Figure 1.

Flies correct for external rotations by using polarized sky light (A) The rotating arena used in experiment 1. The fly is glued to a steel pin and suspended between two magnets. It can view the sky through a glass window (spanning the region 30.6° to 58.5° elevation from vertical, inset), above which various filters were placed. A camera below the fly records its azimuthal orientation. The entire arena was rotated about its vertical axis by 90° every 3 minutes. (B) An example trace showing 24 minutes of flight orientation in arena coordinates (above) and outside world coordinates (below). Changes in background grayscale level indicate when the arena was rotated. Only the first 12 minutes were used in subsequent analyses, in order to increase rate of data collection. (C) Circular mean (colored line) and circular variance (gray patch) of change in heading with respect to arena after a rotation at time t=0. A change of 90° would indicate perfect compensation for external rotation. For each fly, a single response was calculated by averaging its responses to all three rotations during the experiment. The mean and variance of these single fly responses are displayed. In this and subsequent panels, different experimental conditions and sample sizes (N, the total number of individual flies tested), from left to right, were as follows: orange: complete darkness (experiment conducted indoors) N = 18; red: arena covered with circular polarizer, N = 21; green: only glass window between the fly and the sky, N = 21; blue: blue bandpass filter above glass window, N = 19; gray: neutral density filter above glass window, N = 12. See also Figure S1. (D) Circular mean of change in heading between 10 and 30 seconds after rotation of arena. Bars indicate 95% confidence intervals as computed by bootstrap method in [22]. Asterisks indicate with what confidence mean is different from zero (*** p<.001, ** p<.01, NS p>.05). 95% confidence intervals include 90° for no filter and gray filter conditions, 99% confidence interval includes 90° for blue filter condition. (E) Fictive trajectories assuming constant forward flight speed of 0.5 m s⁻¹ in world coordinates. Gray background circles indicate radius of 100 m. Black circles indicate position at the end of the experiment for each fly. (F) Fictive distance traveled at the end of 12 minute experiment (distance from the origin of the black dots in E). A fly orienting perfectly in one direction would ‘travel’ 360 m. Median indicated by horizontal red line, box extends from lower to upper quartile values. Vertical black lines extend to most extreme data point within 150% of the interquartile range. Outliers, defined as any points outside the range of the black lines, are shown as crosses. Lowercase letters above the plot indicate different groups at the p<.05 level as computed by the Bonferroni-corrected one-tailed Mann-Whitney U test.