Fig. 387.—Perla bicaudata.
| Fig. 388.—Nemoura variegata. | Fig. 388.—Nemoura variegata (larva). |
The Ephemeridæ, or May-Fly family, have long, slender bodies, provided with two or three long silky hairs. Their name indicates the short duration of their existence. They appear in great numbers at certain seasons of the year. Their hatching takes place at sunset; they have coupled and laid their eggs by sunrise next day, and have ceased to live; so that the banks of rivers, of ponds, of lakes, are strewed with their bodies. Their number is sometimes so considerable that, according to Réaumur, the soil seems as if it were covered with snow, and they are gathered up for manure. The common Ephemera, or May-Fly (Ephemera vulgata, [Fig. 390]), is of a brown colour, banded with yellow, and the wings smoky, with brown spots. These insects are remarkable for their elegant flight; they are continually rising and falling. When they move their wings they rise; but if their wings, though spread out, remain motionless, as also the silky hairs which form their tail, they fall again. They may be seen in myriads in places where there is much water.
Fig. 390.
Ephemera vulgata, imago.
We have said that the Ephemeræ live only for a few hours. This is the general rule; but their existence can be prolonged for ten or fifteen days by preventing their copulation. If, however, the duration of the life of these insects is so short when they have reached the perfect state, and when the conformation of the mouth prevents them from taking any nourishment, their larvæ state is of very long continuance. Swammerdam says, in his curious Memoir, entitled "Vita Ephemeri," it is not less than three years.
The females lay their eggs in one single mass, and let them fall into the water, in the form of a packet. The larvæ which come out of them are very active, and swim with great ease; but generally conceal themselves under the pebbles at the bottom. The sides of their abdomen are provided with gills, very much fringed, which serve them, not only for breathing the air under the water in the same way that fish do, but also for swimming. The larvæ have, at the extremity of their body, two or three hairs, like the perfect insect. They hollow out galleries in the beds of rivers and ponds, and live on small insects. The pupa ([Fig. 392]) differs only from the larva ([Fig. 391]) in having the rudiments of wings. When about to undergo their metamorphosis, they come out of the water and cling to plants, &c. The skin cracks on the back when it is dry, and there comes out a heavy insect, which flies feebly, and has opaque wings. It is still enveloped in a very thin skin, of which a last moult, after a few hours, frees it. This skin remains sticking to the plant on which the moulting was effected, preserving the shape of the insect. This moult is peculiar to the Ephemeræ; it is the transition from the false imago (pseudo-imago) to the imago.
In the same family is the genus Cloëon, whose larvæ prey on minute insects. The Cloëon diptera ([Fig. 393]), which has only two wings, is often to be met with in houses, resting on the window panes and curtains. All these insects keep badly in collections; they lose their shape, and their members are so fragile that the least shock suffices to break them.