586 



THE EYE IN EVOLUTION 



Calliphora 



in others the human Hght-adapted state (Pieris) (SchHeper, 1927-28 ; 

 Use, 1932). The electroretinogram obtained on stimulating the 

 retina with different wave-lengths also shows a curve resembling 

 the absorption-curve of visual purple in Vertebrates (the grasshopper, 

 Melanoj)lus — Jahn, 1946). The occurrence of a Purkinje shift towards 

 shorter wave-lengths in decreasing intensity of light in some insects 

 suggests the presence of two receptor mechanisms {Drosophila — 

 Fingerman and Brown, 1952-53) ; in this connection the presence 

 of twin-peak sensitivities in electroretinograms is also of interest (at 

 630 and 540 mfx in Calliphora — Antrum and Stumpf, 1953). These, 

 of course, are measurements of the threshold of physiological response, 

 not of sensation. 



YELLOW. YELLOW- GREEN CREEN- 

 -CREEN -BLUE 



Fig. 733. — Colour Vision in Insects. 



A chart showing the relative number of visits of Gonepteryx r^^anini to 

 papers of different colours during the feeding phase (after Use). 



Cetonia 



Geotrupes 



The capacity for colour vision in insects has given rise to some 

 controversy. It would seem reasonable to suppose that the brilliant 

 colours of flowers would be oecologically linked with the insect visitors 

 on which so many plants depend for their propagation. kSuch a sugges- 

 tion demands that flower-visiting insects, which reciprocally depend 

 on the flowers for their food, should appreciate and differentiate the 

 variegated riot of colour evolved for the mutual benefit of both. It 

 must not be thought, however, that colour vision in insects is confined 

 to those that visit flowers or that its function has been evolved specific- 

 ally for this purpose and none other ; the flower-visiting beetle, Cetonia, 

 for example, is colour-blind, whereas the dung-beetle, Geotrupes, is 

 endowed with a well-developed colour sense. However that may be, 

 it has long been accepted for this reason that most insects are possessed 

 of colour vision. The first to extricate this problem from the vagueness 

 of speculation and subject it to scientific analysis was Sir John Lubbock 

 (1885) who applied the relatively simple but somewhat inconclusive 

 technique of " preferential choice." ^ On exposing honey on coloured 

 cards and recording the frequency with which each was visited, he 

 found that the honey-bee exhibited a substantial degree of colour 

 differentiation with a marked preference for blue. At a considerably 



1 p. 568. 



