Platyblemnus: Westwood, 'Modern Classification,' vol. i. p. 447.) is furnished with "a long membranous appendage, which falls over the face like a veil;" but what its use may be, is not known.
ORDER, NEUROPTERA.
Little need here be said, except as to colour. In the Ephemeridae the sexes often differ slightly in their obscure tints (49. B.D. Walsh, the 'Pseudo-neuroptera of Illinois,' in 'Proceedings of the Entomological Society of Philadelphia,' 1862, p. 361.); but it is not probable that the males are thus rendered attractive to the females. The Libellulidae, or dragon-flies, are ornamented with splendid green, blue, yellow, and vermilion metallic tints; and the sexes often differ. Thus, as Prof. Westwood remarks (50. 'Modern Classification,' vol. ii. p. 37.), the males of some of the Agrionidae, "are of a rich blue with black wings, whilst the females are fine green with colourless wings." But in Agrion Ramburii these colours are exactly reversed in the two sexes. (51. Walsh, ibid. p. 381. I am indebted to this naturalist for the following facts on Hetaerina, Anax, and Gomphus.) In the extensive N. American genus of Hetaerina, the males alone have a beautiful carmine spot at the base of each wing. In Anax junius the basal part of 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; for in the true Libellulae, when the sexes differ in colour, the females are orange or yellow; so that supposing Agrion to be descended from some primordial form which resembled the typical Libellulae in its sexual characters, it would not be surprising that a tendency to vary in this manner should occur in the females alone.