ELEMENTARY ENTOMOLOGY 



The surface of the compound eye is composed of numerous 

 hexagonal facets, each of which is the end of a single eye element 



called an ommatidium, which is prac- 

 tically a separate and distinct eye. 

 Each ommatidium is composed of the 

 various optical elements necessary for 

 vision, but it receives impressions only 

 in a straight line, which form only a 

 very small part of the total field of the 

 insect's vision. This is due to the fact 

 that each ommatidium is surrounded 

 by black pigmented cells, which ab- 

 sorb or reflect the light, as shown in 

 Fig. 46, so that only those rays which 

 come in a straight line impress the 

 retina. Thus the whole view formed 

 by the images from all the ommatidia 

 as they reach the optic nerve must be 

 like that of a mosaic. 



Insects are able to distinguish forms 

 at but relatively short distances, vary- 

 ing from two to five feet, and to 



nc 



FIG. 45. Portion of compound 



eye of fly (Calliphora vomitoria), 



radial section 



, cornea ; 2, iris pigment ; n, nerve 

 fibers ; nc, nerve cells ; r, retinal pig- 

 ment; t, trachea. (After Hickson, 

 from Folsom) 



see distinctly only near-by objects. 

 Large eyes, as those of the dragon- 

 fly, give a wide field of vision, 

 and numerous facets would give a 

 greater distinctness of vision. In- 

 sects' eyes are well adapted to 

 detect motion, as a moving object 

 affects the facets in succession, and 

 motion is thus observed without 

 moving the eyes. They are able 

 to distinguish colors and often 

 respond quite definitely to them, 

 but their color sense seems to 

 have a different range from that 

 of man, as ants are sensible to the 

 ultra-violet rays. 



FIG. 46. Illustrating mode of vision 

 in compound eye 



" The light enters through the cornea. 

 The rays which strike the sides of each 

 tube or cone are absorbed by the black 

 pigment which surrounds the tube. Ac- 

 cordingly those rays of light^ only which 

 pass through the crystalline cones directly 

 (or are reflected from their sides), such 

 as a-a', b-b' ', c-c 1 ', d-d' , e-e', will ever affect 

 the nerves at a', b' ', c* , d' ', e'" (After 

 Lubbock, from S. J. Hunter) 



