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. [Illustration: Fig. 391.--Larva of an Ephemera.] [Illustration: Fig. 392.--Pupa of an Ephemera] 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. The _Libellulas_, or Dragon-Flies, are insects of a well-defined type. The elegance of their shape, the grace of their movements, have won for them among the French their common appellation of "Demoiselles." They are always of largish size. Many are of bright and metallic colours, which are not inferior in beauty to those of butterflies. Their wings, of an extreme delicacy, always glossy and brilliant, present varied tints; sometimes they are completely transparent, and have all the colours of the rainbow. Often, the colour of the males differs from that of the females. They may be seen fluttering about on the water during the whole summer, especially when the sun is at its highest. They fly with extreme rapidity, skimming over the water at intervals, and escaping easily when one wishes to catch them. Nothing is prettier than a troop of dragon-flies taking their sport on the side of a pond or on the banks of a river, on a fine summer's day, when a burning sun causes their wings to shine with most vivid colours. [Illustration: Fig. 393.--Cloëon diptera.]