Fig. 2. Loosely tethered D. melanogaster orient upwind. (A) Flies randomly presented with wind velocities between 0 and 1.0 m s^–1 orient progressively more tightly around 0° (upwind) with increasing wind velocity. The heavy black lines indicate the time course of wind velocity. (B) Orientation changes were quantified by an orientation response metric. The mean circular orientation was calculated over the first 100 ms (initial orientation) and the final 2 s (final orientation) of each trial. Orientation response is then given by |initial orientation|–|final orientation|. For example, a fly responded to the onset of a 0.2 m s^–1 wind by turning from an initial angle of –150° to a final angle of –12°; an orientation response of 138° (red arrows). In the absence of wind, the same fly turned from –140° to –169°; an orientation response of –29° (blue arrows). (C) Plotting orientation response as a function of the absolute value of the initial orientation provides evidence for orientation to wind (arrowheads indicate the fly whose responses are shown in B). Responses falling along the upper solid line represent perfect upwind orientation, while those along the lower line indicate responses diametric from upwind. (D) A second metric, the response index, quantified responses independently of initial orientation. The response index was calculated as (90° – |final orientation|)/90° where +1 indicates a response with a final orientation of 0°, –1 corresponds to a final orientation of 180° and 0 indicates a response with a final orientation of ±90°. The response index is thus (90°–12°)/90°=0.86 for the fly represented by the red arrows in B (dashed line indicates response index = 0). (E) Response index varied significantly with wind velocity between 0.2 and 1.0 m s^–1, with responses at all velocities being significantly greater than in no wind.