Figure 3. Histogram showing the distribution of the horizontal velocity of fruitflies (*v*_{x}, *v*_{y}) in each of the arenas. The distribution of *v*_{x} is shown by solid symbols (red triangles, 0*g** (11.5 T); green circles, 1*g** (16.5 T); blue inverted triangles, 2*g** (11.5 T); black squares, 1*g* control (0 T)) and *v*_{y} by open symbols. Here *v*_{x} = *Δ**x*/*Δ**t*, where *Δ**x* is the displacement of the fly along the *x* direction between consecutive video frames, and *Δ**t* is the time interval between the capture of one video frame and the next, and similarly for *v*_{y}. Hence (*v*_{x}, *v*_{y}) is the mean velocity of the fly during the interval *Δ**t*, not the instantaneous velocity of the fly. The data show the distribution for the population, normalized such that *p*(*v*_{x})*Δ**v*_{x} is the probability of finding a fly, randomly selected from the population, with a velocity between *v*_{x} − *Δ**v*_{x}/2 and *v*_{x} + *Δ**v*_{x}/2 at any particular time; similarly for *v*_{y}. The bin-width *Δ**v*_{x} of the histograms are 2 mm s^{−1}. The solid lines are fits to the tails of the distributions with *p*(*v*_{x}) ∼ exp(−*b*|*v*_{x}|); see §4.4. (Online version in colour.)