cult to believe that the two forms can belong to the same
species; and this diversity in the 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.[118] In some of the freshwater
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 (Diptera), 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 is smaller and much
less protectively formed and coloured. In the bees and wasps
(Hymenoptera) it is also the rule that the sexes are alike in colour,
though there are several cases among solitary bees w