160 



THE EYE IN EVOLUTION 



Scutigera 



Phronirna 



Dineutus 



Stylocheiron 



Cut 



Chilopods and Diplopods), in the centipede, Saifigem, and its close 

 allies ; in Arachnids an atypical form occurs in the lateral eyes of the 

 king-crab, but their full development is characteristic of Crustaceans 

 and Insects, in which they are found in the most varied forms. Of 

 these, the most elaborate is the composite compound eye wherein 

 the organ is formed by the apparent fusion of two compound eyes, 

 usually a frontal and a lateral. 

 Among Crustaceans this is seen in 

 pelagic Schizopods (Hesse, 1908), or 

 in some Amphipods such as Phronirna 

 sedentaria (Claus, 1879). Among 

 Insects a frontal and lateral combina- 

 tion is seen in some flies (Diptera) 

 and mayflies (Ephemeroptera), a 

 dorsal and ventral in wasps 

 (Vespoidea) and longhorn beetles 

 (Cerambycidse) (Fig. 140). Such an 

 arrangement undoubtedly increases 

 the visual field and may also serve as 

 an accommodative device providing 

 two focusing mechanisms, one anato- 

 mically adjusted for distant and the 

 other for near vision (Dietrich, 1909 ; 

 Weber, 1934). A further example is 

 the dorsal and ventral eyes of the 

 whirligig beetle, Dineutus, the former 

 for aerial vision and the latter for 

 vision under water (Fig. 231). 



A final complication is seen in some 

 abyssal Crustaceans wherein a frontal 

 portion of the compound eye contains few 

 ommatidia provided with little pigment, 

 obviously adapted for dim light, a lateral 

 portion has many small ommatidia each of 

 which is ensheathed in pigment so as to be 

 effective in brighter light, while immediately 

 below this a third part is adapted as a 

 luminous organ ^ {Stylocheiron mastigo- 

 phorum— Chun, 1896) (Fig. 141). 



Fig. 139. — The Eye of the 

 CoPEPOD, CopiLiA (foreshort- 

 ened). 



Cut, cuticle ; L, lens ; N, nerve 

 fibre to epidermis ; C, crystalline 

 cone ; O, optic nerve ; R, rhab- 

 dome with surrounding sensory 

 cells, encased in a pigment mantle ; 

 M, muscle ; A, antennae (after 

 Grenacher). 



THE COMPOUND EYES OF ARACHNIDS 



In general Arachnids are provided with ocelli, but in a few cases — 

 the scorpion, the median eyes of the whip-scorpion and of the king-crab — 

 the eye is of the type wherein the visual cells are arranged in groups, 



1 p. 736. 



