Flight behaviour of tsetse flies in host odour plumes: the initial response to leaving or entering odour

ABSTRACT. Free‐flying, wild male and female Glossina pallidipes Aust. and G. m. morsitans Westw. were video‐recorded in the field in Zimbabwe as they entered or left the side of a host‐odour plume in cross‐wind flight, or as they overshot a source of host odour in upwind flight (camera 2.5 m up looking down at a 3 times 2.5 m field of view at ground level). 80% of cross‐wind odour leavers turned sharply (x̄ turns 95o), but without regard to wind direction (overshooters behaved essentially the same except that nearly 100% turned). Many fewer flies entering a plume cross wind turned (c. 60%), and when they did they made much smaller turns (x̄ 58o); these turns were, however, significantly biassed upwind (c. 70%). All three classes of fly had similar groundspeeds (x̄ 5.5–6.5 m s_1) and angular velocities (x̄ 350–400o s‐1). Clear evidence was obtained of in‐flight sensitivity to wind direction: significantly more flies entering odour turned upwind than downwind, and odour losers turning upwind made significantly larger turns than average. The main basis for the different sizes of turn was the different durations of the turning flight, rather than changes in angular velocity or speed. No evidence was found of flies landing after losing contact with odour.