THE INTERNAL ANATOMY OF INSECTS 



143 



B 



of that ommatidkim reaches its rhabdom. The image formed in 

 such an eye is termed by Exner an apposed image; because it is formed 

 by apposed points of Hght, falling side by side and not overlapping. 

 Such an image is a, distinct one. 



Night-eyes. — In the night-eyes the envelope of pigment surround- 

 ing the transparent parts of each ommatidium is incomplete ; so that 

 rays of light entering several adjacent corneas can reach the same 

 retinula. In such an eye there will be an overlapping of the points of 

 light; the image thus formed is termed by Exner a superimposed 

 image. It is obvious that such an image is not as distinct as an ap- 

 posed image. It is also obvious that a limited amount of Hght will . 

 produce a greater impression in this type of eye than in one where a 

 considerable part of the light is absorbed by pigment. Night-eyes are 

 fitted to perceive objects and the movement of objects in a dim light, 

 but only the more general features of the object can be perceived by 

 them. 



Eyes with double function. — It is a remarkable fact that with 

 many insects and Crustacea the compound eyes function in a bright 

 light as day-eyes and in a dim light as night- 

 eyes. This is brought about by movements in 

 the pigment. If an insect having eyes of this 

 kind be kept in a light place for a time and then 

 killed while still in the Hght, its eyes will be found 

 to be day-eyes, that is eyes fitted to form apposed 

 images. But if another insect of the same 

 species be kept in a dark place for a time and 

 then killed while still in the dark, its eyes wih be 

 found to be night-eyes, that is,, eyes fitted to 

 form superimposed images. 



Figure 157 represents two preparations 

 showing the structure of the compound eyes of 

 a diving-beetle, studied by Exner. In one 

 (Fig. 157, A), each rhabdom is surrounded by an 

 envelope of pigment, so that it can receive only 

 Fig. 157.— Ommatidia ^^^ ^^^^^ passing through the crystalline-cone of 

 from eyes of Colym- the ommatidiimi of which this rhabdom is a part. 



hetes; A, day-eye /pj^jg -g ^^^ condition found in the individual 



condition; B, nignt- 



eye condition (From killed in the light, and illustrates well the struct- 



■'^'''^^^)- ure of a day-eye. In the other preparation (Fig. 



157, B), which is from an individual kiHed in the dark, it can be seen 



that the pigment has moved up between the cry stalHne -cones so that 



