THE COMPOUND EYE 



167 



at the end of the seventeenth century. Each individual ommatidium 

 has a relatively simple structure similar to that already described in 

 Crustaceans. Most externally is the focusing apparatus, made up 

 from without inwards of a cuticular lens-like formation (the corneal 

 lens or facet) under which lies the crystalline cone surrounded by 

 nucleated hypodermal cells which do not form a complete layer as in 

 the typical crustacean eye (Fig. 151). 



Such a dioptric apparatus forms the typical arrangement (the 

 EUCONE eye) ; but variations occur in which the entire refractive 



Colorado beetle 

 (Coleoptera) 



Figs. 151 and 152.^Schematic Structure of the Two Types of 

 Ommatidia of Insects. 



1/ 



/- 



^\) 



I 



Fig. 151. — The apposition eye, with 

 (alongside and below) a section 

 through the retinule. 



Fig. 152. — The superposition eye in 

 the dark-adapted condition with the 

 pigment in the iris cells almost 

 entirely withdrawn into their upper 

 extremities. 



a, corneal facet ; b, corneal cells ; c, crystalline cone ; d, iris piginent 

 cells ; e, rhabdome ; /, sensory cells of the retinule ; g, retinal pigment cells ; 

 h, fenestrated basement membrane ; i, eccentric retinal cell ; k, filament 

 connecting crystalline cone with rhabdome ; I, nerve fibre (after Weber and 

 Snodgrass). 



function is taken over by the cornea. In place of a separate crystalline 

 cone secreted by special crystalline cells (vitrellce), these cells may 

 merely secrete an accumulation of fluid (the pseudocone eye), as 

 occurs in Muscids. In other types, such as beetles (Coleoptera), some 

 bugs (Hemiptera) and crane-flies (Tipulids), the cones remain cellular 

 and non-refringent (the acone eye). Alternatively, the refractive 



(1921). Cornell (1924), Kuhn (1926), Gotze (1927), Bott (1928), Friederichs (1931), 

 Nowikoff (1931), Werringloer (1932), Weber (1934), Llidtke (1935-51), Wundrig (1936), 

 Vidal and Courtis (1937), Zankert (1939), Verrier (1940), Lhoste (1941), Roonwal (1947), 

 Ehnbom (1948), Tuurala (1954), Fernandez-Moran (1956). 



Crane-fly 

 (Tipulidse) 



