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 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 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 (Libellulidas) are sometimes sensibly 
larger, and never smaller, than the females ; 16 and 
they do 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 accor- 
dance 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 acervorum , and amongst the 
Fossores, the males of the Methoca ichneumonides , are 
larger than the females. The explanation of this ano- 
maly is that a marriage-flight is absolutely necessary 
36 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. 
