i64 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 oh 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, e.g. 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 



