This is difficult to explain succinctly, but biochemical processes can set up periodic events with a fair amount of accuracy. For instance, your heart beats rou

This is difficult to explain succinctly, but biochemical processes can set up periodic events with a fair amount of accuracy. For instance, your heart beats roughly once a second when in a resting state. Some of these processes use physical latency to time the events: if the fireflies were roughly of the same maturity, they would be of similar size and thus have clocks of similar duration.