COLOUR IN NATURE 
CHAP. 
I S° 
warm climates. The Heliconidae exhibit similar 
types of colour-schemes. 
In the Pieridae the pigments, according to Hop¬ 
kins (1896), occur in three ways— (a) 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 
