174 THE FUNDAMENTAL ASPECT OF COLORATION 



4. Selective reflection of light from extremely opaque highly 

 reflecting surfaces. 



The history of the subject need not detain us here, but those 

 who are specially interested should refer to Onslow's paper 

 previously mentioned. The phenomena involved are so diverse, 

 and the wealth of material for study so immense, that the tendency 

 to generalisation involving the application of any one single 

 theory to account for all cases is beset with pitfalls. 



Diffraction. The scales of most insects, whether iridescent or 

 not, bear striations parallel to their long axes and numbering 

 almost anything between 4,000 to 16,000 to a centimetre. Onslow 

 has described a method of taking scale impressions on colourless 

 collodion films, by which the effect of any surface pattern can be 

 analysed and isolated from colours produced in other ways. 

 These replicas show that the upper surfaces of most scales give 

 good " gratings." Onslow's observation that all insects, the 

 impressions of whose scales yield spectra, do not themselves 

 necessarily exhibit iridescence makes it doubtful whether diffrac- 

 tion is ever a main source of colour, especially as he found that 

 the brilliant scales of the butterfly Morpho cypris, and others with 

 the same structure, give an almost flat impression. It is also 

 noteworthy that the changing colours of the unpigmented wings 

 of dragon-flies and other insects cannot be explained upon the 

 basis of a diffraction grating, since striae are entirely wanting in 

 these cases. The conclusions of Onslow, Suffert, and Mason are 

 contrary to any idea that diffraction plays much part in insect 

 coloration. That it may be responsible in certain special cases, 

 however, cannot be excluded. Mason (1927), who is a very 

 decided opponent to diffraction being an explanation of iridescence, 

 states that an exception occurs in the Lamellicorn beetle Sericea 

 sericea. The elytra of this insect show brilliant iridescence in 

 direct sunlight and much less in ordinary light. The variations 

 in iridescence under different tests have led him to look for some 

 oriented structure as the cause of the phenomenon. This was 

 found in the presence of fine striae running transversely to the 

 length of the elytra. The striae are shallow (0-5 — 1-0 /x) and 

 very evenly spaced apart (1 — 1-5 /x or about 20,000 per inch). 



