The respiratory exchange of mature males of the Desert Locust Schistocerca gregaria (Forskål) has been studied during tethered flight in a small container and compared with the exchange before and after flights of varying duration. All determinations were based on gas analyses, and so the CO2 output and the O2 uptake were determined simultaneously. The accuracy of the analytical procedure has been discussed and the determinations of the R.Q. before and during flight found to be valid. The figures of the oxygen consumption are reduced to N.T.P.

1. During rest at 27-30°C the oxygen consumption amounted to 0.63 l. O2/kg./hr., which means that an average male of 1.8 g. consumed 18 O2 per min. During flight, however, the consumption increased fifteen to fifty times, the corresponding figures being 10-30 l. O2/kg./hr. or 300-900 O2 per min. in an average male.

2. After even a short period (10 min.) of flight in a roundabout or in front of a wind tunnel a distinct ‘oxygen debt’ was demonstrated. After prolonged flight (90-195 min.) the ‘debt’ was nearly doubled. It corresponded to 0.3-0.7 l. O2/kg. or to the oxygen consumption during only 0.5-1.5 min. of flight, so that anaerobic processes could not amount to much. It was characteristic that the recovery lasted at least 1 hr.

3. The R.Q. in resting animals averaged 0.82, and during the first 30 min. of flight the same value was obtained, but during the following 60 min. a statistically significant decrease of the R.Q. could be demonstrated, the average value in this period being 0.75. This unexpected result strongly indicates that, unlike other insects investigated so far, locusts utilize mainly fat as a source of energy during sustained flight.

4. It is suggested that a surplus of ketone bodies caused by the intensive breakdown of fats might explain the prolonged recovery of 1-2 hr. duration in Schistocerca, the recovery after flight in Drosophila which fly on carbohydrate lasting only 2 min. or less in spite of the same relative rate of combustion.