But now let us consider by what characteristics the reader is to recognise his Libellulæ.
The eyes are very large, the size of the insect considered; of a brown colour, and nearly joined together at the top of the head; in front of this point of junction a tiny vesicle is visible, carrying three ocelli (i.e., little, simple, glossy eyes), two on each side, and the third on the anterior margin: the thorax is large, hairy, and composed of two yellow plates; the abdomen laterally depressed, in such a manner as to give great prominence to the medial line. This last and well-marked feature it is which has procured for the Eleanora the scientific name of Libellula depressa,—a name proposed by Linnæus, and unanimously adopted; such unanimity being a rare occurrence among naturalists, though, to parody a phrase of Sheridan's, when naturalists do agree, it is something wonderful!
In the male Libellulæ, the upper surface of the abdomen is bluish in hue, and covered as it were with an ashy dust, while in the female it is olive; in both sexes the first and last abdominal segment are of a deeper shade than the other segments. The feet are black, and bristle all over with stiff hairs; the thighs are of a brownish red. The two pairs of wings, each strongly reticulated, present, towards the extremity of the upper border, a black rectangular spot; their base, moreover, is edged with brown spots; the spots of the lower pair are triangular and larger than those of the upper pair, which are nearly linear. They form, to a certain extent, the reservoir of the liquid which nourishes and maintains the circulation of the network of the wings.
The Libellula, which Geoffroy calls (in his "Histoire des Insectes") the Philinta, is simply the male of the species we have just been describing. You may see them
"Where water-lilies mount their snowy buds,"
pursuing one another with an abrupt, jerking—I had almost said staccato—flight.
A species less common than the preceding, but closely resembling it, is the Françoise of Geoffroy, the Libellula quadrimaculata of Linnæus. It owes its descriptive or specific designation to the colours which diversify its wings. On the outer edge of each, two brown marginal spots are conspicuous: the first at the place where in the Eleanora (and other species) the black spot is found, and the second nearly in the centre of the external border, which, at this point, is considerably compressed; moreover, the lower wings are marked, beneath their yellow base, with a kind of triangular spot of blackish brown, finely reticulated with yellow. Externally, there is no difference of appearance between male and female, except that the abdomen of the latter is somewhat the larger.
Now we come to the Sylvia, the Libellula cancellata of Linnæus. Its eyes and thorax wear a greenish hue; the diaphanous wings are spotted with brown near the outer margin; the abdomen is of a bluish-gray; the extremity of the sixth segment, and the following segments, are wholly black.