FUNCTIONS OF PIGMENTS 195 



to appear white and their iridescence to be greatly reduced. 

 The blue scales, it may be added, can be made to appear similar 

 to the white ones by treatment with bleaching agents. In 

 the white and in dark purple areas of the wings of the male of 

 Hypolimnas bolina the same phenomenon also presents itself. 

 Onslow also describes the means of colour production in the case 

 of the brilliant green seen in the male of Ornithoptera poseidon. 

 This he finds is due to a structural blue combined with a yellow 

 pigment diffused in the cuticular parts of the scales themselves. 

 In Teracolous phlegyas a pigmentary red in the scale- wall, combined 

 with a structural purple, produces a magenta. Also in Callitcera 

 esmeralda a dull reddish-brown pigment alone is present, but, in 

 such areas of the wings where it occurs, structural features of the 

 scales combined with it produce vivid magenta red spots. The 

 actual colour is probably largely structural, the pigment behaving 

 more especially as an absorptive layer which serves to enhance the 

 iridescence. A number of other examples of combination colours 

 will be found in the literature already quoted, and do not require 

 mention here. 



Further Remarks on Insect Coloration 



Much has been written regarding the relations of the colours of 

 insects to the environment in which those animals live. It is 

 often comparatively easy to give an explanation of the way in 

 which a particular colour or type of coloration may be useful to a 

 given species. Entomological literature abounds with empirical 

 deductions of this kind. Such explanations are largely concerned 

 with superficial appearances and do not penetrate very far into 

 the fundamental nature of the problems involved. The more 

 difficult subject of the function and significance of pigments, in the 

 physiological processes of the individual, tends to be relegated to 

 the background in such cases. Possibly, more often than not, 

 paucity of biochemical knowledge is one of the contributing 

 causes. Colour, it may be said, arises as a necessary result of the 

 complex chemico-physical constitution of the organism ; it is the 

 outward expression, or the indicator, of a chain of processes within. 

 It is primarily determined by physiological causes and it remains 



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