Roll-Stabilization During Flight of the Blowfly's Head and Body by Mechanical and Visual Cues

Flying animals have six degrees of freedom of movement in space: three of translation along their body axes (lift, slip, thrust), and three of rotation about these axes (yaw, pitch, roll). For aerodynamical reasons, however, they maintain on average a characteristic flight attitude, i.e. a particular orientation of their body with respect to the gravity field. Translatory and yaw movements have little influence upon flight stability, but pitch and roll movements involve the risk of crashing. Consequently such movements must be rigidly controlled, especially in highly manoeuvrable animals like flies. Flying, like any other form of locomotion, causes the image of the environment to drift across the retina, particularly when objects are nearby, and especially during rotatory movements. Since the visual field of a fly comprises virtually the whole solid angle around the animal, any flight manoeuvre generates a complex optical flow-pattern in the compound eyes. Consequently different kinds of visual disturbances occur simultaneously in different parts of the fly's visual field. Rolling, for example, causes maximal retinal slip speed along the transverse meridian of the eyes, but much less straight ahead of the fly. At the same time, rolling disturbs the angular alignment of the visual field with the environment, but predominantly so in frontal parts of the visual field. Rolling, however, does not directly disturb the fly's orientation e.g. towards an object of interest. Obviously, all such disturbances of vision can be simultaneously counteracted by compensatory head/eye movements and/or corrective flight manoeuvres. This article deals mainly with the question how the blowfly Calliphora erythrocephala L. stabilizes its head during flight with respect to the environment. From the arguments above it is evident that this task is closely interrelated with the more general task of maintaining the whole fly's orientation with respect to the vertical.