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Video records were made of the blowfly Calliphora erythrocephala L. mainly during tethered flight in a wind-tunnel, to study its movements about the longitudinal body axis (roll). During undisturbed flight, flies hold their head on average aligned with the body but may turn it about all three body axes. Pitch and yaw turns of the head are comparatively small (20 degrees), whereas roll turns can be large (90 degrees), and fast (1200 degrees s^-1}). When passively rolled, flies produce compensatory head movements during walking or flight; at rest this reflex is turned off. Flies perceive a static misalignment relative to the vertical, as well as roll motion up to 10 000 degrees s^{-1}. Within this range flies counteract an imposed roll with maximal gain at about 1000 degrees s^{-1}. Compensatory head movements are made with very low latency (down to Δ t \approx 5 ms), and with considerable speed (up to ω = 1000 degrees s^{-1). Flies may `disregard an apparent deviation from their correct orientation, and may superimpose spontaneous head movements on those elicited by a stimulus. Compensatory head movements generally undercompensate the imposed misalignment. Simultaneously, however, flies modify their wing pitch and wingbeat amplitude to produce a compensatory roll torque. Since head and body roll act simultaneously and in the same direction, the overall speed and degree of head realignment, relative to external coordinates, increase considerably. This is certainly an advantage for flight in turbulent air. In still air, without need to correct an imposed misalignment, flies nevertheless produce spontaneous fluctuations of their flight torque, and head roll movements in the opposite direction. This is to be expected if flies intend to keep their eyes aligned with the coordinates of the environment while spontaneously performing banked turns. The limits of fly vision and the advantages of compensatory head movements for different visually guided behaviour are discussed. Compensatory head roll movements give flies greater manoeuvrability when cruising than the visual system would allow, without such a stabilizing reflex.
