Fig. 2. Basic kinematics of free flight trajectories. (A) A sample trajectory lasting 17 s within a textured background demonstrating how a fly explores its environment using a series of straight flight segments separated by saccades. Note that the pattern on the walls was randomly filled, instead of the regular pattern that is shown in the figure. (B) Angular, horizontal and vertical velocity plotted as a time series for the trajectory shown in A. Spikes in the angular velocity trace indicate saccades (B, upper). Horizontal and vertical velocities (B, lower) change in concert with angular velocity. (C) Event-triggered averages of angular velocity and horizontal and vertical velocity over all flies. Traces were aligned using the point of maximum angular velocity. Each plot of horizontal and vertical velocity shows two traces representing the mean ± S.E.M, mean line not plotted. The velocities come from 1523 saccades from 36 flies. All saccades are plotted as if they occurred in the same direction, with the sign of angular velocity reversed for saccades to the right. Horizontal velocity (blue) decreases slowly before each saccade, rises rapidly afterwards and then returns to its pre-saccade level. Vertical velocity (red) increases slightly before the saccade and decreases after the saccade.