O2 consumption was measured in 5th and 6th instar larvae of S. exempta (Walker) (Lepidoptera, Noctuidae) at 25.degree. C using Warburg manometry. The mean rate of O2 consumption while feeding on maize or sorghum leaves was approximately twice that at rest. The increment in O2 consumption attributable to feeding was 583 .+-. 207 .mu.l g-1 h-1 (mean .+-. 95% confidence limits) or, for maize, 6.1 .+-. 3.5 .mu.l O2/mg leaf eaten.