37S THE DESCENT OF MAN. 



to depend on their developing an enormous number of ova; and 

 this may to a certain extent hold good with insects. But Dr. 

 V/allace has suggested a much more probable explanation. He 

 finds, after carefully attending to the development of the cater- 

 pillars of Bombyx cynthia and yamamai, and especially to that 

 of some dwarfed caterpillars reared from a second brood on un- 

 natural food, "that in proportion as the individual 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."" 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 impreg- 

 nated 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," 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 ma- 

 tured later would leave ^ewer offspring. 



There are, however, exceptions to the rule of male insects be- 

 ing smaller than the females: and some of these exceptions are 

 Intelligible. Size and strength would be an advantage to the 

 males, which fight for the possession of the females; and in these 

 cases, as with the stag-beetle (Lucanus), the males are larger 

 than the 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 soraa 

 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-fiies (Libellu- 

 lidas) are sometimes sensibly larger, and never smaller, than the 

 females;" and as Mr. MacLachlan believes, they do not generally 

 pair with the females until a week or fortnight has elapsed, and 

 until they have assumed their proper masculine colors. But the 

 most curious cases, showing on what complex and easily over- 

 looked relations, so trifiing a character as difference in size be- 

 tween the sexes may depend, is that of the aculeate Hymenop- 



" 'Transact. Bnt. Soc' 3ra series, vol. v. p. 486. 



« 'Journal of Proo. Ent. Soc.' Feb. 4th, 1867, p. Ixxl. 



" For this and other statements on the size of the sexes, see Kirby 

 and Spence, ibid. vol. iii. p. 300; on the duration of life in Insects, 

 see p. 344. 



