164 
COLOUR IN NATURE 
CHAP. 
is well differentiated, and this is also the condition 
necessary in the general case for the development 
of optical colour. Mr. Poulton speaks of larval 
Lepidoptera exhibiting the dark cuticular pigments 
in the places where the cuticle is thickest because the 
epidermal pigments would be useless in these spots, 
the cuticle being too thick to allow them to shine 
through ; a comparative survey, however, makes it 
more probable that there is a direct association 
between the pigment and the thickened cuticle. 
An excellent example is afforded by the dark 
colours of many beetles with their thickened elytra, 
which form a marked contrast to the browns or greens 
of the Orthoptera with their relatively little developed 
cuticle. 
Another important point about the colours of 
insects is the relation of food to colour. Mr. Poulton’s 
experiments on caterpillars have shown that the pig¬ 
ments of the food may find their way to the blood 
or to the tissues, and owing to the thinness of the 
cuticle be instrumental in producing the typical 
coloration. The prevalence of green colours among 
herbivorous forms with little developed cuticle, e.g. 
the Aphides or plant-lice, certainly suggests that 
this also occurs elsewhere. The question has been 
considerably confused by the common habit of 
calling such derived pigments chlorophyll. In the 
Crustacea green pigments occur, eg. in Virbius , 
which there is little doubt are merely combinations 
of lipochromes with other substances, perhaps bases, 
and there seems no reason why similar combinations 
should not occur in insects, with the aid of the 
derived pigments. At the same time there is no 
