The host-orientated behaviour of tsetse flies (Glossina): the interaction of visual and olfactory stimuli

ABSTRACT. Studies were made in Zimbabwe of the propensity of Glossina pallidipes Austen and G. morsitans morsitans Westwood to divert from flying upwind in plumes of host odour to various visual features (termed targets). Using various arrangements of electrocuting nets with targets placed downwind of an odour source it was found that 45% diverted to a square target, c. 30% diverted to a black vertical oblong and there was no significant diversion to a bark-coloured vertical oblong that simulated the bole of a tree. The relative propensity of tsetse to divert to variously coloured targets decreased in the order: black = blue > red > yellow; for different shapes it decreased in the order: circle > square > horizontal oblong = vertical oblong. Changes in the composition or concentration of the odour, or loss of contact with it, did not markedly affect the percentage that diverted. Tsetse that diverted to a target and subsequently flew away from it showed an upwind bias in the presence of odour. In the absence of odour there was a slight crosswind bias. If these crosswind fliers then flew into a plume of host odour they turned c. 50 upwind.