The sunset activity of tsetse flies: a light threshold study on Glossina morsitans

ABSTRACT. At dusk, tsetses fly from their day-time resting sites on tree trunks to spend the night on leaves and twigs. At the end of the day in the laboratory, they show a few minutes of heightened activity which apparently represents this behaviour. This occurs immediately after lights-out in a square-wave LD cycle, but just before the end of a 30-min artificial 'dusk' which mimics the natural change in light intensity at sunset. The activity is triggered when the declining light falls to a mean value of c. 350 mW m^-2. Accurate, 24-h light measurements made in Zimbabwe show that in tsetse 'bush' this intensity occurs close to sunset. Neither the initial photophase light intensity (900 and 2500 mW m^-2 were tested) nor the rate of dimming affect the critical value, which is also the same whether arrived at within c. 10 min in a 'logarithmic dusk' or within c. 20 min in a 'linear dusk'. In newly-emerged or recently fed flies, however, it is lower ( c. 50–100 mW m^-2); and when a similar activity burst is induced by a 'dusk' at midday (i.e. at the flies' phase of minimum activity), the threshold is c. 200 m W m^-2. It is concluded that this short behavioural programme is primarily a direct response to a specific, low light intensity, but that the threshold is modified by circadian and other physiological inputs.