Tethered flying desert locusts, Schistocerca gregaria , generate yaw-torque in response to rotation of a radial grating located beneath them. By screening parts of the pattern, rotation of the unscreened grating turned out to induce a compensatory steering (by pattern motion within transversally oriented 90-degrees wide sectors) as well as an upwind/downwind turning response (by pattern motion within the anterior ventral 90-degrees wide sector). The strength and polarity of responses upon the unscreened grating results from a linear superposition of these two response components. The results are discussed with regard to a functional specialization of eye regions. In a typical experiment, 3 consecutive flight-phases, assumed to mirror start, long-range flight, and landing of a free-flying locust, were distinguished. They may result from a time dependent variation of the polarity and relative strength of upwind/downwind turning and compensatory steering responses. Starting and landing phases were under strong optomotor control and were dominated by the high-gain compensatory steering. In contrast, the phase of long-range flight was under weak optomotor control resulting from a low gain in both of the two response components. The biological significance of this variable strength of optomotor control on free flight orientation of swarming locusts is discussed.
