The Descent of Man

difference in colour between the sexes of many
Libellulidae, it is often difficult to say which is
the more brilliant; and the ordinary coloration
of the two sexes is reversed, as we have just
seen, in one species of Agrion. It is not probable
that their colours in any case have been gained
as a protection. Mr. MacLachlan, who has closely
attended to this family, writes to me that
dragon-flies—the tyrants of the insect-world—
are the least liable of any insect to be attacked
by birds or other enemies, and he believes that
their bright colours serve as a sexual attraction.
Certain dragon-flies apparently are attracted by
particular colours: Mr. Patterson observed (52.
'Transactions, Ent. Soc.' vol. i. 1836, p. lxxxi.)
that the Agrionidae, of which the males are
blue, settled in numbers on the blue float of a
fishing line; whilst two other species were attracted
by shining white colours.
It is an interesting fact, first noticed by Schelver,
that, in several genera belonging to two