Photoreception 



389 



some sort of biological timing mechanism which controls the pigment mi- 

 gration. The role of the sinus gland in controlling pigment migration is dis- 

 cussed in Chapter 22. 



Image Fonuation. The crystalline cone is the principal focusing device of 

 the compound eye, and it is not adjustable. Each ommatidium, therefore, 

 has a fixed focus. The crystalline cone is made up of concentric lamellae, the 

 more central layers of which have a higher refractive index than the more 

 peripheral ones. The cone is therefore a virtual lens cylinder. Figure 104 

 shows light passing through such lenses. If the length of the cylinder is 

 equal to its focal length (A) a beam of parallel light (rwpw) which enters 

 perpendicular to one end of the cylinder (edge ah) is focused on the other 

 end (edge cd), and emerges as a divergent beam (^n'p'm'). Other beams of 

 light Qq) entering at an angle to the axis of the cylinder emerge at an anole 



<^' ^P 



Fig. 104. Diagram of the path of light rays (A) through a lens cylinder of a length 

 equal to the focal length; (B) through a lens cylinder of a length equal to twice the focal 

 length. After Exner." 



Cq')- This permits image formation, but since this image is limited to only 

 seven retinula cells and is also subject to considerable distortion by the rhab- 

 dom, it is probably useful merely as a spot of light rather than as an image. 

 If the lens cylinder is twice as long as its focal length (B) the beam 

 emerges as a parallel beam whose angle of emergence (y3) is equal and op- 

 posite to the angle of incidence (a). In arthropod eyes there are two types 

 of image formation, superpositional and appositional, which correspond to 

 these two possible types of lenses. In both types the light entering a single 

 facet is probably detected as a single spot of light. 



There are two chief types of ommatidia as shown in Figure 105. In A, 

 the iris pigment completely sheathes the ommatidium and the only light 

 which strikes the rhabdom and retinula cells is that which enters almost 

 parallel to the axis. In B, the pigment cells do not completely sheathe the 

 ommatidium and, depending on the angle of incidence, light which passes 

 through the crystalline cone may strike neighboring rhabdoms as well as the 



