Among other Neuroptera which undergo incomplete metamorphoses, we may mention, first, the genera _Perla_ and _Nemoura_,[117] (Figs. 384, 385, and 386), which flutter about the banks of rivers, and settle on stones, shrubs, and aquatic plants. Their larvæ are naked, without cases, and always live in the water, hiding themselves under stones, to watch for small insects, for they are carnivorous. One sees them often balancing their bodies, holding on to a pebble. They go through the winter, and only become pupæ in the spring. After moulting, they have the rudiments of wings. Very soon afterwards the pupæ leave the water, and undergo their metamorphosis. The adult lives only a few days, for its mouth is not suited for receiving food. The larvæ have, at the end of their bodies, two long threads, which remain in the perfect _Perla_, but not in the perfect _Nemoura_; the latter lose the two caudal hairs when they arrive at the adult state. One species of _Perla_ is very common on the quays of Paris. [117] From [Greek: nêma], a thread; and [Greek: oura], a tail.--ED. [Illustration: Fig. 387.--Perla bicaudata.] [Illustration: Fig. 388.--Nemoura variegata.] [Illustration: Fig. 389.--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. [Illustration: 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.