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Show 330 SEXUAL SELECTION. PART II. In the higher crttstaceans the anterior legs form a pair of chelre or pincers, and these are generally larger in the male than in the female. In many species the chelre on the opposite sides of the body are of unequal size, the right-hand one being, as I am in- Fig. 4. Anterior part of body of Callianassa (from Mllne-Edwards), showing the unequal and differently-constructed right and left-hand chelre of the male. N.B.-The artist by mistake bas reversed the drawing, and made the left-band chela the largest. Fig. 5. Fig. 6. Fig. 5. Second leg of male Orchestia Tucuratinga (from Fritz Milller). · Fig. 6. Ditto of female. formed by Mr. C. Spence Bate, generally, though not invariably, the largest. This inequality is often much greater in the male than in the female. The two chelre also often differ in structure (figs. 4 and 5), the smaller one resembling those of the female. What advantage CJTAP. IX. CRUSTACEANS. 331 is gained by their inequality in size on the opposite sides of the body, and by the inequality being much greater in the male than in the female; and why, when they are of equal size, both are often much larger in the male than in the female, is not known. The chelre are sometimes of such length and size that they cannot possibly be used, as I hear from Mr. Spence Bate, for carrying food to the mouth. In the males of certain freshwater prawns (Palremon) the right leg is actually longer than the whole body.6 It is probable that the great size of one leg with its chelre may aid the male in fighting with his rivals; but this use will not account for their inequality in the female on the opposite sides of the body. In Gelasimus, according to a statement quoted by MilneEd wards,7 the male and female live in the same burrow, which is worth notice, as shewing that they pair, and the male closes the mouth of the burrow with one of its chelre, which is enormously developed; so that here it indirectly serves as a means of defence. Their main use, however, probably is to seize and to secure the female, and this in some instances, as with Gammarus, is known to be the case. The sexes, however, of the common shore-crab (Oarcinus mrenas), as Mr. Spence Bate informs me, unite directly after the female has moulted her hard shell, and when she is so soft that she would be injured if seized by the strong pincers of the male; but as she is caught and carried about by the male previously to the act of moulting, she could then be seized with impunity. Fritz Miiller states that certain species of Melita are 6 See a paper by Mr. C. Spence Bate, with figures, in 'Proc. Zoolog. Soc.' 1868, p. 363; and on the nomenclature of the genus, ibid. p. 585. I am greatly indebted to Mr. Spence Bate for nearly all the above statements with respect to the cbelre of the higher crustaceans. 7 'Hist. Nat. des Crust.' tom. ii. 1~37, p. 50. |
