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Show .314 THE PRINCIPLES OF PAHT II. With Siagonium (Staphylinidre), in which the males arc furnished with horns "the females are far more numerous than the opposite «:sex." M~. Janson stated at the Entomological Society that the females of the bark-feedinO' 'Tomicus villosus are so common as to be a plague, whilst the m:les are so rare as to be har~ly lmo~n. In other Orders, from unknown causes, but apparently m some mstances owing to parthenogenesis, the males of certain species have never been discovered or are excessively rare, as with several of the Cynipidre.67 In all the gall-making Cynipidm known to Mr. Walsh, the females are four or five times as numerous as the males ; and so it is, as he informs me, with the gall-making Cecidomyiire (Diptera). With some common species of Saw-flies ('l'enthredinm) Mr. F. Smith has reared hundreds of specimens from larvro of all sizes, but has never reared a single male : on the other hand Curtis says/8 that with certain species (Athalia), bred by him, the males to the females were as six to one; whilst exactly the reverse occurred with the mature insects of the same species caught in the fields. With the Neuroptera, Mr. ·walsh states that in many, but by no means in all, the species of the Odonatons groups (Ephemerina), there is a great overplus of males: in the genus Hetrerina, also, the males are generally at least four times as numerous as the females. In certain species in the genus Gomphus the males are equally numerous, whilst in two other species, the females are twice or thrice as numerous as the males. In some European species of Psocus thousands of females may be collected without a single male, whilst with other species of the same genus both sexes are common.69 In England, Mr. MacLachlan has captured hundreds of the female Apatania muliebris, but has never seen the male ; and of Boreus hyemalis only four or five males have been here seen.70 With most ·Of these species (excepting, as I have heard, with the Tenthredinre) there is no reason to suppose that the females are subject to parthenogenesis; and thus we see how ignorant we are on the causes of the .apparent discrepancy in the proportional numbers of the two sexes. In the other Classes of the Articulata I have been able to collect still less information. With Spiders, Mr. Blackwall, who has carefully attended to this class during many years, writes to me that the males from their more erratic habits are more commonly seen, 67 Walsh, in 'The American Entomologist,' vol. i. 1869, p. 103. F. Smith, ·'Record of Zoological Literature,' 1867, p. 328. 68 ' Farm Insects,' p. 45-46. 69 'Observations on N. American Neuroptera,' by H. Hagen and B. D. Walsh, 'Proc. Ent. Soc. Philadelphia,' Oct. 1863, p. 168, 223, 239. 10 'Proc. Ent. Soc. London,' Feb. 17, Hl68 . . Cnu. VJII. SEXUAL SELECTION. 315 .and therefore appear to be the more num~rous. This is ac~ual~y t~c case with a few species; but he mentwns several spectes m stK <J'enera in which the females appear to be much more numerous than the males.n 'J'he small size of the males in comparison with the females which is sometimes carried to an extreme degree, and their wiclel; different appearance, may account in some instances for their rarity in collections.72 Some of the lower Crustaceans are able to propagate their kind asexually, and this will account for the extreme rarity of the males. With some other forms (as with 'l'anais and Cypris) there is reason to believe, as Fritz Muller informs me, that the male is much shorterlived than the female, which, supposing the two sexes to be at first -equal in number, would explain the scarcity of the males. On the other hand this same naturalist has invariably taken, on the shores <Jf Brazil far more males than females of the Diastylidro and of Cypridin~ ; thus with a species in the latter genus, 63 specime~s cau<Yht the same clay, included 57 males; but he suggests that th1s })re;onderance may be due to some unknown difference in the habits <lf the two sexes. With one of the higher Brazilian crabs, namely a Gelasimus Fritz Miiller found the males to be more numerous than the fe~ales. The reverse seems to be the case, according to the large experience of Mr. C. Spence Bate, with six common British crabs, the names of which he has given me. On the Power of Natural Selection to regulate the proportional Numbers of the Sexes, and General Fertility.ln some peculiar cases, an excess in the number of one sex over the other might be a great advantage to a species, as with the sterile females of sorial insects, ~r with those animals in which more than one male 1s requisite to fertilise the female, as with certain cirripedes and perhaps certain fishes. .An inequality between the sexes in these cases might have been acquired through natural selection, but from their rarity they need not here be further considered. In all ordinary il Another great authority in this class, Prof. Thorell of Upsala ('On European Spiners,' 1869-70, part i. p. 205) speaks 11s if female spiders were O'enerally commoner than the males. "' i2 See, on this subject, Mr. Pickard-Cambridge, as quoted in 'Quarterly Journal of Science,' 1868, p. 429. |