There are many documented accounts (including video footage) of large groups of 'Fireflies' (typically beetles of the Lampyridae family) flashing their bio-luminescent lights in a synchronised display.

It's not known why they have developed the synchronised displays, and recent research is suggesting that the methods used to synchronise are less straightforward than was previously assumed.

Despite casual descriptions of collective flashing and hasty analogies with models of coupled oscillators, careful observations and quantitative analysis suggest that the underlying mechanisms of synchrony are complex and remain poorly understood. By using stereoscopic recordings of the collective flashing display of synchronous fireflies in their natural environment as well as in controlled experiments, we reconstruct flashing swarms in three dimensions and investigate local interactions and collective patterns. We show in particular that flashing information propagates across the swarm along a network of visual connections, and we search for signatures of heterogeneities that could suggest social differentiation."

Source : American Physical Society, March meeting, 2021

New (2021) research, which monitored thousands of fireflies in a 360 degree 3-D environment suggests that :

[…] flash bursts nucleate and propagate across the swarm in a relay-like process."

Source : Self-organization in natural swarms of Photinus carolinus synchronous fireflies Science Advances, Vol. 7, no. 28

Note: The exact biochemical processes which produce the firefly's light have still not been determined. (ref.)