Visual speed compensation of pitch-constrained blue-bottle flies under retinal-image velocity perturbation in a flight mill

In the presence of wind or background image motion, flies are able to maintain a constant retinal-image velocity via regulating flight speed to the extent permitted by their locomotor capacity. Here we investigated the speed regulation of semi-tethered blue-bottle flies (Calliphora vomitoria) flying along an annular corridor in a magnetically levitated flight mill enclosed by two motorized cylindrical walls. We perturbed the flies retinal-image motion via spinning the cylindrical walls, generating bilaterally-averaged velocity perturbations from -0.3 to 0.3 m{middle dot}s-1. Flies compensated retinal-image velocity perturbations by adjusting airspeed up to 20%, thereby maintaining a relatively constant retinal-image velocity. When the retinal-image velocity perturbation became greater than [~]0.1 m{middle dot}s-1, the compensation weakened as airspeed plateaued, suggesting that flies were unable to further change airspeed. The compensation gain, i.e., the ratio of airspeed compensation and retinal-image velocity perturbation, depended on the spatial frequency of the grating patterns, being the largest at 12 m-1.