ANATOMY AND PHYSIOLOGY 



" superposition " image being correspondingly bright an ad- 

 vantage, 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 impres- 

 sion, 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 ne- 

 cessary; while if, on the other hand, 

 each cornea with its crystalline cone's 

 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 Lepi- 

 doptera, 68 cm. for Diptera and 58 cm. 

 for Hymenoptera. 



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 ob- 

 servation ; often, 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 

 9 



Diagram of outer, trans- 

 parent portion of an omma- 

 tidium to illustrate the 

 transmission of an axial ray 

 (A) and the repeated reflec- 

 tion and absorption of an 

 oblique ray (B), which at 

 length emerges at C. p, iris 

 pigment. 



