Fig. 2. Typical recording of a hyperoxic switch recording of Drosophila melanogaster. Eleven flies, total mass 9.41 mg, at 20°C and 40 ml min^–1 STP. The recording begins and ends with baselines. Two water excretion events can be seen at 20–30 min into the recording. The large dip in _CO2 that follows hyperoxia is plainly visible at 65 min. A modest drop in _H2O coincides with the drop in _CO2. To minimize the influence of long-term drift on our measurements, we used only the magnitude of the _H2O drop immediately coinciding with the _CO2 drop. Following the introduction of nitrogen, a rapid rise in both _H2O and _CO2 are evident. _CO2 rapidly declines to near baseline levels, but _H2O plateaus at an intermediate level; see text.