198 THE PHYSIOLOGY OF INSECT SENSES 



those rhabdoms. Microscopic examination of the image at the level 

 of the rhabdom reveals a large, somewhat fuzzy, erect image. As the 

 microscope is brought to focus closer and closer to the crystalHne 

 cones, the rays of light forming the image are seen to retreat into each 

 cone. The image at the level of the rhabdoms is a compound of super- 

 imposed images; hence, this type of eye is called a superposition eye. 

 When the pigment in the superposition eye migrates proximally so that 

 the rhabdoms are shielded from one another the eye functions Hke an 

 apposition eye. It is noteworthy in this connexion that Antrum (1961) 

 in measuring the visual acuity of a white mutant of CaUiphora in which 



Fig. 93. Concentric lamellae 

 of different refractive 

 index in the dioptric 

 apparatus of the om- 

 matidium. The refrac- 

 tive index is greatest 

 along the axis xy. (Re- 

 drawn from Exner, 

 1891.) 



Proximal Surface 



there was no screening pigment in the eye found no difference between 

 the mutant and the wild type. Since the superposition is not total, that 

 is, not all the images are congruent, it could be argued that the differ- 

 ence between the mosaic image and the superposition image is one of 

 degree. 



In order to explain image formation in the two types of eyes Exner 

 (1891) proposed that the crystalline cone acted as a lens cyHnder. If it 

 were constructed of concentric lamellae of different refractive indices 

 (Fig. 93), the refractive index would be greatest along the longitudinal 

 axis and would decrease towards the periphery. The path of a ray of 



