Stochastic insect navigation in complex rapidly fluctuating odor plumes*

How insects navigate complex odor plumes, where the location and timing of odor packets are uncertain, remains unclear. Here, we imaged complex odor plumes simultaneous with freely-walking flies, allowing us to quantify how behavior is shaped by discrete encounters with odor packets. Combining measurements, dynamical models, and statistical inference we found that navigation was stochastic, and did not rely on the continuous modulation of speed or orientation. Instead, flies turned stochastically with stereotyped saccades, whose direction was biased upwind by the timing of prior odor encounters, while the magnitude and rate of saccades remained constant. Further, the timing of encounters was used to modulate the transition rates between walks and stops. Thus, while in more regular environments, flies are known to continuously modulate speed and orientation, our results show that in less predictable environments walking flies instead navigate with random walks biased by the timing of encounters.