However it can be applied if there is an integer relationship of frequency, such that the cyclic signals share a repeating sequence of phase angles over consecutive cycles.
These integer relationships are called Arnold tongues which follow from bifurcation of the circle map.
At dusk, the flies begin to flash periodically with random phases and a gaussian distribution of native frequencies.
After some time all the fireflies within a given tree (or even larger area) will begin to flash simultaneously in a burst.
One way to keep a local oscillator "phase synchronized" with a remote transmitter uses a phase-locked loop.