There are usually found in the month of June, especially near water, certain insects that are called Ephemera, and which long ago acquired true celebrity, and furnished material for comparison to poets and philosophers. Indeed, in the adult state they live but one day, a fact that has given them their name. They appear for a few hours, fluttering about in the rays of a sun whose setting they are not to see, as they live during the space of a single twilight only. These insects have very short antennae, an imperfect mouth incapable of taking food, and delicate, gauze like wings, the posterior ones of which are always small, or even rudimentary or wanting. Their legs are very delicate--the anterior ones very long--and their abdomen terminates in two or three long articulated filaments. One character, which is unique among insects, is peculiar to Ephemerids; the adults issuing from the pupal envelope undergo still another moult in divesting themselves of a thin pellicle that covers the body, wings, and other appendages. This is what is called the subimago, and precedes the imago or perfect state of the insect. The short life of adult May-flies is, with most of them, passed in a continual state of agitation.
They are seen rising vertically in a straight line, their long fore-legs stretched out like antennae, and serving to balance the posterior part of the body and the filaments of the abdomen during flight. On reaching a certain height they allow themselves to descend, stretching out while doing so their long wings and tail, which then serve as a parachute. Then a rapid working of these organs suddenly changes the direction of the motion, and they begin to ascend again. Coupling takes place during these aerial dances. Soon afterward the females approach the surface of the water and lay therein their eggs, spreading them out the while with the caudal filaments, or else depositing them all together in one mass that falls to the bottom.
These insects seek the light, and are attracted by an artificial one, describing concentric circles around it and finally falling into it and being burnt up. Their bodies on falling into the water constitute a food which is eagerly sought by fishes, and which is made use of by fishermen as a bait.
But the above is not the only state of Ephemerids, for their entire existence really lasts a year. Linnaeus has thus summed up the total life of these little creatures: "The larvae swim in water; and, in becoming winged insects, have only the shortest kind of joy, for they often celebrate in a single day their wedding, parturition, and funeral obsequies." The eggs, in fact, give birth to more or less elongated larvae, which are always provided with three filaments at the end of the abdomen, and which breathe the oxygen dissolved in the water by tracheo-branchiae along the sides of the body. They are carnivorous, and live on small animal prey. The most recent authors who have studied them are Mr. Eaton, in England, and Mr. Vayssiere, of the Faculte des Sciences, at Marseilles.
A propos of the larvae of Ephemera or May-flies, we must speak of one of the entomological rarities of France, the nature and zoological place of which it has taken more than a century to demonstrate. Geoffroy, the old historian of the insects of the vicinity of Paris, was the first to find in the waters of the Seine a small animal resembling one of the Daphnids. This animal has six short and slender thoracic legs, which terminate in a hook and are borne on the under side of the cephalic shield. This latter is provided above with two slender six-jointed antennae, two very large faceted eyes at the side, and three ocelli forming a triangle. The large thoraceo-abdominal shield is hollowed out behind into two movable valves which cover the first five segments of the abdomen (Fig. 1). The last four segments, of decreasing breadth, are retractile beneath the carapax, as is also the broad plume that terminates them, and which is formed of three short, transparent, and elegantly ciliated bristles. These are the locomotive organs of the animal, whose total length, with the segments of the tail expanded, does not exceed seven to eight millimeters. The animal is found in running waters, at a depth of from half a meter to a meter and a half.
It hides under stones of all sizes, and, as soon as it is touched, its first care is to fix itself by the breast to their rough surface, and then to swim off to a more quiet place. It fastens itself so firmly to the stone that it is necessary to pass a thin knife-blade under it in order to detach it.
FIG. 1.--LARVA OF MAY FLY. (Magnified 12 times.)
Geoffroy, because of the two large eyes, and without paying attention to the ocelli, named this larva the "feather-tailed binocle." C. Dumeril, in 1876, found it again in pools that formed after rains, and named the creature (which is of a bluish color passing to red) the "pisciform binocle." Since then, this larva has been found in the Seine at Point-du-Jour, Bas-Meudon, and between Epone and Mantes. Latreille, in 1832, decided it to be a crustacean, and named it Prosopistoma foliaceum. In September, 1868, the animal was found at Toulouse by Dr. E. Joly in the nearly dry Garonne. Finally, in 1880, Mr. Vayssiere met with it in abundance in the Rhone, near Avignon.
The abnormal existence of a six-legged crustacean occupied the attention of naturalists considerably. In 1869, Messrs. N. and E. Joly demonstrated that the famous "feather-tailed binocle" was the larva of an insect. They found in its mouth the buccal pieces of the Neuroptera, and, under the carapax, five pairs of branchial tufts attached to the segments that are invisible outwardly. Inside the animal were found tracheae, the digestic tube of an insect, and malpighian canals. Finally, in June, 1880, Mr. Vayssière was enabled to establish the fact definitely that the insect belonged among the Ephemerids. Two of the larvae that he raised in water became, from yellowish, gradually brown. Then they crawled up a stone partially out of water, the carapax gradually split, and the adults readily issued therefrom--the head first, then the legs, and finally the abdomen. At the same time, the wings, which were in three folds in the direction of their length, spread out in their definite form (Fig. 2). The insects finally flew away to alight at a distance from the water. The wings of the insect, which are of an iron gray, are covered with a down of fine hairs.
The posterior ones soon disappear.
FIG. 2.--MAY-FLY (adult magnified 14 times).
Perhaps the subimago in this genus of Ephemerids, as in certain others, is the permanent aerial state of the female.--La Nature.
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