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Show 336 SEXUAL SELECTION. PAnT II. minute shields or cell-like bodies, which exhibit beautiful changing colours; these being absent in the females, and in the case of one species in both sexes.n It would, however, be extremely rash to conclude that these curious organs serve merely to attract the females. In the female of a Brazilian species of Gelasimus, the whole body, as I am informed by Fritz Muller, 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 g1·een, 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." The males apparently are much more numerous than the females. It deserves especial notice that they do not acquire their bright colours until they become mature. They differ also from the females in the larger size of their chelre. 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 highly 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 the general rule in the whole class with the many remarkable differences in structure between the two sexes. Yv e shall hereafter £nd 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 11 Claus, ' Die freilebendeu Copepoden,' 1863, s. 35. Cn.\r. IX. SPIDERS. 337 acquired through sexual selection. Fritz Muller 12 gives some striking instances of this law; thus the male sand-hopper (Orchestia) does not acquire his large claspers, which are very differently constructed from those of the female, until nearly full-grown; whilst young his claspers resemble those of the female. Thus, again, the male Brachyscelns possesses, like all other amphipods, a pair of posterior antennre; the female, and this is a most extraordinary circumstance, is destitute of them, and so is the male as long as he remains immatm·e. Class, Arachnida (Spiders).-The males are often darker, but sometimes lighter than the females, as may . be seen in Mr. Blackwall's magnificent work.13 In some species the sexes differ conspicuously from each other in colour; thus the female of Sparassus smaragdulus is dullish-green; whilst the adult male has the abdomen of a £ne yellow, with three longitudinal stripes of rich red. In some species of Thomisus the two sexes closely resemble each other ; in others they differ much; thus in T. citreus the legs and body of the female are pale-yellow or green, whilst the front legs of the male are reddish-brown: in T. jloricolens, the legs of the female are pale-green, those of the male being ringed in a conspicuous manner with various tints. Numerous analogous cases could be given in the genera Epeira, N ephila, Philodromus, Theridion, Linyphia, &c. It is often difficult to say which of the two sexes departs most from the ordinary coloration of the genus to which the species belong; but Mr. Blackwall 12 ' Facts nnd Arguments,' &c .. p. 7!). 13 'A History of the Spiders of Great Britain,' 1861-64. For the following facts, seep. 102, 77, 88. VOL. L Z |
