patterns

As they flash, fireflies interact with one another through visual connections.⊕ They also interact with a complex environment, crowded with grass, bushes and trees, which restricts their vision and movement. The network of interactions, in conjonction with specific behavioral responses, can sometimes generate harmonious collective patterns.

The most spectacular example of such patterns is synchrony. Synchronous fireflies blink in concert on the same tempo, creating a collective beat of light. A few rare species have been proven to synchronize, but it is not clear how many are actually capable of it. Those who synchronize usually start in pairs and trios, and at sufficiently high density their rhythm will spread across the swarm.

But synchrony is not a monolith. It can exhibit different variations and modulations, just like musicians on the same tempo can still play a wide repertoire.

Synchrony

There are at least 3 different synchronous patterns produced by North American species. The easiest way to characterize a pattern is to record a movie of the collective display and simply count how many flashes appear in each frame. This outlines a temporal signal (time series) which translates a complex phenomenon into a simple motif.

snap synchrony

Snap synchrony denotes sharp and precise coordinated blinks, like a troupe snapping fingers. Flashes repeat continuously and periodically, typically every 0.5s to 1s, for hours. This is the pattern created by Photuris frontalis fireflies, which also happen to have been nicknamed "Snappies"! You can observe Snappies at several different spots across the Southeast US.

⊕ Snap synchrony in Photuris frontalis fireflies.

breath synchrony

Breath synchrony is a variation from snap synchrony where the spikes widen, like a respiration. Individual flashes last longer and the group is less precise in the timing of their flashes, which creates a bit more of a curve in the pattern. But these spikes are also precisely periodic. Breath synchrony can be seen in Photinus concisus fireflies in Texas and Photuris tremulans in the Southeast.

⊕ Breath synchrony in Photinus concisus.

burst synchrony

Burst synchrony is a more complex motif. It consists of broad spikes happening within periodic bursts of activity. Between the burst there is (almost) absolute darkness. Both the spikes and bursts are periodic. Burst synchrony has been observed in popular species Photinus carolinus which displays all around the Smoky Mountains and central Appalachians; and also in a distant cousin, Photinus knulli, which is currently found exclusively in Arizona, some 2000 miles away!

⊕ Burst synchrony in Photinus carolinus.

Spatio-temporality

What matters is not just when flashes occur, but also where. Indeed, a firefly blinks at a certain instant t in conjonction with a specific location (x,y,z) in three dimensional space. The spatial distribution of flash occurrences may form an interesting pattern in itself. Combined, space and time correlations offer a glimpse at how fireflies interact.

How does one evaluate the 3D position of a flash? Using stereoscopic video recordings. Just like with two eyes you achieve depth perception, with two cameras filming the same scene a computer can reconstruct the distances of the features. This is a process called triangulation.

By filming firefly swarms with two cameras, one can obtain the (x,y,z,t) coordinates of each flash. In synchronous displays, this technique has revealed some propagation mechanisms akin to a wave. The flash information nucleates in a small region of the swarm, and then propagates from there. This illuminates a network of visual connections, along which information is carried away in a relay-like manner.

Below is a very compelling manifestation of flash propagation in a synchronized swarm:

⊕ Flash propagation in Pteroptyx malaccae (Thailand).

Adapted from Synchronicity by Robin Meier & Andre Gwerder.