Note 60 — Chapter V · EVOLUTION.

This beautiful and transparent larva reminds one in many respects of the Pike as it poises itself in the water awaiting its prey. It is enabled to do so without the slightest exertion by the air-bladders which it possesses in the two kidney-shaped rudiments of tracheæ, and which in the gnat become developed into the respiratory apparatus. The resemblance to the air-bladder of fishes is not simply that it serves a similar purpose of sustaining the body in the water, it is in both cases a rudiment of the respiratory apparatus, which in the fish never becomes developed. Weismann calls attention to an organ in the larvæ of certain insects (the Culicidæ), which have what he calls a tracheal gill, which gill has this striking analogy with the fish-gill that it separates the air from the water, and not, as a trachea, direct from the atmosphere. See his remarkable memoir Die nachembryonale Entwickelung des Muscidens, in Siebold und Kölliker’s Zeitschrift, 1864, p. 223.