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Show 346 SEXUAL SELECTION. P.un II. between the male and female cocoons of the silk-moth (Bombyx mori), that in France they are separated by a particular mode of weighingP In the lower classes of the animal kingdom, the greater size of the females seems aenerally to depend on their developing an enor- o . mous number of ova; and this may to a ce1·tam extent hold good with insects. But Dr. Wallace has suggested a much more probable explanation. He finds, after carefully attending to the development of the caterpillars of Bombyx cynthia and yarnarnai, and. especially of some dwarfed caterpillars reared from a second brood on unnatural food, "that in proportion as the indivi" dual moth is finer, so is the time required for its " metamorphosis longer; and for this reason the female, "which is the larger and heavier insect, from having to "carry her numerous eggs, will be preceded by the "male, which is smaller and has less to mature." 14 Now as most insects are short-lived, and as they are exposed to many dangers, it would manifestly be advantageous to the female to be impregnated as soon as possible. This end would be gained by the males being first matured in large numbers ready for the advent of the females; and this again would naturally follow, as Mr. A. R. Wallace has remarked/5 through natural selection; for the smaller males would be first matured, and thus would procreate a large number of offspring which would inherit the reduced size of their male parents, whilst the larger males from being matured later would leave fewer offspring. There are, however, exceptions to the rule of male insects being smaller than the females ; and some of I~ Robinet, 'Versa Soie,' 1848, p. 207. 14 ''l'ransact. Ent. Soc.' 3rd series, vol. v. p. 486. 1• 'Journal of Pl'oc. Ent. Soc.' Feb. 4th, 1867, p. lxxi. CHAP. X. INSECTS. 347 these exceptions are intelligible. Size and strength would be an advantage to the males, which fight for the possession of the female ; and in these cases the males as with the stag-beetle (Lucanus), are larger than th~ females. There are, however, other beetles which are not known to fight together, of which the males exceed the females in size ; and the meaning of this fact is not known; but in some of these cases, as with the huge Dynastes and Megasoma, we can at least see that there would be no necessity for the males to be smaller than the females, in order to be matured before them, for these beetles are not short-lived, and there would be ample time for the pairing of the sexes. So, again,. male dragon-flies (Libellulidre) are sometimes sensibly larger, and never smaller, than the females ; 16 and th~y d? not, as Mr. MacLachlan believes, generally pair With the females, until a week or fortnight has elapsed, and until they have assumed their proper masculine colours. But the most curious case, shewing on what complex and easily-overlooked relations so trifling a character as a difference in size between' the sexes may depend, is that of the aculeate Hymenoptera ; for Mr. F. Smith informs me that throughout nearly the whole of this large group the males, in accordance with the general rule, are smaller than the females and emerge about a week before them ; but amongst the Bees, the males of Apis mellifica, Anthidium manicatum and Anthophora acervon~m, and amongst the Fossores, the males of the Methoca ichneurnonides, are larger than the females. The explanation of this anomaly is that a marriage-flight is absolutely necessary 16 For this and other statements on the size of the sexes, see Kiruv and Spence, ibid. vol.:'iii. p. 300; on the duration of life in insect;, seep. 344. |