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Comparative Animal Physiology 



rhabdom of the crystaline cone through which the hght has passed. There 

 fore, there are two possible types of image formation, as shown in Figure 

 106. In A is shown the so-called apposition image, in which each rhabdom 

 receives light which enters the ommatidium parallel to its axis. In this type 

 the length of the crystalline cone may be equal to its focal length, as in 

 Figure 105, A. In B is shown an image formed by superposition, in which 

 light from a point (e) may pass through any of a number of crystalline 

 cones and be bent (as in Figure 105, B), so that it is focused on a single 



Fig. 105. Diagram of the two types of ommatidium. A, From eye forming apposition 

 image (after Snodgrass); B, From eye forming superposition image (after Weber), a, 

 Corneal lens; h, matrix cells of cornea; c, crystalline cone; d, iris pigment cells; e, rhabdom; 

 f, retinal cells; g, retinal pigment cells; h, fenestrated basement membrane; i, eccentric 

 retinal cell; k, translucent filament connecting crystalline cone with rhabdom; I, nerve 

 fibers. From Wiggles worth ."^ 



rhabdom. In this type the length of the crystalline cone is twice the focal 

 length. If the secondary iris pigment should migrate, as shown in Figure 

 105, B, then the superposition eye may function as an apposition eye. 



In general, most diurnal insects have apposition eyes, and nocturnal ones 

 have superposition eyes. Undoubtedly there are many intermediate types. In 

 some insects the cone cells are not refractile and apparently do not serve as 

 lenses. The apposition eye is adapted to function at high light intensity and 

 is really very inefficient as a light-gathering device. The superposition eye is 

 adapted to low light intensities and is much more efficient for gathering light 

 than the apposition eye, but is still much less efficient than most vertebrate 

 eyes. 



Because of the fact that the image formed by the apposition eye is made up 

 only of spots of light of different intensities, each of which has entered 



