Fig. 1. C. elegans crawls towards its set-point (T_S) from higher temperatures but not from lower temperatures. (A) Wild-type (WT) worms that were grown overnight at 15°C were initially placed near the middle of a linear thermal gradient spanning 18–22°C across a 9 cm plate, and their instantaneous positions over time were recorded by video microscopy. Each upper panel shows overlaid snapshots of instantaneous worm positions from six independent plates containing 100 worms in total, with the position of each worm indicated by an open circle. Lower panels show the corresponding histograms of worm positions. The evolution of the worm distributions over time indicates that these worms migrate towards the previous cultivation temperature in what is called cryophilic movement. (B) When wild-type worms that were grown overnight at 25°C were initially placed near the middle of the linear thermal gradient, they exhibit random dispersal.