Shock avoidance experiments

For Fig. ​Fig.3d,3d, flies were transferred to an odourless tube (not lined with filter paper) on which three red LEDs were mounted. Flies were exposed to 500 Hz red light for 2 min, then transferred to the T-maze elevator and immediately allowed to choose in darkness for 1 min between two copper-lined tubes, one of which delivered 90 V electric shocks of 1.5 s duration at 0.2 Hz. Control groups were not exposed to red light before testing. For the naive shock avoidance experiment in Fig. ​Fig.3e,3e, flies were directly transferred to the T-maze elevator without training and immediately allowed to choose in darkness between two copper-lined tubes (each mounted with three green LEDs), one of which was coupled with 90 V shocks. Both tubes were illuminated with continuous green light throughout testing. For both experiments, the preference index (PI) was calculated as the number of flies in the shock tube minus those in the control tube, divided by the total number of flies. Each experiment contributed a single PI value (rather than the mean scores of two experiments), but the tube conducting electric shocks alternated between experiments.