The earlier stages of the life of Ephemeridae are, it is believed, in the case of all the species, aquatic. May-flies, indeed, during the period of their post-embryonic development are more modified for an aquatic life than any other Insects, and are provided with a complex apparatus of tracheal gills. The eggs are committed to the waters without any care or foresight on the part of the parent flies, thus the embryonic development is also aquatic; little, however, is known of it. According to Joly[[350]] the process in Palingenia virgo is slow. The larva on emerging from the egg has no respiratory system, neither could Joly detect any circulation or any nervous system. The creature on emergence is very like Campodea in form, possessing long antennae and tails—caudal setae. Owing to the organisation being inferior, the creature in its earlier stages is called a larvule; in its later stages it is usually spoken of as a nymph, but the term larva is also frequently applied to it. Soon the gills begin to appear in the form of small tubular caeca placed in the posterior and upper angles of the abdominal rings; in fifteen days the gills begin to assume their characteristic form, are penetrated by tracheae, and the circulation can be seen. The amount of growth accomplished after hatching between March and September is but small.
Fig. 276.—Nymph of Cloëon dipterum.[[351]] Wing-sheath of left side, gills of right side, removed; g, tracheal gills. (After Vayssière.)
Fig. 277.—Larvule of Cloëon dimidiatum. (After Lubbock.)
The metamorphosis of Cloëon has been described by Sir John Lubbock; he informs us that the young creature undergoes a constant and progressive development, going through a series of more than twenty moults, each accompanied by a slight change of form or structure. His observations were made on captured specimens, so that it is not certain that what he calls[[352]] the first stage is really such. He found no tracheae in the earliest stages; the small first rudiments of the gills became visible in the third stage, when there were no tracheae; the fourth instar possessed tracheae, and they could be seen in the gills. The wing rudiments could first be detected in the ninth and tenth stages. The changes of skin during the winter months are separated by longer intervals than those occurring at other periods of the year.
Fig. 278.—Adult nymph of Ephemera vulgata. (After Eaton.) Britain.
The nymphs differ greatly in the structure and arrangement of their tracheal gills, and display much variety in their general form and habits; some of them are very curious creatures. Pictet[[353]] divides them in accordance with their habits into four groups: (1) Fossorial larvae: these live in the banks of streams and excavate burrows for shelter; they are of cylindrical form, possess robust legs, abundant gills at the sides of the body, and frequently processes projecting forwards from the head: examples, Ephemera (Fig. 278) and Palingenia. (2) Flat larvae: these live attached to rocks, but run with rapidity when disturbed; they prefer rapid streams, have the breathing organs attached to the sides of the body and not reposing on the back; they are exclusively carnivorous, while the fossorial forms are believed to obtain their nutriment by eating mud: example, Baëtis. (3) Swimming larvae: elongate delicate creatures, with feeble legs, and with strongly ciliated caudal setae: example, Cloëon (Fig. 276). (4) Climbing larvae: these live in slowly-moving waters, especially such as have much slimy mud in suspension, and they have a habit of covering themselves with this mud sometimes to such an extent as to become concealed by it: example, Potamanthus.
