Title: Thermotactic behavior in C. elegans and Drosophila larvae
Description: Thermotactic behavior is one of the most robust forms of exploratory navigation in C. elegans. Above their cultivation temperature, C. elegans migrate towards colder temperatures by using a biased random walk. Far below their cultivation temperature, their random walk is unbiased. Near their cultivation temperature, C. elegans has the ability to track isotherms. In this thesis, I analyze the ability of C. elegan to crawl isothermally by quantifying the trajectories of individual worms responding to defined thermal gradients. Sensorimotor control during isothermal tracking may be summarized as a strategy in which the worm changes the curvature of its propulsive undulations in response to temperature changes measured at its head. A concise mathematical model is consistent with the exquisite, stability of the worm's isothermal alignment in spatial thermal gradients as well as its more complex trajectories in spatiotemporal thermal gradients. In the second portion of this thesis, I show the response hierarchy between thermosensory and olfactory inputs in C. elegans. The worm suppresses its aversive response to the removal of olfactory attractants when it is subjected to cooling, and suppresses its aversive response to warming when it is subjected to the addition of olfactory attractants.

Interneuronal circuits downstream of the sensory layer allow C. elegans to respond to overall change in its environment on the basis of simultaneous variation in different sensory inputs. To investigate whether the biased random walk -- the algorithm used by motile bacteria and C. elegans to navigate towards more favorable environments -- is shared by larger animals, I developed a tracking microscope to study thermotaxis in Drosophila larvae. Drosophila larvae crawl towards preferred temperatures between 25 and 30 °C when placed on spatial temperature gradients. However, unlike motile bacteria and C. elegans, larval thermotaxis has both stochastic and deterministic components. The quantitative understanding of larval behavioral strategy that I obtained by quantifying larval movements on defined temperature gradients places strong constraints on how the underlying neural circuits generate thermotactic behavior.