Speed dependent descending control of innate freezing behavior in Drosophila melanogaster

The most fundamental choice an animal has to make when it detects a predator, or other threats, is whether to freeze, reducing its chances of being noticed, or to flee to safety. Here we show that Drosophila melanogaster exposed to looming stimuli in a confined arena either froze or fled. The probability of freezing versus fleeing was modulated by the fly9s walking speed at the time of threat, demonstrating that freeze/flee decisions were context dependent. We describe a pair of descending neurons crucially implicated in freezing. Genetic silencing of P9 descending neurons disrupted freezing yet did not prevent fleeing. Optogenetic activation of both P9 neurons induced running and freezing in a state-dependent manner. Our findings establish walking speed as a key factor in defensive response choices and reveal a pair of descending neurons as a critical component in the circuitry mediating selection and execution of freezing or fleeing behaviors.