The sensory and behavioral basis of Drosophila larval phototaxis

The avoidance of light by fly larvae has been studied for over a century. Early 20th-century investigators found that larvae crawled away from light sources incident at an angle (e.g. a sunlit window). Contemporary studies project light from directly above or below and find that larvae accumulate in shadows and have stereotyped responses to sudden changes in light intensity. Now, as then, both the sensory and behavioral mechanisms for phototaxis remain controversial. Here, I unify the historic and modern approaches in the Drosophila larva using a novel apparatus and high-resolution behavioral analysis to allow for the precise quantification of larval movement in response to photosensory inputs. Larval locomotion is composed of sequences of runs (periods of forward movement) that are interrupted by abrupt turns, where the larva pauses and sweeps its head back and forth (head-sweeping) until it begins a new run in a new direction. My analysis reveals that the larva uses head-sweeps as spatiotemporal probes of local light information to determine the direction of successive runs. I find all forms of phototaxis are mediated by the same sensorimotor transformation and establish the necessity of the larval eye to decode the direction of incident light. This work provides the necessary foundation for the decryption of the neural circuits controlling phototaxis.