The Descent of Man

the sexes. So again, male dragon-flies (Libellulidae)
are sometimes sensibly larger, and never
smaller, than the females (16. 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.); 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 colours. But the most curious
case, shewing on what complex and easilyoverlooked
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 amongst the Bees, the males of Apis
mellifica, Anthidium manicatum, and Anthophora
acervorum, and amongst the Fossores, the
males of the Methoca ichneumonides, are lar-