| Fig. 391.—Larva of an Ephemera. | Fig. 392.—Pupa of an Ephemera |
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.
Fig. 393.—Cloëon diptera.
In the perfect state, as well as in that of the larva and the pupa, the Libellulæ are carnivorous. Their rapid flight makes them expert hunters, and their enormous eyes embrace the whole horizon. They seize, while on the wing, flies and butterflies, and tear them to pieces immediately with their strong mandibles. Sometimes, the ardour of the chase leading them on far from the streams, they are met with in the fields. The female lays her eggs in the water, from which emerge larvæ which remind one somewhat of the form of the insect, only their body is more compact and their head flattened. The larvæ and pupæ inhabit the bottom of ponds and streams, where, keeping out of sight in the mud, they seek for insects, molluscs, small fish, &c. If any prey passes within their reach, they dart forward, like a spring, a very singular arm, which represents the under lip. It is a sort of animated mask, armed with strong jagged pincers and supported by strong joints, the which, taken together, is equal to the length of the body itself. This mask acts at the same time as a lip and an arm; it seizes the prey on its passage, and conveys it to the mouth. "When any aquatic insect approaches them at a time when they are in a humour for eating," says Charles de Geer, "they shoot the mask forward very suddenly and like a flash of lightning, and seize the insect between their two pincers; then, drawing back the mask, they bring the prey up to their mandibles, and begin to eat it. I have remarked that they do not spare those of their own kind, but that they eat each other up when they can, and I have also seen them devouring very small fish which I put by them. It is very difficult for other insects to avoid their blows, because, walking along generally in the water very gently, and, as it were, with measured steps—almost in the same way a cat does on the look-out for birds—they suddenly dart forward their mask and seize their prey instantaneously." [118] [Fig. 394] represents, to the left, the larva of the dragon-fly, with the instrument of attack which we have called a "mask," and which it is making use of for seizing a small insect; on the right, the adult dragon-fly coming out of the nymph.
Fig. 394.—Larva of the Libellula, and the perfect insect emerging.
The respiration of these larvæ is very singular. Their abdomen is terminated by appendages, which they open to allow the water to penetrate into the digestive tube, the sides of which are furnished with gills communicating with the tracheæ. The water, deprived of oxygen, is then thrown out, and the larva advances thus in the water by the recoil. It has no tufts of external lateral gills, which in the case of the Ephemeræ do the duty of fins. The pupa already presents stumps of wings. To effect its metamorphosis it drags itself out of the water, where it has lived for nearly a year, climbs slowly to some neighbouring plant, and hangs itself there. Very soon the sun dries and hardens its skin, which all of a sudden becomes crisp, and cracks. The dragon-fly then sets free its head and its thorax, and its legs; its wings, still soft and wanting in vigour, gain strength by coming in contact with the air, and, after a few hours, they have attained their full development. Immediately the insect abandons, like a worn-out suit, the dull slimy skin which had covered it so long, and which still preserves its shape ([Fig. 394]), and dashes off in quest of prey.