168 



THE EYE IN EVOLUTION 



Dytiscus 



functions of the crystalline structure may be replaced by the cuticular 

 cornea which itself forms a cone-like invagination, as occurs in fire-flies 

 (Lampyrids) or the water-beetle, Dytiscus (the exocone eye), an 

 arrangement reminiscent of that seen in the king-crab (Fig. 143). 



In most diurnal insects the retinule with its tubular arrangement 

 of a group (usually 7 or 8) of elongated sensory cells arranged around 

 the central rhabdome lies immediately underneath the lens, resting 

 upon a fenestrated basement membrane through which pass nerve 

 fibres which run to the outermost nucleus of the optic lobe.^ Around 

 the bases of the retinular cells in close association with the basement 

 membrane are refractile trachea which increase the optical efficiency 



Fig. 153. — Image Formation in the Compound Eye. 



A, apposition eye. Only the rays of light falling normally (or practically so) 

 (a, b, c) reach the rhabdomes and retinular cells so that each ommatidium 

 functions as a unit. The ray from b deviating to the left is absorbed by the 

 pigment sheath, P. (Compare Fig. 156.) 



B, superposition eye. The main part of the diagram shows the pigment 

 in the dark-adapted position drawn up between the cones in which case the 

 superposition optical system is effective ; thus the rays from d and e can 

 traverse many ommatidia to become focused on one rhabdome, Rh. In the 

 two ommatidia on the right the pigment is in the light-adapted position so 

 that all rays except those entering normally (or nearly so) on the facet are 

 intercepted by the pigment, P. 



of the eye by reflecting the light back through the rhabdome. thus 

 serving the function of a ta/petum. As in Crustaceans, pigment is 

 usually a prominent feature. In most diurnal insects each ommatidium 

 is entirely ensheathed by pigmented cells arranged in two sections, the 

 iris 2^igment cells or primary iris cells lying distally surrounding the 

 crystalline cones, and the retired pigynent cells or secondary iris cells 

 lying proximally which encircle the retinule ; the ommatidium thus 

 act.s optically as an isolated unit. The iris cells contain not only 

 black absorbing pigment but also pale or coloured reflecting granules 

 witi) i tapetal function. 



1 p. 521. 



