170 



THE EYE IN EVOLUTION 



Butterfly 

 ( Vanessa) 



by a migration of pigment, thus effecting an adaptive process in species 

 which are active both by day and by night : in dim hght the pigment 

 becomes concentrated anteriorly so that the eye can function as a super- 

 position eye and make full use of all the available light ; while in 

 bright illumination it disperses and migrates posteriorly surrounding 

 each retinule with an opaque mantle intercepting all lateral rays 

 (Parker, 1932). Thus in the dark-adapted state examination of the 

 eyes of certain noctuid moths with an ophthalmoscopic mirror shows a 

 luminous red reflection from a group of ommatidia ; in the light - 

 adapted state there is a minute glow from one central ommatidium 

 only (Demoll, 1917 ; Horstmann, 1935). This pigmentary migration 

 in some butterflies and moths begins from half to one hour before 

 sunrise or sunset and the change occupies an interval varying from r. 

 few minutes to an hour (Merker, 1929-34 ; Collins, 1934). The 

 excised eye always adopts the light-adapted distribution of pigment. 

 The migratory response is abolished by narcosis nor does it occur in 

 butterflies when the insect is at rest and inactive (Demoll, 1909-11 ; 

 Day, 1941). Its mechanism is unknown ; a purely hormonal control is 

 improbable since individual ommatidia may respond to localized 

 illumination (Day, 1941) ; but whether the migration of pigment is 

 dependent upon nervous reflexes from the retinule or is initiated by 

 photochemical reactions within the pigmentary cells is controversial. 



Notonecta 



Pigmentary migration of a less dramatic kind occurs in certain purely 

 apposition eyes of diurnal species as a response to rapid changes in illumination. 

 These are associated chiefly with the pigment in the cells around the basement 

 meinbrane (butterflies — Demoll, 1909 ; the water-boatman, Notonecta — Bedau, 

 1911). In the latter the visual cells also elongate in the dark-adapted state 

 (Liidtke, 1951-53). 



The Optical System of the Coni2)ound Eye 



The optical system of the compound eye has always excited 

 considerable interest since it was first studied by Johannes Miiller 

 (1826) ; Fig. 156, taken from his classical work on^ this subject, 

 indicates characteristically his conception of the optical mechanism 

 whereby a point source of light excites only one (or two) ommatidium. 

 In his Mosaic Theory he showed that an image of considerable definition 

 would be formed by the juxtaposition of the many small luminous 

 stimuli received by the ommatidia, each of them the impression of the 

 corresponding projection in the visual field, each of them varying 

 acocjrding to the pattern of the incident light. Such an image, in 

 CO])- r?) distinction to that formed by the eye of Vertebrates, is erect, and 

 the -f can be easily simulated by allowing light to traverse a bundle 

 of Dj tubes and fall upon a plate of ground-glass, an arrangement 



