62 BRITISH LEPIDOPTERA. 



us to be red, if blue, then the colour appears to be blue, and so on. 

 Substances which are thus able to select certain light waves for 

 absorption, and to reflect others to our eyes, are termed pigments, and 

 the fact that most scales of Lepidoptera contain substances that can 

 do this, causes us to term the colours thus produced pigmentary 

 colours. 



But colours are also obtained by the refraction, interference and 

 diffraction of white light. Scratched and striated surfaces diffract 

 light. Diffraction breaks up the bent part of a ray of light into its 

 component parts, and, dispersing the waves, gives, on the edge of 

 each bright space between the slits or striations, a fringe of colour. 

 The exposed surface of the scales of many Lepidoptera are striated, 

 both longitudinally and transversely, hence these produce surface 

 colours by diffraction. One of the best-known examples of this kind 

 of coloration in British insects is the purple of the male of Apatura 

 iris. Such colours as these are usually termed in entomological 

 magazines, non-pigmentary colours. 



Having thus briefly stated the phenomena by means of which, 

 practically, all the colours of the scales of butterflies and moths 

 are derived, we see that the colours are due either to the selective 

 power of the pigment contained in the scales or membrane of the 

 wing, or they are due to the peculiarities of structure and form of the 

 scale. 



We have already stated that variation is general in all insects, no 

 two butterflies or moths of the same species being exactly alike. 

 Sometimes this general variation in a particular species is so marked 

 and conspicuous, that the most casual observer notices the fact. Such 

 species are then said to be polymorphic. In a less degree, however, it 

 may be accepted as a general fact that all species of insects are 

 polymorphic. 



The enemies of butterflies and moths are very numerous insec- 

 tivorous birds, reptiles, mammals, other insects and as they have 

 practically no weapons of offence, their safety lies in their resemblance 

 to their surroundings. Danger, to them, is probably more real when 

 they are at rest, hence, when at rest in a natural attitude, one is 

 at once struck by the marvellous resemblance which most butterflies 

 and moths bear to the surface (or to some common object on the sur- 

 face) on which they rest. With the initial general variation which we 

 have observed to occur in all insects, it is pretty certain that some 

 individuals will be more readily detected than others, some peculiarity 

 of tint, some mark or spot of colour, maybe, rendering them a little 

 more conspicuous. These will fall a more ready prey to the enemies 

 that are searching for them, and they are, as a rule, the first eaten. 

 Those that are best protected are most likely to be left, the laws of 

 heredity step in, and a larger proportion of well-protected specimens 

 results in the progeny. Of course, the general variation which must 

 exist in all broods, and between all individuals, the tendency to 

 atavism, and similar causes, will always result, even then, in producing 

 some less favoured individuals. Still the general result will be that a 

 well protected race, suited to the particular environment by which it 

 is surrounded, will be developed. 



It is evident, when we consider the different habits of insects, that 

 the particular habit and environment of each species, will determine 



