224 



THE EYE IN EVOLUTION 



simple pair of visual cells with two overlying pigment cells which form 

 the eye of the larva of the midge, Ceratojjogon, or the few light-sensitive 

 cells lying in a shaded pocket in the pharyngeal skeleton of the larva of 

 the house-fly, Musca (Fig. 99) (Welsh, 1937 ; Debaisieux, 1939). 



THE DORSAL OCELLI OF ADULTS were described and figured as early 

 as 1678 by the French scientist, de la Hire (Figs. 149, 227, 230). They 

 are usually three in number arranged in triangular form, one median 

 and anterior and two lateral and posterior on the dorsal aspect of the 



Figs. 230 and 23 L — Unusual Compound Eyes in Insects (Natural 

 History Museum, London). 



Fig. 230. — The aphid, Dnctynotus 

 obscuras (Homoptera). 



There are 2 compound eyes (C) one 

 on each side of the head, and, in 

 winged forms, 3 ocelh on tlie vertex 

 of the head, the median one of which 

 is marked S. In the family Aphididae 

 there is in addition a prominence, the 

 triommatidion (T), of unknown func- 

 tion, bearing 3 facets at the base of 

 each compound eye. This organ is 

 always present, even in those forms in 

 which a comjjound eye is lacking. 



Fig. 231.— The Whirligig beetle, 

 Dineutus grossus (Coleoptera). 



There are 2 compound eyes on each 

 side of the head, one dorsal (D) for 

 aerial vision, and one ventral (V) for 

 vision under water. 



head between the compound eyes ; but they are small and incon- 

 spicuous, being often hidden by scales as in moths or hairs as in bees. 

 In some species of ants belonging to the sub-family Myrmicinae, the 

 anterior ocelli are double or binary in type (Weber, 1947). In others, 

 such as Orthoptera, the ocelli are vestigial ; in general, their degree of 

 development shows some correlation with that of the wings (Kalmus, 

 1945). As a rule they resemble in structure the more simple type of 

 stemmata, being comjirised merely of a group of visual cells associated 

 with rb-'bdomes lying beneath a common cuticular lens (Fig. 108). 



T OMPOUND EYES OF ADULTS are laterally situated on the head 

 and foi be essential visual organ (Fig. 149). They are large and 



