182 THE FUNDAMENTAL ASPECT OF COLORATION 



on the multiple film theory, only the film layer is thicker in these 

 cases. He remarks that the " colours and their changes, with angle 

 of incidence, pressure, swelling or ' faults,' the complementary 

 character of the transmission colours, and their low saturation, 

 the loss of colour on penetration, after treatment with HNO3, the 

 persistence after bleaching or in different light — all these criteria 

 are fully satisfied and they could not be met by any other known 

 type of structural colour." The existence of minute rod-like 

 structures, which penetrate the cuticula perpendicular to the 

 surface, modifies the properties of the multiple film layer : they 

 increase the lustre and may impart a bluish-white appearance 

 owing apparently to the scattering of light by the ends of the 

 rods (about 0-3 ju, in diameter) which function as individual 

 particles. Mason's conclusions are in accord with those of 

 Biedermann before him, but it may be said that neither the theory 

 of selective reflection, nor that of multiple films, can be regarded 

 as a fully established explanation of iridescent integuments. The 

 problem, so briefly outlined here, awaits further evidence for its 

 solution. Mason's contention that the existence of multiple 

 films has to be tested by optical methods rather than by micro- 

 scopical examination cannot be controverted. Nevertheless, it 

 must be remembered that microscopical evidence of the existence 

 of a given theoretical type of structure is of extreme importance. 

 No very large amount of study has been directed to the histological 

 aspects of the problem by way of applying refined methods of 

 technique. In this respect the work of Suffert is a definite 

 advance upon that of others, and he was able to show that definite 

 stratification occurs in the scale-wall in species of Urania, Papilio 

 and other genera, thereby satisfying theoretical requirements. 

 He was also the first to make the important discovery that in 

 scales of the Morpho type the ribs themselves actually show a 

 stratified composition — a fact brought out by observation upon 

 broken or split scale-ribs and by the use of polarised light. Not- 

 withstanding Mason's belief to the contrary, some indication of 

 lamination, if it exists, in iridescent insect integuments should 

 prove capable of detection by the application of suitable technique 

 to specially favourable types. 



