one species. (16. Claus, Die freilebenden Copepoden, 1863, s. 35.) It
would, however, be extremely rash to conclude that these curious organs
serve to attract the females. I am informed by Fritz Muller, that in
the female of a Brazilian species of Gelasimus, the whole body is of a
nearly uniform greyish-brown. In the male the posterior part of the
cephalo-thorax is pure white, with the anterior part of a rich green,
shading into dark brown; and it is remarkable that these colours are
liable to change in the course of a few minutes the white becoming
dirty grey or even black, the green losing much of its brilliancy. It
deserves especial notice that the males do not acquire their bright
colours until they become mature. They appear to be much more numerous
than the females; they differ also in the larger size of their chelae.
In some species of the genus, probably in all, the sexes pair and
inhabit the same burrow. They are also, as we have seen, highly
intelligent animals. From these various considerations it seems
probable that the male in this species has become gaily ornamented in
order to attract or excite the female.
It has just been stated that the male Gelasimus does not acquire his
conspicuous colours until mature and nearly ready to breed. This seems
a general rule in the whole class in respect to the many remarkable
structural differences between the sexes. We shall hereafter find the
same law prevailing throughout the great sub-kingdom of the Vertebrata;
and in all cases it is eminently distinctive of characters which have
been acquired through sexual selection. Fritz Muller (17. Facts and
Arguments, etc., p. 79.) gives some striking instances of this law;
thus the male sand-hopper (Orchestia) does not, until nearly full
grown, acquire his large claspers, which are very differently
constructed from those of the female; whilst young, his claspers
resemble those of the female.
CLASS, ARACHNIDA (SPIDERS).
The sexes do not generally differ much in colour, but the males are
often darker than the females, as may be seen in Mr. Blackwall s
magnificent work. (18. A History of the Spiders of Great Britain,
1861-64. For the following facts, see pp. 77, 88, 102.) In some
species, however, the difference is conspicuous: thus the female of
Sparassus smaragdulus is dullish green, whilst the adult male has the
abdomen of a fine yellow, with three longitudinal stripes of rich red.
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