of the animal of primitive type. For instance, in the 

 sponge, where the body is Jike a sack, the body consists 

 of three layers, two of them in contact with the water in 

 which the sponge lives. The outer one of these layers, 

 the ectoderm, discharges the functions of our tactile 



Fig. 3. — Median ocellus of honey-bee, 

 Apis mellifera, in sagittal section, h, 

 hypodermis; I, lens; n, nerve; p, iris pig- 

 ment; r, retinal cells; v, vitreous body. 



