The Descent of Man

In this Order slight differences in colour, according
to sex, are common, but conspicuous differences
are rare except in the family of Bees;
yet both sexes of certain groups are so brilliantly
coloured—for instance in Chrysis, in which
vermilion and metallic greens prevail—that we
are tempted to attribute the result to sexual
selection. In the Ichneumonidae, according to
Mr. Walsh (59. 'Proceedings of the Entomological
Society of Philadelphia,' 1866, pp. 238, 239.),
the males are almost universally lightercoloured
than the females. On the other hand,
in the Tenthredinidae the males are generally
darker than the females. In the Siricidae the
sexes frequently differ; thus the male of Sirex
juvencus is banded with orange, whilst the female
is dark purple; but it is difficult to say
which sex is the more ornamented. In Tremex
columbae the female is much brighter coloured
than the male. I am informed by Mr. F. Smith,
that the male ants of several species are black,
the females being testaceous.