Chasing and pursuit in the dolichopodid fly Poecilobothrus nobilitatus

Abstract1.Male Poecilobothrus nobilitatus show two distinct kinds of pursuit. Females are “shadowed” at a distance of a few cm, using both rotational and lateral movements. Other males are chased, in a pursuit that involves only rotation and fast forward flight. The rotational component of pursuit appears to have the same control system in both types of tracking, and it is best described as a continuous translation of the error angle between the direction of the target and the pursuing fly's body axis into the pursuing fly's angular velocity. The constant of proportionality is 30–40°·s−1 per degree, and the delay in the system is about 15 ms. Pursuit on the ground is 2–3 times slower than in flight, although the delay seems to be similar.2.Attempts were made to see whether the aerial pursuits could be modelled effectively by a saccadic or discontinuous control system, as suggested for Musca pursuit (Wagner 1986). It was found that the velocity profiles of the chases could be fitted by an overlapping series of plausible saccade-like events, However, the correlation between visual information (error angle and error angular velocity) available just before each fictive saccade correlated poorly with saccade peak velocity. It is thus concluded that Poedlobothrus pursuit is basically continuous in nature, but it is argued that the two types of mechanism are hard to distinguish in natural behaviour.