Fig. 4. Isothermal tracking does not affect the speed, wavelength, or frequency of propulsive undulations. (A) Using video microscopy, we quantified the propulsive undulations of C. elegans. Here, we show the representative path traced by the head of a worm crawling along an isotherm near 20.1°C on a spatial thermal gradient with 0.7°C cm^-1 steepness. Based on the video images and movies of C. elegans crawling freely on agar surfaces and tracking defined spatial thermal gradients, we quantified the wavelength of sinusoidal bending (B), the temporal frequency of the propulsive undulation (C), and the center- of-mass speed of the movement of the animal's body. In (D), black circles represent direct measurements of the center-of-mass speed, and the white circles are the product of the wavelength and frequency measurements from (B) and (C). Each data point corresponds to measurements with 20 animals. Error bars represent ± 1 s.d., which reflect the worm-to-worm variabilities in the speed and geometry of propulsive undulations.