ch. x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 269 
colours of the sexes does not bear any constant relation 
to affinity or systematic position. In both insects and birds 
we find examples of complete identity and extreme diversity 
of the sexes ; and these differences occur sometimes in the 
same tribe or family, and sometimes even in the same 
genus. 
It is only among the higher and more active animals that 
sexual differences of colour acquire any prominence. In the 
mollusca the two sexes, when separated, are always alike in 
colour, and only very rarely present slight differences in the 
form of the shell. In the extensive group of Crustacea the 
two sexes as a rule are identical in colour, though there are 
often differences in the form of the prehensile organs ; but in 
a very few cases there arc differences of colour also. Thus, in 
a Brazilian species of shore-crab (Gelasimus) the female is 
grayish-brown, while in the male the posterior part of the 
cephalo-thorax is pure white, with the anterior part of a rich 
green. This colour is only acquired by the males when they 
become mature, and is liable to rapid change in a few 
minutes to dusky tints. 1 In some of the fresh-water fleas 
(Daphnoidse) the males are ornamented with red and blue 
spots, while in others similar colours occur in both sexes. In 
spiders also, though as a rule the two sexes are alike in colour, 
there are a few exceptions, the males being ornamented with 
brilliant colours on the abdomen, while the female is dull 
coloured. 
Sexual Coloration in Insects. 
It is only when we come to the winged insects that we find 
any large amount of peculiarity in sexual coloration, and 
even here it is only developed in certain orders. Flies (Dip- 
tera), field-bugs (Hemiptera), cicadas (Homoptera), and the 
grasshoppers, locusts, and crickets (Orthoptera) present very 
few and unimportant sexual differences of colour ; but the last 
two groups have special musical organs very fully developed 
in the males of some of the species, and these no doubt enable 
the sexes to discover and recognise each other. In some cases, 
however, when the female is protectively coloured, as in the 
well-known leaf-insects already referred to (p. 207), the male 
1 Darwiu’s Descent of Man, p. 271. 
