194 THE FUNDAMENTAL ASPECT OF COLORATION 



position of nucleic acids which occur with a protein group to 

 form nucleoprotein. Pigments, therefore, with purine bases are 

 excretory products which, in the Pieridae, become deposited in 

 the wings. With respect to the white colour of these insects, it 

 has already been pointed out that, according to Mason, it is due 

 to structural features rather than to the presence of urates. 



Combination Colours 



Combination colours are much more prevalent among insects 

 than colours solely due to structural features. In other words, 

 pigment very commonly co-exists with structures that are, in 

 themselves, colour-producing. The structural features involved 

 produce iridescence which is due to one or more of the four causes 

 already discussed. Whenever a pigment is suspected to occur in 

 combination with a structural colour its effect can be eliminated 

 by bleaching (preferably with hydrogen peroxide) ; similarly, 

 the absence of pigment can be demonstrated by the same means, 

 when the original colour will reappear unaltered after the specimen 

 has been dried. Iridescent colours are very often devoid of the 

 presence of all pigment excepting black or dark brown. The dark 

 pigment acts as a background or absorbing screen, thus inter- 

 cepting white light which would otherwise reach the eye. The 

 amount of desaturation structural colours undergo is largely 

 proportional to the extent to which the dark pigment is developed. 

 As a rule a background of this nature is necessary to the j^roduction 

 of the brilliant and intense structural blues and metallic greens. 

 The pigmentary substance may be deposited in the same scales 

 which bear the special structural features concerned, or in a layer 

 of scales beneath them ; it may occur either in the scale cavity or 

 diffused in the cuticular substance of the scale. 



Onslow's researches have brought to light interesting examples 

 of combination colours among insects, besides other cases where 

 the causes of the colours observed are entirely obscure. In the 

 butterfly Morpho cypris, for example, there are brilliant iridescent 

 blue scales and also white ones, and the white scales only differ 

 in the entire absence of brown or black pigment. The absence 

 of this absorbing screen causes areas composed of these scales 



