Fig. 2. Side-slip manoeuvre of Drosophila and body orientation. (A) Sequence of video images showing a side-slip manoeuvre of a flying fruit fly in front of the rotating pattern. The red arrow indicates body orientation that is derived from the length and width ratio of the fly's image (grey blob). The small red dot indicates the position of the fly's head. Sampling rate, 62.5 Hz; rotational speed of environment, 180° s^–1. (B) Data of a single fly showing the relationship between body orientation derived from blob analysis (x-scale, orientation I, b/a ratio <0.8, where b is the lateral and a the longitudinal extension of the blob ellipsis, inset) and body orientation reconstructed from the position of the fly on two successive video frames (y-scale, orientation II). In the experiment, the fly responded to an optomotor pattern rotating at 500° s^–1. Flight time was 7.54 s. Linear regression fit (reduced major axis, model II regression, y=0.995x–2.12, R²=0.94, N=775 sample points, P<0.001, red) shows a high degree of conformance between the two methods, suggesting that side-slip manoeuvres are rare in freely flying Drosophila. See text for details.