We studied whether oxygen uptake from the surrounding water might enhance survival in submerged third instar larvae of Phaeoxantha klugii, a tiger beetle from the central Amazonian floodplains. Local oxygen partial pressures (Po₂) were measured with microcoaxial needle electrodes close to larvae submerged in initially air-saturated still water. The Po₂ profiles showed that the larvae exploit oxygen from the aquatic medium. Metabolism in the air of more or less resting larvae was determined by measuring the rate of CO₂ production (s(V) over dot CO₂) with an infrared gas analyzer at 29 degreesC. The s(V) over dot Co₂ was around 1.8 μL g⁻¹ min⁻¹, equivalent to an oxygen consumption rate (s(V) over dot O₂) of 1.8-2.6 μL g⁻¹ min⁻¹. Oxygen consumption ((V) over dot O₂) of individually submerged larvae measured in closed respiration chambers at 19 - 10.3 kPa Po₂ (initially air saturated, 29 degreesC) ranged between 0.05 and 0.2 μL min⁻¹ and was not correlated with body mass. The s(V) over dot O₂ ranged between 0.1 and 0.4 μL min⁻¹. that is, 4% - 22% of the metabolic rate measured in air. Mean (V) over dot o₂ decreased with declining Po₂; however, some individuals showed contrary patterns. (V) over dot o(2) was additionally measured in dormant larvae, in larvae submerged for 1 - 2 d in open water or for 30 - 49 d within sediment, as well as in larvae exposed to anoxia before the measurements. The range of (V) over dot O₂ was similar in all groups, indicating that the larvae exploit oxygen from the water whenever available. Similar (V) over dot o₂ across the whole range of body mass investigated (0.31 - 0.76 g) suggests that oxygen uptake occurs by spiracular uptake. Assuming that larvae survive for some time at rates comparable to depressed metabolic rates reported for other insect species, it can be concluded that oxygen uptake from water can sustain aerobic metabolism even under quite severe hypoxia. It might therefore play an important role for survival during inundation periods
