Resting insects are extremely tolerant of hypoxia. However, oxygen requirements increase dramatically during flight. Does the critical atmospheric P O 2 ( P c) increase strongly during flight, or does increased tracheal conductance allow even flying insects to possess large safety margins for oxygen delivery? We tested the effect of P O 2 on resting and flying CO 2 emission, as well as on flight behavior and vertical force production in flying locusts, Schistocerca americana. The P c for CO 2 emission of resting animals was less than 1 kPa, similar to prior studies. The P c for flight bout duration was between 10 and 21 kPa, the P c for vertical force production was between 3 and 5 kPa, and the P c for CO 2 emission was between 10 and 21 kPa. Our study suggests that the P c for steady-state oxygen consumption is between 10 and 21 kPa (much higher than for resting animals), and that tracheal oxygen stores allowed brief flights in 5 and 10 kPa P O 2 atmospheres to occur. Thus, P c values strongly increased during flight, consistent with the hypothesis that the excess oxygen delivery capacity observed in resting insects is substantially reduced during flight.