Tracheal volume in the pupa on the saturniid moth Hyalophora cecropia determined with inert gases

Abstract

Tracheal volume (Vtr) was measured in pupae of the Giant silkworm moth Hyalophora cecropia (Saturniidae, Lepidoptera, Insecta) using inert gas wash-out techniques. The animal was placed in a small vessel that was continuously ventilated (rate V̊) by a gas mixture containing 20% in N 2; the inflowing (FI i) and outflowing gas fractions (F e) of the vessel could be continuously measured by a respiratory mas spectrometer. At the onset of a spiracular constriction period, which was evidenced from the F e CO2 emerging from the animal, M N2, was calculated from V̊ and the difference (F E−F I) N2 . V tr was calculated from M N2 and the N 2 concentration in the tracheal system before constriction (assumed to equal that in the ventilating gas before replacement by Ar). Measurements were repeated with N 2 and Ar replacing each other. V tr averaged 48 μl·g −1 (range 39 to 59) for animals of 5.8 g average body weight (range 3.4 to 9.9), when inert gas solubility in body fluids was accounted for. Both size and stage in pupal development appear to affect V Tr. These values show reasonable agreement with literature data, mostly obtained by emptying the tracheal gas space by mechanical compression.