Figure 4. Isothermal tracking in C. elegans includes diffusion in the $x$-direction.

(A) Experimental trajectories under temperature gradient conditions, with worms initially placed at their cultivation temperature of 15°C. Colors are used for the reader to distinguish between individual tracks. (B) The diffusion constants along the $x$ and $y$ directions, with $D_y$ dominating, but $D_x$ highly significant. Error bars are $\pm$s.e.m. (C) MSM-generated equilibrium probabilities along the temperature gradient ($x$ axis) showing the long time scale distribution of the population of worms, and perpendicular to the temperature gradient along isotherms ($y$ axis). In $x$, worms avoid regions with low temperatures, but freely explore regions with higher temperature; in $y$, worms explore the axis with approximately equal probability. Analysis based on 25 experiments, with 688 tracks.