148 
COLOUR IN NATURE 
CHAP. 
thought to be of importance in colour-production. 
The two membranes are connected by bridges of 
chitin, and pigment granules may be deposited both 
in these bridges and in the outer membrane. 
Having regard to the small size of the scales, it 
will be readily understood that there is frequently 
great difficulty in determining whether a particular 
colour is due to pigment or structure ; the fact that 
a scale showing structural colour frequently contains 
pigment in addition further complicates the matter. 
We thus find that there iS much difference of opinion 
among observers as to the cause of particular colours. 
This occurs especially in the case of dichroic scales, 
those displaying one colour by transmitted light and 
the complementary colour by reflected light. Some 
regard this effect as purely structural, others main¬ 
tain that the colours are due to dichroic pigments. 
Making due allowance for difficulties of observation, 
it seems, however, certain that blue in butterflies is 
always a structural colour, while green, black, and 
white are at least usually due to structure. Other 
of the structural colours are readily recognised by 
their metallic brilliancy and changing glow, which 
give to the butterflies possessing them an appear¬ 
ance of surpassing beauty. Many will recall Mr. 
Wallace’s description of how his heart beat fast 
and his brain reeled when his perseverance was 
rewarded, and he captured with his own hands 
one of the finest of these living gems in the Malay 
Archipelago. 
In connection with these colours Urech(i893) 
notices one little point of some interest. He found 
that in some cases the scales display under the 
