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In Tennessee, Fireflies Are Beacon for Tourists:

The secret is out about this marvelously rare and very brief annual spectacle. About a thousand tourists a night come to Elkmont, a small trailhead in Great Smoky Mountains National Park, during the two weeks each June when the country’s largest population of synchronous fireflies puts on what locals call “the light show.”

That is not exactly true, but the finer details of Photinus carolinus remain mysterious. The synchrony is more a race than a drum beat. One dominant male blinks and then all of the others flash almost instantly too. They follow a pattern, Morse-code-like, of roughly six seconds of flashing and then six seconds of darkness that lasts for about two hours after dusk.