Visual control of flight behaviour in the hoverflySyritta pipiens L.

Summary

• 1. The visually guided flight behaviour of groups of male and femaleSyritta pipiens was filmed at 50 f.p.s. and analysed frame by frame. Sometimes the flies cruise around ignoring each other. At other times males but not females track other flies closely, during which the body axis points accurately towards the leading fly.

• 2. The eyes of males but not females have a forward directed region of enlarged facets where the resolution is 2 to 3 times greater than elsewhere. The inter-ommatidial angle in this “fovea” is 0.6°.

• 3. Targets outside the fovea are fixated by accurately directed, intermittent, open-loop body saccades. Fixation of moving targets within the fovea is maintained by “continuous” tracking in which the angular position of the target on the retina (Θ _e) is continuously translated into the angular velocity of the tracking fly ($\dot \Phi _p$) with a latency of roughly 20 ms ($\dot \Phi _p = k \Theta _e$, wherek≏30 s⁻¹).

• 4. The tracking fly maintains a roughly constant distance (in the range 5–15 cm) from the target. If the distance between the two flies is more than some set value the fly moves forwards, if it is less the fly moves backwards. The forward or backward velocity ($\dot F_p$) increases with the difference (D-D ₀) between the actual and desired distance ($\dot F_p = k^\prime (D - D_0 )$), wherek′=10 to 20 s⁻¹). It is argued that the fly computes distance by measuring the vertical substense of the target image on the retina.

• 5. Angular tracking is sometimes, at the tracking fly's choice,supplemented by changes in sideways velocity. The fly predicts a suitable sideways velocity probably on the basis of a running averageΘ _e , but not its instantaneous value. Alternatively, when the target is almost stationary, angular tracking may bereplaced by sideways tracking. In this case the sideways velocity ($\dot S$) is related toΘ _e about 30 ms earlier ($\dot S_p = k\prime \prime \Theta _e$, wherek″=2.5 cm · s⁻¹ · deg⁻¹), and the angular tracking system is inoperative.

• 6. When the leading fly settles the tracking fly often moves rapidly sideways in an arc centred on the leading fly. During thesevoluntary sideways movements the male continues to point his head at the target. He does this not by correctingΘ _e , which is usually zero, but by predicting the angular velocity needed to maintain fixation. This prediction requires knowledge of both the distance between the flies and the tracking fly's sideways velocity. It is shown that the fly tends to over-estimate distance by about 20%.

• 7. When two males meet head on during tracking the pursuit may be cut short as a result of vigorous sideways oscillations of both flies. These side-to-side movements are synchronised so that the males move in opposite directions, and the oscillations usually grow in size until the males separate. The angular tracking system is active during “wobbling” and it is shown that to synchronise the two flies the sideways tracking system must also be operative. The combined action of both systems in the two flies leads to instability and so provides a simple way of automatically separating two males.

• 8. Tracking is probably sexual in function and often culminates in a rapid dart towards the leading fly, after the latter has settled. During these “rapes” the male accelerates continuously at about 500 cm · s⁻², turning just before it lands so that it is in the copulatory position. The male rapes flies of either sex indicating that successful copulation involves more trial and error than recognition.

• 9. During cruising flight the angular velocity of the fly is zero except for brief saccadic turns. There is often a sideways component to flight which means that the body axis is not necessarily in the direction of flight. Changes in flight direction are made either by means of saccades or by adjusting the ratio of sideways to forward velocity ($\dot S/\dot F$). Changes in body axis are frequently made without any change in the direction of flight. On these occasions, when the fly makes an angular saccade, it simultaneously adjusts$\dot S/\dot F$ by an appropriate amount.

• 10. Flies change course when they approach flowers using the same variety of mechanisms: a series of saccades, adjustments to$\dot S/\dot F$, or by a mixture of the two.

• 11. The optomotor response, which tends to prevent rotation except during saccades, is active both during cruising and tracking flight.