Fig. 441.—Nymph of Prosopistoma punctifrons: o, upper orifice of the respiratory chamber.—After Vayssière.
Fig. 442.—Filamentous tracheal gill and part of a trachea of Pteronarcys.—After Newport from Sharp.
Finally, in the highly modified forms Bætisca and Prosopistoma the tracheal gills are entirely concealed and protected by mesothoracic projections so as to form a true respiratory chamber, to which the water has access either by an opening behind, as in Bætisca, or by three openings, two ventral and one dorsal (Fig. 441), as in Prosopistoma.
The slender cylindrical tracheal gills of Heptagenia in the third or fourth nymphal stage are 2–jointed, and the first abdominal pair in Cænis are said by Palmén to be finger-shaped and 2–jointed. In Polymitarcys virgo the gills do not appear until the eighth or tenth day after hatching.
Dewitz found that young nymphs of Ephemerids will well endure the amputation of their gills, while fully grown ones die. Amputation of the lateral gills hastens ecdysis. After the change of skin, the gills are smaller than before, and at first contain no tracheæ, but in a few weeks they develop as completely as in normal individuals. The caudal gills were also renewed.
Fig. 443.—A, larva of Sisyra, enlarged. B, one of the hinder gills, with its tracheæ.—After Westwood, from Sharp. C, a gill, showing the branched tracheæ.—After Grube.
In the nymphs of Perlidæ the tracheal gills are usually present, and are either foliaceous (Nemoura) or more commonly filamentous in shape (Fig. 442). They are situated either on the prosternum (Nemoura and Pteronarcys), or on each side of the thorax, or on the sides of the abdomen, or are restricted to a tuft on each side of the anus at the base of the caudal stylets (Pteronarcys and Perla). Unlike the Ephemeridæ the gills persist in certain genera throughout life.