132 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



However, when the pigment surrounding the ommatidia 

 is drawn back from their anterior ends, light rays can pass 

 through the lateral walls of the ommatidia from the lenses 

 of adjoining ommatidia, and thus the reflected rays from a 

 single point in an object may reach and stimulate several 

 adjacent rhabdomes, forming a picture in a somewhat different 

 way from that by the strict mosaic method. This picture is 

 called a superposition image as contrasted with the apposition 

 image of the true mosaic vision. 



The focal distance of the lenses in the compound eyes is 

 usually about two yards, so that these eyes see objects best at 



that distance from the insect. 

 The sharpness or clearness of the 

 image formed depends, too, on 

 the number and size of the sepa- 

 rate ommatidia. The smaller 

 and the more numerous they 

 are the more perfect will be the 

 mosaic; that is, the more complete 

 and clear will be the picture seen. 

 The number of facets in the com- 

 pound eyes of insects varies from 

 three or four to twenty thousand 

 or more. 



The simple eyes, or ocelli, are 

 very different from the com- 



FIG. 54. Part of corneal 

 cuticle, showing facets, of the 

 compound eye of a horse-fly, 

 Therioplectes sp. (Greatly 

 magnified.) 



pound eyes in make-up. Each ocellus has but one lens, but 

 behind it is a varying number of sensitive or optic cells each 

 with anterior crystalline part and posterior retinal or percip- 

 ient part. But the very short focus of the lens, usually but a 

 few inches, and the primitive character of the structure of the 

 part behind the lens, limit the vision probably to little more 

 than a perception of shadows in imperfect outline. The 

 ocelli can only perceive objects very close to the insect, and 

 then with but little clearness. In fact the vision of insects, 

 either by means of compound or simple eyes, is at best imperfect 

 when compared with that of the vertebrate animals. Although 

 observation and experiment have shown that insects can dis- 



