the abdomen in the male is a vivid ultramarine blue, and in the female
grass-green. In the allied genus Gomphus, on the other hand, and in
some other genera, the sexes differ but little in colour. In
closely-allied forms throughout the animal kingdom, similar cases of
the sexes differing greatly, or very little, or not at all, are of
frequent occurrence. Although there is so wide a 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 sub-families, the males on first emergence from
the pupal state, are coloured exactly like the females; but that their
bodies in a short time assume a conspicuous milky-blue tint, owing to
the exudation of a kind of oil, soluble in ether and alcohol. Mr.
MacLachlan believes that in the male of Libellula depressa this change
of colour does not occur until nearly a fortnight after the
metamorphosis, when the sexes are ready to pair.
Certain species of Neurothemis present, according to Brauer (53. See
abstract in the Zoological Record for 1867, p. 450.), a curious case
of dimorphism, some of the females having ordinary wings, whilst others
have them very richly netted, as in the males of the same species.
Brauer explains the phenomenon on Darwinian principles by the
supposition that the close netting of the veins is a secondary sexual
character in the males, which has been abruptly transferred to some of
the females, instead of, as generally occurs, to all of them. Mr.
MacLachlan informs me of another instance of dimorphism in several
species of Agrion, in which some individuals are of an orange colour,
and these are invariably females. This is probably a case of reversion;
320