THE COMPOUND EYE 



161 



Figs. 140 and 141. — The Composite Compound Eye, 



Fig. 140. — Frontal section of the eye Fig. 141. — The faceted eye of the 



of the male April fly, Bibio marci. Schizopod, Stylocheiroyi mastigo- 



F, frontal eye ; L, lateral eye (after phorum. 



Hesse). F, frontal eye ; L, lateral eye ; 



c, corneal lens ; k, crystalline cone ; 

 r, rhabdomes. The luminous organ is 

 not shown (after Hesse). 



each around a rhabdome, the whole collection lying underneath a 

 common lens (Fig. 142). The large lateral eyes of the king-crab, 

 however, are unique and merit a special description. 



The compound (lateral) eyes of the king-crab, Limulus, are of a 

 relatively simple but unique structure, but are of unusual interest since 

 they have been widely used by Hartline and his collaborators as a 

 means of studying the electrical activity of photoreceptor cells ; their 

 choice was determined by the fact that one fibre only of the optic 

 nerve apparently acts as a conductor on stimulation of an ommatidium. 

 A considerable amoimt of work has been done on the minute structure 

 of this eye, but some points in the anatomy, particularly of its nervous 

 connections, still remain obscure ^ (Fig. 143). 



Although the ej'e show s wide differences in size and complexity of structure 

 with growth and between species (Waterman, 1954), as a rule it consists of some 

 600 ominatidia, the whole being covered with a continuous corneal stratum of 

 transparent chitin ; on its inner surface this presents a series of papilliform 

 downgrowths which act as corneal lenses to the barrel-shaped retinules which 



1 See Lankester and Bourne (1883), Watase (1890), Miller (1952), Waterman and 

 Wiersma (1954). 



S.O.— VOL. I. 11 



Limulus 



