9 o 



ENTOMOLOGY 



a microscope and found that the images made by the separate ommatidia 

 were either very close together or else overlapped one another, and that 

 in the latter case the details corresponded; in other words, as many as 

 twenty or thirty ommatidia may co-operate to form an image of the same 

 portion of the field of vision; this "superposition" image being corres- 

 pondingly bright an advantage, probably, in the case of nocturnal insects. 

 Large convex eyes indicate a wide field of vision, while small numerous 

 facets mean distinctness of vision, as Lubbock has pointed out. The 



closer the object the better the sight, for the 

 greater will be the number of lenses employed to 

 produce the impression, as Mollock says. If 

 Mtiller's theory is true, an image may be formed 

 of an object at any reasonable distance, no power 

 of accommodation being necessary; while if, on 

 the other hand, each cornea with its crystalline 

 cones had to form an image after the manner of 

 an ordinary hand-lens, .only objects at a definite 

 distance could be imaged. 



The limit of the perception of form by insects 

 is placed at about two meters for Lampyris, 1.50 

 meters for Lepidoptera, 68 cm. for Diptera and 

 58 cm. for Hymenoptera. 



It is generally agreed, however, that the com- 

 pound eyes are specially adapted to perceive 

 movements of objects. The sensitiveness of in- 

 sects to even slight movements is a matter of 

 common observation; often, however, these in- 

 sects can be picked up with the fingers, if the 

 operation is performed slowly until the insect is 

 within the grasp. A moving object affects different facets in succession, 

 without necessitating any turning of the eyes or the head, as in verte- 

 brates. Furthermore, on the same principle, the compound eyes are 

 serviceable for the perception of form when the insect itself is moving 

 rapidly. 



The arrangement of the pigment depends adaptively upon the quality 

 of the light, as Stefanowska and Exner have shown; thus, when the light 

 is too strong, the iris and retinal pigment cells elongate around the om- 

 matidium and their pigment granules absorb from the cone cells and rhab- 

 dom the excess of light. If the light is weak, they shorten, and absorb 

 but a minimum amount of light. 



FIG. 143. Diagram of 

 outer transparent portion 

 of an ommatidium to illus- 

 trate the transmission of an 

 axial ray 0*4) and the re- 

 peated reflection and ab- 

 sorption of an oblique ray 

 (B), which at length emerges 

 at C. p, iris pigment. 



