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 are 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 

 (Daphnoidae) 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 Darwin's Descent of Man, p. 271. 



