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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.
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