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Show 328 SEXUAL SELECTION. PAltT lL s~£b-kingdom of the Arthropoda: Class, Crustacea.In this great class we first meet with undoubted secondary sexual characters, often developed in a remarkable manner. Unfortunately the habits of crustaceans are very imperfectly known, and we cannot explain the uses of many structures peculiar to one sex. \Vith the lower parasitic species the males are of small size, and they alone arc furnished with perfect swimminglegs, antennro and sense-organs ; the females being destitute of these organs, with their bodies often consisting of a mere distorted mass. But these extraordinary differences between the t\vo sexes are no doubt related to their widely different habits of life, and consequently do not concern us. In various crustaceans, belonging to distinct families, the anterior antennro are furnished with peculiar thread-like bodies, which are believed to act as smelling-organs, and these are much more numerous in the males than in the females. As the males, without any unusual development of their olfactory organs, would almost certainly be able sooner or later to find the females, the increased number of the smelling- threads bas probably been acquired through sexual selection, by the better provided males having been the most successful in finding partners and in leaving offspring. Fritz l\fuller has described a remarkable dimorphic species of Tanais, in which the male is represented by two distinct forms, never graduating into each other. In the one form the male is fUl'nished with more numerous smelling-threads, and in the other form with more powerful and more elongated chelm or pincers which serve to hold the female. Fritz Muller suggests that these differences between the two male forms of the same species must have originated in certain individuals having varied in the number of the smelling-threacls1 whilst other individuals varied in the shape and size of CHAP. IX. CRuSTACEANS. 329 their chelm; so that of the former, those which were best able to find the female, and of the latter, those which were best able to hold her when found, have left the greater number of progeny to inherit their respective advantages.4 In some of the lower crustaceans, the right-hand anterior antenna of the male differs greatly in structure from the left-hand one, the latter resembling in its simple tapering joints the antennro of the female. In the male the modified antenna is either swollen in the middle or angularly bent, or converted (fig. 3) into an elegant, and sometimes wonderfully complex, prehensile organ.5 It serves, as I hear from Sir J. Lubbock, to hold the female, a Fig. 3. Labidocera Darwinil, (from and for this same purpose one Lubbock). of the tY\'O posterior legs (b) on the same side of the body is t:!onverted into a forceps. In another family the inferior or a. Part of right-hand anterior an· tenna of mt\le, formiug a prehensile organ. b. Post.erior pair of thoracic legs of male. c. Ditto of female. posterior antennro are "curiously zigzagged" in the males alone. 4 'Facts a~d Ar~1men~s for Darwin,' English translat. 1869, p. 20. Se~ the p1·evwus discusston on the olfactory threads. Sars has descnbed. a somewha~ analogous case (as quoted in 'Nature,' 1870, p. 455) m a Norweg1an crustacean, the Pontop01·eia aflinis. 5 See Sir J. Lubbock in 'Annals. and Mag. of Nat. Hist.' vol. xi. 1853, pl. i. and x.; and vol. xii. (185R) pl. vii. Sec also Lubbock in 'Transn?t. Ent. Soc.' vol. iv. new. series, 1856-1858, p. 8. With respect to the ztgzagged antennre mentwned below, see Fritz Miiller, 'Facts and Arguments for Darwin ' 1869, p. 40, foot-note. |