Supplementary Figure 1: Design of the arena used to measure heat avoidance behavior in planarians (the Planariometer).

a) Planariometer design. In each trial two opposing tiles are set to 24°C and two to 32°C (noxious heat). Animal movement is then recorded for 4 minutes. The spatial configuration of hot and cool tiles is then reversed for 4 additional minutes (and a second movie acquired) to control for potential spatial biases. The hot and cold tiles are covered by thin anodized aluminum foil; on it, a hydrophobic ink pen is used to create a circular barrier. A thin film of water (1-2 mm) forms a central pool in which the worms can move freely. The experiment is conducted in the dark, under IR-LED illumination; movies are acquired using an IR-sensitive CCD camera. b) Images acquired using a thermal imaging camera, showing sharp thermal boundaries between hot and cool quadrants in the two spatial configurations used for each experiment. c) Snapshots from video recordings of planarian movement, corresponding to the thermal configurations shown in b.