FIELD ZOOLOGY. 



I •'•-.. 



in some insects numbering as high as thirty thousand. 

 Behind each facet is an eye element which is supposed to 

 enter the nerve tract by a separate nerve fiber. This eye 

 element includes a cone of crystalline clearness, with the 

 tapering end enveloped more or less completely by pigment. 

 Behind each cone are retinal cells, ganglionated nerve 



cells, and nerve fibers, all finally 

 passing into the optic tract, or 

 the tract of the optic nerve. 



Each of these eye elements 

 is supposed to perceive only that 

 part of the object which is 

 directly in front of its surface. 

 Since the nerve fibers pass 

 . separately from each eye ele- 

 ment into the optic nerve, it is 

 possible that the sensations are 

 received separately by the optic 

 lobes of the brain. Whether 

 they are put together by the 

 interpreting head ganglion to 

 form an image as in the human' 

 eye, is very doubtful. If the 

 impressions are interpreted 

 separately, the insect has sight 

 in no such sense as has the human being; but the visual 

 impression must be a composite of the image, made up of 

 as many parts as there are corneal facets on the side of 

 the head next to the object. Such sight is called mosaic 

 sight. It would follow, then, that insects whose eyes pro- 

 ject much beyond the head contour, which possess the 

 largest number of facets, and which, in addition, can turn 

 the head through the greatest possible angle, must have the 



Fig. 2. — Portion of compound 

 eye of fiy, Calliphora vomitoria, 

 radial section, c, cornea; i, iris 

 pigment; n, nerve fibers; nc, 

 nerve cells; r, retinal pigment; 

 (, trachea. {Folsorfi, after Hick- 

 son.) 



