The control of optic flow during learning flights

Hymenopteran insects perform systematic learning flights on departure from their nest, during which they acquire a visual representation of the nest environment. They back away from and pivot around the nest in a series of arcs while turning to view it in their fronto-lateral visual field. During the initial stages of the flights, turning rate and arc velocity relative to the nest are roughly constant at 100–200° s⁻¹ and are independent of distance, since the insects increase their flight speed as they back away from the pivoting centre. In this paper I analyse how solitary wasps control their flight by having them perform learning flights inside a rotating striped drum. The wasps' turning velocity is under visual control. When the insects fly inside a drum that rotates around the nest as a centre, their average turning rate is faster than normal when they fly an arc into the direction of drum rotation and slower when they fly in the opposite direction. The average slip speed they experience lies within 100–200° s⁻¹. The wasps also adjust their flight speed depending on the rotation of the drum. They modulate their distance from the pivoting centre accordingly and presumably also their height above ground, so that maximal ground slip is on average 200°␣s⁻¹. The insects move along arcs by short pulses of translation, followed by rapid body turns to correct for the change in retinal position of the nest entrance. Saccadic body turns follow pulses of translation with a delay of 80–120 ms. The optomotor response is active during these turns. The control of pivoting flight most likely involves three position servos, to control the retinal position of both the azimuth and the altitude of nest and the direction of flight relative to it, and two velocity servos, one constituting the optomotor reflex and the other one serving to clamp ground slip at about 200° s⁻¹. The control of ground slip is the prime source of the dynamic constancy of learning flights, which may help wasps to scale the pivoting parallax field they produce during these flights. Constant pivoting rate may in addition be important for the acquisition of a regular sequence of snapshots and in scanning for compass cues. Accepted : 31 July 1996