For decades, scientists have speculated about why some fireflies exhibit synchronous flashing, in which large groups produce rhythmic, repeated flashes in unison – sometimes lighting up a whole forest at once. Now, the first experiments on the function of this phenomenon suggest that synchronous flashing preserves female fireflies' recognition of suitable mates. The results are reported in the July 9 issue of Science. In this video, fireflies in the great Smoky Mountains National Park display flashing synchrony, with a period of flashes before a characteristic pause. Credit: Andrew Moiseff