Posterior pair of thoracic legs of male. c. Ditto of female.]
In some of the lower crustaceans, the right anterior antenna of the
male differs greatly in structure from the left, the latter resembling
in its simple tapering joints the antennae of the female. In the male
the modified antenna is either swollen in the middle or angularly bent,
or converted (Fig. 4) into an elegant, and sometimes wonderfully
complex, prehensile organ. (9. See Sir J. Lubbock in Annals and Mag.
of Nat. Hist. vol. xi. 1853, pl. i. and x.; and vol. xii. (1853), pl.
vii. See also Lubbock in Transactions, Entomological Society, vol.
iv. new series, 1856-1858, p. 8. With respect to the zigzagged antennae
mentioned below, see Fritz Muller, Facts and Arguments for Darwin,
1869, p. 40, foot-note.) It serves, as I hear from Sir J. Lubbock, to
hold the female, and for this same purpose one of the two posterior
legs (b) on the same side of the body is converted into a forceps. In
another family the inferior or posterior antennae are curiously
zigzagged in the males alone.
[Fig. 5. Anterior part of body of Callianassa (from Milne-Edwards),
showing the unequal and differently-constructed right and left-hand
chelae of the male. N.B. The artist by mistake has reversed the
drawing, and made the left-hand chela the largest.
Fig. 6. Second leg of male Orchestia Tucuratinga (from Fritz Muller).
Fig. 7. Ditto of female.]
In the higher crustaceans the anterior legs are developed into chelae
or pincers; and these are generally larger in the male than in the
female, so much so that the market value of the male edible crab
(Cancer pagurus), according to Mr. C. Spence Bate, is five times as
great as that of the female. In many species the chelae are of unequal
size on the opposite side of the body, the right-hand one being, as I
am informed by Mr. Bate, generally, though not invariably, the largest.
This inequality is also often much greater in the male than in the
female. The two chelae of the male often differ in structure (Figs. 5,
6, and 7), the smaller one resembling that of the female. What
advantage 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
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