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Abstract1. The method of dual quantification was used to study the effect of courtship of both receptive and non-receptive females on the subsequent behaviour of the male Mormoniella vitripennis. 2. The male's responsiveness to successive non-receptive females waned when the time between presentations was short. The extent of this waning was less with longer time intervals. 3. When many females were presented to a male one after another the male courted almost all of them if they were receptive females but only a few if they were non-receptive females. 4. A single courtship of either a receptive or a non-receptive female had a similar effect on the male's subsequent behaviour and recovery occurred in a similar way. 5. Courtship of 20 non-receptive females reduced the male's response to further females more than did courtship of 20 receptive females. 6. The significance of these observations is discussed with reference to the use of dummy animals and to the recent ethological concepts of reaction specific energy, motivational impulses, specific action potentiality and consummatory act. 7. An endogenous central nervous influence on the male's readiness to respond is postulated. Courtship has a short-term response-specific effect (receptive or non-receptive females) and an inhibitory stimulus-specific effect (non-receptive females). With receptive females the inhibitory effect is absent and/or mating has an excitatory effect. The stimuli provided by a receptive female must direct nervous activity rather than release a limited amount of stored energy.

Female tsetse flies (Glossina morsitans Westw.) were more active in an experimental chamber in the laboratory: (a) at high than at low temperatures, (b) in high than in low humidities, and except at high temperatures, (c) when hungry than when replete. Flies were also more active in the chamber when a small area of black surface (1·6%) was present than when a larger black surface (12·8% or 50%) was available. Both black and white surfaces were provided in all experiments, and over the temperature range 25–40°C flies settled on black more frequently than would be expected by chance. This preference for a black resting surface was most marked when the flies were most active: (a) above 35°C in all experiments, (b) at high humidities with teneral flies, and (c) when the flies had not fed. Flies settling on black usually rested longer than those settling on a white surface.