Sexual dimorphism in the visual system of flies: The free flight behaviour of male bibionidae (Diptera)

Summary

• 1. The free flight behaviour of swarming male Bibionidae (Diptera) has been filmed in the field with two cameras simultaneously at 50 frames/s. A method is described which allows the reconstruction of the three-dimensional trajectories of flies from the two films.

• 2. Males track other flies and dummy targets from below and from downwind (Figs. 5–8; 10a). They keep pointing into the wind and control flight direction and their position relative to the target by way of translational movements (forward, side-ways and vertical movements) (Fig. 9; Fig. 16b–d). Angular tracking is minimal, there is evidence, however, that the flies roll and pitch.

• 3. When interacting with a target, the flies keep the target's image close to and within the zone of binocular vision of their dorsal compound eyes (Figs. 3; 10c). There is some evidence that the flies' sideways acceleration is determined by the error angle velocity with a delay of 90 ms (Fig. 12). Underneath the target the flies perform sideways oscillations at 3–5 Hz (Fig. 13).

• 4. The flies repeatedly home in on and retreat from a target. At close range (0–20 cm) their vertical and forward velocity is linearily related to the target distance (Figs. 14a, 15a). Switches from ascending to descending flight occur whenever the target is lost from the visual field of the dorsal eyes.

• 5. Both forward and vertical velocity are to some extent dependent on the altitude of the target above the horizon of the fly (Figs. 14b, 15b).

• 6. Rotational body movements in response to rotating striped patterns can only be elicited by stimulating the ventral eyes, the dorsal eyes do not mediate the optomotor response (Fig. 17).

• 7. In bibionids, male-specific visually guided behaviour is mediated through the dorsal eyes only. The visual tasks of swarming males are discussed. The complex behaviour seen in males interacting with a dummy target might reflect their need to distinguish other males in a swarm from females and to keep the surrounding airspace clear of unwanted targets.