316 THE DESCENT OF MAN. 



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 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 wliich 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 

 (Libellulide) 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 case show- 

 ing on what complex and easily overlooked relations so 

 trifling a character as 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 among the bees, the males of Apis 

 mellificay Anthidium 7)ia)iicatum, and AntJiopliora acervo- 

 rum, and among the Fossores the males of the Metliora 

 ichneumonid<is are larger than the females. The explana- 

 tion of this anomaly is that a marriage flight is absolutely 

 necessary with these species, and the male requires great 

 strength and size in order to carry the female through the 

 air. Increased size has here been acquired in opposition to 

 the usual relation between size and the period of develop- 

 ment, for the males, though larger, emerge before the 

 smaller females. 



We will now review the several orders, selecting such 

 facts as more particularly concern us. The Lepidoptera 



* For tins and otber 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. 



