PHOTORECEPTION 197 



gathering light from a narrow sector of the visual field and projecting 

 it as a point on the retinal field. Since each ommatidium sampled, as it 

 were, a mean intensity from a particular area, the point of light pro- 

 jected by all the ommatidia composed an erect mosaic image. 



Nocturnal and crepuscular insects characteristically possess eyes in 

 which the rhabdoms are situated at a considerable distance from the 

 crystalline cone. The enveloping pigment migrates back and forth in 

 their respective cells as the eye is light- or dark-adapted (Fig. 92). 



A B 



Fig. 92. Image formation in the apposition eye, A, and the superposition 

 eye, B. a-f, paths of light rays; P, pigment; IP, position of pigment 

 when eye is in the light adapted condition; Rh, rhabdom. (Redrawn 

 from Wigglesworth, 1939 after Kuhn.) 



When this eye is dark-adapted the iris pigment moves distally into a 

 position surrounding the cones. In this position each cone is isolated 

 from its fellows, but the rhabdoms are now no longer shielded. Under 

 these conditions light entering one ommatidium at an angle is not 

 confined to that ommatidium as in the apposition eye, but may travel 

 to neighbouring rhabdoms (Fig. 92). The image produced by a single 

 ommatidium is an erect image which becomes larger the farther away 

 from the crystalline cone it is projected. At the plane of the rhabdoms 

 it is so large that it overlaps many rhabdoms where it is superimposed 

 on the images similarly produced by the other lenses corresponding to 



