Fig. 437.—A, nymph of Ephemerella ignita, with gills of left side removed; g, gills. B, nymph of Tricorythrus (sp), with gill-cover of right side removed; gc, gill-cover; g, g′, gills.—After Vayssière.
Fig. 438.—Left maxilla of Jolia weselii, with the cephalic tracheal gill (h) inserted at the base on the under side.—After Vayssière.
Palmén has shown that these tracheal gills, as he calls them, are not developed on the same segments as the stigmata, and that the two structures have no genetic connection with each other. It is evident that these gills are secondary, adaptive organs.
In some cases (see p. 475) the tracheæ are wanting, but as such gills are filled with blood, the air contained in the water must pass in through their delicate walls.
In the Plectoptera (Ephemeridæ) the tracheal gills are either foliaceous or filamentous; when foliaceous they form simple or double leaves, with or without branches, or with a fringe of tubules, or under the leaf-like cover-bearing tufts of filaments. They are situated on the (usually) basal seven abdominal segments, at their hinder edge (Figs. 435, 436). In Oligoneuria and Jolia a pair occurs on the under side of the head, attached to the maxillæ, while in Jolia there is a pair on the under side of the first thoracic segment at the insertion of each of the legs. In certain genera (Heptagenia, Oligoneuria, and Jolia), they are in the form of a flat cover, under which lies a tuft of respiratory tubes, or (Ephemerella) a small bifid cluster of very delicate leaves (Fig. 437, A). In Cœnis and Tricorythus the tracheal gills of the second pair are modified to form plates covering all the succeeding pairs, those of the first pair being nearly atrophied and well-nigh functionless. (Fig. 437, B.)
Fig. 439.—Inner side of a gill-cover of the first pair, of Ephemerella, with the tracheal gills.—After Vayssière.
Fig. 440.—Nymph of Bætisca: III, section of abdomen; a, gills; b, flap; 1–9, abdominal segments.—After Walsh.