FLYING MATE DETECTION AND CHASING BY TSETSE-FLIES (GLOSSINA)

Male tsetse flies, probably Glossina morsitans morsitans Westw., were video-recorded in the field as they took off and chased other tsetse flies. Chasers responded (took off) to a target fly at a maximum distance of c. 55 cm, when it subtended c. 1.6-degrees to their eye (congruent-to 1 foveal ommatidial subtense). Chased targets were always within this range (mean subtense at take-off = 3.2-degrees) and approaching the chaser. The most significant difference between chased and non-chased targets was in the rate of approach of the target fly in terms of the increase in its image size immediately before the chaser took off (xBAR 21-degrees s-1), especially as its relative increase (xBAR 690% s-1, P < 0.005). No feature of the target's translational velocity, nor any relationship between that and the image size approached this level of significance. Chasers seemed to 'slipstream' their target at c. 20 cm directly behind it, perhaps suggesting target identification by speed matching. Chases were apparently abandoned when the target image shrank from covering at least two of the chaser's foveal ommatidia to covering only one. Parallax-free measurements of flight speeds indicated a preferred, stable mean groundspeed of 4.8 +/- 0.1 m s-1 (SE), at a mean wing-beat frequency of 209 +/- 3 Hz.