Fig. 279.—Nymph of Oligoneuria garumnica, France. g2 and g7, two of the dorsal tracheal gills. (After Vayssière.)
The anatomy of the nymphs has been treated by Vayssière,[[354]] who arranges them in five groups in accordance with the conditions of the tracheal gills: (1) The gills are of large size, are exposed and furnished at the sides with respiratory fringes: example, Ephemera (Fig. 278). (2) The branchiae are blade-like, not fringed, and are exposed at the sides of the body: example, Cloëon (Fig. 276). (3) The respiratory tubes are placed on the under surface of plates whose upper surface is not respiratory: example, Oligoneuria garumnica (Fig. 279). (4) The anterior gill is modified to form a plate that covers the others: example, Tricorythus (Fig. 282, B). (5) The gills are concealed in a respiratory chamber: example, Prosopistoma (Fig. 280). The last of these nymphs is more completely adapted for an aquatic life than any other Insect at present known; it was for long supposed to be a Crustacean, but it has now been shown to be the early stage of a may-fly, the sub-imago having been reared from the nymph. The carapace by which the larger part of the body is covered is formed by the union of the pro- and meso-thorax with the sheaths of the anterior wings, which have an unusually extensive development; under the carapace there is a respiratory chamber, the floor and sides of which are formed by the posterior wing-sheaths, and by a large plate composed of the united nota of the metathorax and the first six abdominal segments. In this chamber there are placed five pairs of tracheal gills; entrance of water to the chamber is effected by two laterally-placed orifices, and exit by a single dorsal aperture. These nymphs use the body as a sucker, and so adhere strongly to stones under water. When detached they swim rapidly by means of their caudal setae; the form of these latter organs is different from that of other Ephemerid nymphs. This point and other details of the anatomy of this creature have been described in detail by Vayssière.[[355]] These nymphs have a very highly developed tracheal system; they live in rapid watercourses attached to stones at a depth of three to six inches or more under the water. Species of Prosopistoma occur in Europe, Madagascar, and West Africa.
Fig. 280.—Prosopistoma punctifrons, nymph. France. (After Vayssière.) o, Orifice of exit from respiratory chamber.
According to Eaton,[[356]] in the nymphs of some Ephemeridae the rectum serves, to a certain extent, as a respiratory agent; he considers that water is admitted to it and expelled after the manner we have described in Odonata, p. [421].
Fig. 281.—A, Last three abdominal segments and bases of the three caudal processes of Cloëon dipterum: r, dorsal vessel; kl, ostia thereof; k, special terminal chamber of the dorsal vessel with its entrance a; b, blood-vessel of the left caudal process; B, twenty-sixth joint of the left caudal process from below; b, a portion of the blood-vessel; o, orifice in the latter. (After Zimmermann.)
The internal anatomy of the nymphs of Ephemeridae shows some points of extreme interest. The long caudal setae are respiratory organs of a kind that is almost if not quite without parallel in the other divisions of Insecta. The dorsal vessel for the circulation of the blood is elongate, and its chambers are arranged one to each segment of the body. It drives the blood forwards in the usual manner, but the posterior chamber possesses three blood-vessels, one of which is prolonged into each caudal seta. This terminal chamber is so arranged as to drive the blood backwards into the vessels of the setae; on the under surface of the vessels there are oval orifices by which the blood escapes into the cavity of the seta so as to be submitted to the action of the surrounding medium for some of the purposes of respiration. This structure has been described by Zimmermann,[[357]] who agrees with Creutzberg[[358]] that the organ by which the blood is propelled into the setae is a terminal chamber of the dorsal vessel; Verlooren,[[359]] who first observed this accessory system of circulation, thought the contractile chamber was quite separate from the heart. The nature of the connexion between this terminal chamber that drives the blood backwards and the other chambers that propel the fluid forwards appears still to want elucidation.
Fig. 282.—A, Nymph of Ephemerella ignita with gills of left side removed; g, gills: B, nymph of Tricorythus sp. with gill cover of right side removed; g.c, gill cover; g, g′, gills. (After Vayssière.)