126 INSECT PHYSIOLOGY 



that the physical mechanism is like that of polaroid. In normal vision 

 the light reaching the rhabdom acts upon the retinal pigment 'retinene' 

 (a derivative of vitamin A) which the rhabdom contains, and the re- 

 sulting breakdown products stimulate the sensory cells of the retina. 



Colour vision 



The visual perceptions, notably in flower-visiting insects, are greatly 

 enhanced if the objects are coloured ; insects possess, in fact, a well- 

 developed colour vision. The histological basis of this colour vision 

 is not certainly known, but there is evidence that the different rhab- 

 domeres of a single rhabdom may contain different visual pigments and 

 react to different wavelengths of light, so that a single retinal element, 

 below a single facet, will be able to distinguish a range of colours. 



The functional significance of colours in the vision of insects is 

 much better understood. Certain responses, such as the fixation reflex, 

 are influenced only by the luminosity of the source of stimulus and not 

 by its colour. Such responses, therefore, afford a method of testing the 

 subjective luminosity of different colours for different insects ; and it 

 has been shown that for many insects {Mantis, Coccinella, Apis) the 

 luminosity of the visible spectrum is the same as for man with the eye 

 dark adapted ; for others (Pieris) it corresponds with the light adapted 

 human eye. Many insects can vary the amount of light entering the 

 eye, by movements of the pigment in the iris cells. These changes, 

 combined perhaps with biochemical changes in the retina, can bring 

 about a 1000-fold increase or decrease in the sensitivity of the eye. 



Corresponding with these differences in apparent luminosity, there 

 are differences in the visibility of the spectrum. Certain insects {Apis, 

 Macroglossa, Lep.) are practically blind to red, but can see far into 

 the ultra-violet. Such insects may be attracted by the ultra-violet 

 light reflected from certain flowers; and they will cluster round 

 screens illuminated with ultra-violet rays which are invisible to us. 

 Indeed, the stimulating efficiency of the ultra-violet part of the spec- 

 trum, for the honey-bee and for Drosophila, may be far greater than 

 that of the regions visible to man ; and although this effect may be 

 due in some degree to the fluorescence of parts of the eye under 

 ultra-violet illumination, this cannot account entirely for the pheno- 

 menon. There must be a true perception of ultra-violet; and it is in- 



