Fig. 453.—Larva of Psephenus, enlarged.
Fig. 454.—Larva of Gyrinus.—After Westwood.
The larvæ of the small water beetles of the family Elmidæ (Elmis, Potamophilus, Macronychus, and Psephenus) have similar habits. That of Elmis has ten dorsally situated pairs of spiracles, and on the end of the body bushy gills which are protruded at pleasure. The young larva is without spiracles, its tracheal system being closed. Macronychus and Potamophilus have similar habits. In the larva of the latter genus, which has nine pairs of spiracles, there are at the end of the body on each side three tufts of thread-like gills which are connected with the two main horizontal tracheæ, while the branches of the abdominal tracheæ are dilated into numerous (64) bladder-like sacs. The larva usually breathes through the caudal gills. When the water is low or dried up, the air is inhaled directly through the spiracles. (Kolbe.)
The larva of Psephenus lecontei, by its broad hemispherical body, is adapted to adhere to the smooth surface of rounded stones, in which situation we have found it. Although it is said by Rolph to have two pairs of spiracles, one pair on the mesothoracic and the other on the 1st abdominal segment, it probably rarely rises to the surface to breathe the air direct.
Fig. 455.—End of body of a Psychodes larva: A, end of body of a young, freshly moulted larva, side view: a, the three anal gills; b, the left air-cavity. B, older larva of the same species, with the open air-cavity seen from above. C, end of larva of another species as it goes down into the water with a bubble of air, b, between the crown of hairs of the air-cavity or tube: a, the two pairs of anal gills; b, the two main tracheæ.—After F. Müller.
It possesses five pairs of gills on the under side of the 2d to the 6th abdominal segments. Each gill has finger-shaped processes on its hinder edge, which are “from their constant motion evidently connected with respiration.” Tracheæ may be seen, according to H. J. Clark, entering the gills, and “the circulation of water among the branchiæ is kept up by the flapping of the tail-pieces.” The larva of Helichus fastigiatus is said by Leconte to be “very nearly allied, while the remotely allied Stenelmis crenatus has no external branchiæ.”[[75]]
The larva of the mosquito also has two modes of respiration, breathing either at the surface of the water through the two spiracles situated on the projection (siphon) at the hinder end of the body which is thrust out into the air; or when at the bottom respiring by tracheal gills. The pupa also has a double mode of respiration, either taking in air at the surface by the two thoracic horns with stigmatic openings, or when submerged using its tracheal gills.
Besides its long caudal tracheal air-tubes, the larval Eristalis is said by Chun to thrust out from the anus a number (20) of short tracheal filaments which float about in the water and serve to absorb the air.