I50 COLOUR IN NATURE chap. 



warm climates. The Heliconidae exhibit similar 

 types of colour-schemes. 



In the Pieridse the pigments, according to Hop- 

 kins (1896), occur in three ways — («) first uniformly 

 distributed through the chitin, apparently of the 

 outer layer of the scale : the black and brown pig- 

 ments occur in this position ; (b) in granules between 

 the two layers of scales, according to Hopkins — that 

 is, probably in the bridges of chitin already described : 

 white, yellow, and red pigments occur in this posi- 

 tion ; (c) between the two chitinous lamellae of the 

 wing : a green pigment is described in this position. 



The black and brown pigments, as elsewhere in 

 butterflies, are characterised chiefly by their insolu- 

 bility and their stability ; they are not attacked by any 

 agents which do not attack chitin. They are possibly 

 the same as the dark cuticular pigments of the larvae, 

 and may have some connection with the substances 

 which occur in the blood and oxidise rapidly when it 

 is exposed to the air. In the Pieridae they are im- 

 portant in directly giving rise to the dark patches 

 and markings on the wing, but in many butterflies 

 they probably aid in the production of structural 

 colours. 



The white pigment of the Pieridae is, as we have 

 seen, uric acid itself. It can be extracted from the 

 tissues by dilute alkaline solutions, which yield a 

 copious precipitate on acidification. The precipitate, 

 if treated with nitric acid and heated, leaves a 

 residue which turns purple with ammonia — the 

 murexide reaction, so characteristic of the uric-acid 

 group of substances. 



The yellow or orange pigment is readily soluble 



