172 



THE EYE IN EVOLUTION 



Musca 



Dragonfly 



Necrophorus 



Apis 



In the compound eye of the winged male of Lam2Jyris there are 

 2,500 ommatidia ; but the number of elements varies considerably 

 between different species depending largely on their habits. Thus, in 

 Solenopsis, the worker-ants which live underground have 6 or 9, 

 while the winged males which pursue the female in tlie air are provided 

 with 400 ; in genera with a high 

 visual acuity the numbers are much 

 higher — in the house-fly, Musca, 

 4,000 ; in the water-beetle, Dytiscus, 

 9,000 ; and in dragonflies (Odonata) 

 up to 28,000 (Demoll, 1917 ; Imms, 

 1935), or the burying beetle, Necro- 

 phorus, 29,300 (Leinemann, 1904). 

 The size of the individual facets re- 

 mains fairly constant (15 to 40/x) ; the 

 size of the eye is determined essenti- 

 ally by their number. 



From the functional point of 

 view, however, the most important 

 feature is the ommatidial angle. 

 that is, the angular extent of the 

 visual field covered by each element. 

 It is obvious that if a pattern is to be 

 resolved, two adjacent ommatidia 

 must be unequally stimulated so that 

 their angular separation must form 

 the anatomical basis of the visual 

 acuity, corresponding in man to the 

 inter-cone distance and determining 

 the fineness of the " grain " of the 

 resulting picture (del Portillo, 1936). 

 As this angle becomes smaller, the 

 resolving power increases, but less 

 light will enter each facet. Thus the 

 angle in the bee. Apis, varies from 



0-9° to 1° in the centre of the eye, and in the earwig, Forficula, is 8°, 

 so that the latter will obtain a single point of light as the image of an 

 object which the eye of the bee will resolve into 64 (Baumgartner, 1928 ; 

 V. Buddenbrock, 1937). In the locust, Locusta, the ommatidial angle 

 is about 21° (Burtt and Catton, 1954). In the periphery of the eye the 

 ommatidial angle is larger than in the centre and the acuity corres- 

 pondingly less; in the anterior region of the eye it is often smaller than 

 in i ' ventral, an arrangement which favours visual acuity in flight 

 (Aut. jiu, 1949) (Fig. 158). 



Fig. 157. — Exner's Classical Photo- 

 graph THROUGH THE OPTICAL SYS- 

 TEM OF THE Compound Eye of 

 Lamp mis splesdidula. 



Showing a window with a letter R 

 on one pane and a church beyond (from 

 Wigglesworth's Principles of Insect 

 Physiologu, Methuen). 



