Chasing behaviour of houseflies (Fannia canicularis)

Summary1.Chases in which male flies (Fannia canicularis) pursue other flies were studied by filming such encounters from directly below. Males will start to chase whenever a second fly comes within 10–15 cm (Fig. 3).2.Throughout these chases there was a continuous relationship between the angle (θe) made by the leading fly and the direction of flight of the chasing fly, and the angular velocity of the chasing fly (ωf). This relation was approximately linear, with a slope of 20 ° s−1 per degree θe (Figs. 4–7).3.The maximum correlation between ωf and θite occurs after a lag of approximately 30 ms, which represents the total delay in the system (Fig. 8).4.In the region close to the chasing fly's axis (θite less than about 35 °) a second mechanism exists in which the angular velocity of the chasing fly (ωf) is controlled by the relative angular velocity of the leading fly (ωe), rather than its relative position. The ratio of ωf to ωe in this region is approximately 0.7.5.Using the results in 2–4 above, and an empirically determined relation between the angular and forward velocities of the chasing fly, it was possible to simulate the flight path of the chasing fly, given that of the leading fly (Fig. 11). Because these simulations predict correctly the manoeuvres and outcomes of quite complicated chases, it is concluded that the control system actually used by the fly is accurately described by conclusions 2–4.6.The physiological implications of this behaviour, and the possible function of chasing, are discussed.