ANATOMY AND PHYSIOLOGY 



99 



tially what the compound eye does, so far as can be inferred from its 

 structure. 



Exner, removing the cones with the corneal cuticula (in Lampyris), 

 looked through them from behind with the aid of 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 por- 

 tion of the field of vision; this "superposition" 

 image being correspondingly bright an advan- 

 tage, probably, in the case of nocturnal insects. 



Large convex eyes indicate a wide field of 

 vision, while small numerous facets mean dis- 

 tinctness of vision, as Lubbock 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 Miiller'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 f IG ; ^s- Diagram of 



J J outer transparent portion of 



at a definite distance could be imaged. an ommatidium to illustrate 



rr\i f 'j f ii j e c i the transmission of an axial 



The limit of the perception of form by insects ray ^ and the repeated 

 is placed at about two meters for Lampyris, i.co reflection and absorption of 



an oblique ray (), which at 



meters for Lepidoptera, 68 centimeters for length emerges at c. p, iris 

 Diptera and 58 centimeters for Hymenoptera. pigment - 



It is generally agreed, however, that the compound eyes are specially 

 adapted to perceive movements of objects. The sensitiveness of insects 

 to even slight movements is a matter of common observation; -of ten, 

 however, these insects 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 vertebrates. 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 



