THE COMPOUND EYE 169 



While this is the most common form of compound eye wherein each 

 ommatidium is designed to act by itself with the result that the optical 

 image resembles a finely grained mosaic (the apjposition eye)} many 

 nocturnal insects show a dramatic contrast wherein light is utilized 

 more effectively by an arrangement which allows incident rays from 

 several facets to reach one rhabdome (the sujierposition eye). The 

 typical structure of this type of eye is seen in beetles and noctuid 

 moths (Fig. 152). In these the retinule is situated far back and the 

 interval between it and the crystalline cone is traversed by a non- 

 refractile translucent filament connecting this structure with the 

 rhabdome. while the pigmented iris cells are concentrated distally 

 between the crystalline cones leaving the retinules without an insulating 

 sheath. 



Figs. 154 ant> 1.5.5. — Superposition Images formed by the Refractive 

 System of LAMpyjtis. 



Fig. 154. — The mosaic of images Fig. 155. — The superimjiosed images 



formed at a level immediately be- at the level of the rhabdome (after 



neath the optical system. Exner). 



The functional contrast between the two types is seen in Fig. 153. 

 In Figs. 154 and 155 are seen the illustrations from Exner's (1891) 

 classical treatise showing the image of a candle flame formed by the 

 corneal facets and cones of the fire-fly, Lampyris. When the microscope 

 is focused just below the dioptric apparatus a multitude of luminous 

 spots is seen all of which become merged into one at the level of the 

 rhabdomes. The light from as many as 30 different facets may thus be 

 concentrated on one of these structures. 



Intermediate forms between these two types of compound eye 

 exist ; nor are they mutually exclusive. Thus in Mantids the two are 

 seen combined in the same eye ; the anterior ommatidia which are 

 used for binocular vision are of the apposition t^'pe while the lateral 

 parts are of the superposition tyjDe (Friza, 1928) — a functionally 

 efficient arrangement. Moreover, as in Crustaceans, the change from a 

 superjDosition eye of the nocturnal tyjje to an apposition eye of the 

 diurnal type with its high degree of resolution can be made functionally 



1 p. 173. 



Fire-fly 



