Excitation-inhibition interactions mediate firefly flash synchronization

Large populations of fireflies can synchronize their bioluminescent flashes with remarkable precision, producing collective rhythms that emerge from interactions among intrinsically variable individuals. In the North American firefly Photuris frontalis, this behavior usually manifests as a stable, population-level single-period beat whose mechanistic origins remain unresolved. To identify the local interaction rules giving rise to this emergent synchrony, we performed controlled perturbation experiments on isolated P. frontalis males using fixed-period light stimuli. By measuring changes in flash period as a function of stimulus timing, we reconstructed the phase-response curve (PRC) governing individual flash dynamics. The resulting PRC exhibits a biphasic structure, revealing phase-advancing (excitatory) and phase-delaying (inhibitory) responses. Using this PRC, we formalized an integrate-and-fire model that quantitatively reproduced the observed adaptable entrainment across tested stimuli. These results establish a direct mechanistic link between phase sensitivity and emergent collective synchronization, demonstrating how excitation-inhibition interactions influence large-scale rhythmic coherence in firefly populations.