Changing visual responsiveness in pregnant and larvipositing tsetse flies, Glossina morsitans

Abstract

ABSTRACT. Direct observations on the spontaneous activity and visually‐evoked responses of pregnant female Glossina morsitans morsitans Westw. were made hourly during the photophase over the 4 days around larviposition. Visual stimulation was by a 5d̀‐wide vertical black stripe moving laterally, 4 times min‐1 at c. 5d̀ s‐1. The numbers of take‐offs and the numbers of females taking off were recorded, both in response to the stimulus and spontaneously in its absence. The visual responsiveness paralled both the day‐to‐day changes and the altered circadian patterns of spontaneous activity reported previously (Brady & Gibson, 1983), being very low for the 48 h before parturition. It did not, however, rise in parallel with the increase in spontaneous activity during the last 3–4 h before parturition, and the proportion of flights oriented towards the moving stripe was also much reduced at that time. The significance of this changing responsiveness to slow movement is discussed, and its potential enhancement of any behavioural avoidance of aerosol, non‐residual insecticide spraying during late pregnancy is noted.