Abstract; ; Flash synchrony within firefly swarms is an elegant but elusive manifestation of collective animal behaviour. It has been observed, and sometimes demonstrated, in a few populations across the world, but exactly which species are capable of large-scale synchronization remains unclear, especially in low-density swarms. The underlying question which we address here is: how does one qualify a collective flashing display as synchronous, given that the only information available is the time and location of flashes? We propose different statistical approaches and apply them to high-resolution stereoscopic video recordings of the collective flashing of; Photinus knulli; fireflies, hence establishing the occurrence of synchrony in this species. These results substantiate detailed visual observations published in the early 1980s and made at the same experimental site: Peña Blanca Canyon, Coronado National Forest, Arizona, USA. We also remark that; P. knulli; ’s collective flashing patterns mirror that observed in; Photinus carolinus; fireflies in the Eastern United States, consisting of synchronous flashes in periodic bursts with rapid accretion and quick decay.;
