Fig. 3. Typical recording of a hyperoxic switch recording of Forelius mccooki. Ten ants, total mass 3.28 mg, at 20°C and 20 ml min^–1 STP. The recording begins and ends with baselines. Intermittent water excretion events can be seen near the start of the recording and at 60 min. The large dip in _CO2 that follows hyperoxia is plainly visible at 50 min. The drop in _H2O coincides with the drop in _CO2 and immediately precedes a small excretion event. We noticed that water excretion events often occurred 5–20 min after initiation of hyperoxia in all species. Because we used only the magnitude of the _H2O drop immediately coinciding with the _CO2 drop, these events, though annoying, usually did not interfere with our measurements. Following the introduction of nitrogen, a rapid rise in both _H2O and _CO2 are evident. _CO2 rapidly declines to near baseline levels, but _H2O starts to plateau at an intermediate level; see text.