170 
COLOUR IN NATURE 
CHAP. 
not infrequently different from the hind, and the 
type of coloration suggests that seen in butterflies. 
Among the pure pigmental colours brown, red, 
yellow, and green are common. A brilliant red 
colour is far from uncommon, and is displayed for 
example on the surface of the abdomen in Nepa 
rubra. 
Among the pigments of the Hemiptera we must 
assign the first place to the carmine of Coccus cacti, 
not on account of its importance in coloration but 
because its commercial value has caused it to be 
somewhat fully investigated. 
Carminic acid occurs in nature in the Coccidae, 
especially Coccus cacti , and, according to Sorby, also 
in various species of Aphis. It is a glucoside, yielding 
a sugar when treated with dilute acid. 
In the case of Coccus cacti the pigment occurs in 
large amount in the female (26 to 50 per cent of the 
body weight according to Krukenberg) and in less 
amount in the male ; to the dried insects at least it 
gives a dull red-gray colour rather than a pure full 
red. According to Mayer it occurs in drops near the 
periphery of the cells of the fatty body, the drops 
being less numerous in the case of the male. It 
occurs also in the yolks of the eggs and in the diffuse 
fatty body in new-hatched larvae. Mayer says ex¬ 
pressly that the pigment does not occur in the gut, 
but in another place he states that it markedly 
colours the faeces ; the anomaly is, however, nowhere 
explained. It is possible that the meaning is that 
the pigment is not found in the anterior part of the 
gut, but is introduced into it by the Malpighian 
tubules. 
