Fig. 451.—Larva (1) and pupa (2a) of Paraponyx pyropalis enlarged: st, stigmata.—After W. Müller.
Of coleopterous larvæ breathing by tracheal gills there are but few. The larva of Gyrinus (Fig. 454) respires by 10 pairs of slender, hairy abdominal gills similar to those of Corydalus, and the stigmata are entirely wanting. Somewhat similar are the tracheal gills of Hydrocharis caraboides. Hydrobius has shorter setose gills, our American species having seven pairs of short setose gills. It has two spiracles at the end of the body, through which the air is taken by thrusting the body out of the water. The larvæ of two other aquatic coleopterous genera, Pelobius and Cnemidotus, also have gills; those of the former situated at the base of the coxæ, and brush-like, but containing no tracheæ, though filled with blood, while those of Cnemidotus are very long, bristle-like, jointed, and arising from the dorsal side of the thoracic and abdominal segments. The stigmata are wanting. (Schiödte.)
The larva of the dipterous genus Tanypus respires by two caudal papilliform processes, in each of which a trachea ramifies.
Fig. 452.—Freshly hatched larva of Hydrobius: t, enlarged tracheæ, the heart between them; g1-g7, the seven pairs of gills. A, end of body, enlarged, showing the two terminal stigmata.—Emerton del.
Certain larvæ with both stigmata and tracheal gills are enabled either to live in or out of water or on the surface, as in the case of certain beetles (Cyphonidæ, Elmidæ, Hydrophilidæ, Fig. 452), or the larval mosquito and Psychodes (Fig. 455); also the nymphs of dragon-flies.
The larvæ of the Cyphonidæ (Helodes, Cyphon, Hydrocyphon) possess but a single pair of stigmata, situated in the penultimate abdominal segment, while at the end of the abdomen are delicate tracheal gills. The two main tracheal trunks are much swollen. When on the surface of the water the larva breathes through the stigmata situated near the end of the abdomen; when floating in the water, the larva, like that of Gyrinus, carries along at the end of its body a bubble of air. The gills are only of use, as Rolph thinks, when the insect is compelled to remain a long time under water.
The larva of our native Prionocyphon discoideus (Say) is described by Walsh as “vibrating vigorously up and down a pencil of hairs proceeding from a horizontal slit in the tail”; this pencil is composed “of three pairs of filaments, each beautifully bipectinate. I presume it is used to extract air from the water.” When the larva is at the surface the pencil of hairs touches the surface of the water, and occasionally a bubble of air is discharged from the tail. “The general habit is to crawl on decayed wood beneath the surface, occasionally swimming to the surface, probably for a fresh supply of air.” (Proc. Ent. Soc. Phil., i, p. 117.)
