Blowfly flight and optic flow. II. Head movements during flight

The position and orientation of the thorax and head of flying blowflies (Calliphora vicina) were measured using small sensor coils mounted on the thorax and head. During flight, roll movements of the thorax are compensated by counter rolls of the head relative to the thorax. The yaw turns of the thorax (thorax saccades) are accompanied by faster saccades of the head, starting later and finishing earlier than the thorax saccades. Blowfly flight can be divided into two sets of episodes: 'during saccades', when high angular velocities of up to a few thousand degrees per second are reached by both the thorax and head, and 'between saccades', when the orientation of the thorax and, in particular, the head is well stabilized. Between saccades, the angular velocities of the head are approximately half those of the thorax and lie mostly in the range 0-100 degrees s-1 for any rotation (yaw, pitch and roll). These velocities are low enough to limit the visual blur attributable to rotation. It is argued that the split into periods during which either rotational optic flow ('during saccades') or translatory optic flow ('between saccades') dominates is helpful for processing optic flow when signals and neurons are noisy.