Observing a swarm of midges, one may think that individuals move randomly, attracted by a landmark as a puddle or a street lamp. Though, it is not so: even if disorder seems to prevail, a strong correlation in fact exists between the insects’ movements.

Irene Giardina from the Physics Department of Sapienza University of Rome, together with the CoBBS group (Collective Behaviour in Biological Systems) led by Andrea Cavagna from the Institute for Complex Systems (ISC-CNR), in their recent work published in Nature Physics show how experimental data, gathered by analyzing midges’ trajectories, tell a truth different from the naïve observation. Their results validate the importance of statistical physics to the understanding of collective phenomena in biology.

After having acquired sequences of stereoscopic images of the midges’ movements and having converted them into 3D trajectories through in-house tracking software, researchers have analyzed them using a statistical approach. The team has then computed the space-time correlations between the midges’ velocities: they have in this way quantified the influence of a given individual in the group on the behavior of another individual, at a specific time and place.

The statistical analysis of such correlations allows measuring how large are the regions of a swarm where midges move in a coordinated way; and how the mutual influence between midges decays as time passes.
This decay time changes in every swarm analyzed, and swarms with larger coordinated regions show longer decay times.

Furthermore, researchers have observed that – once this dependence is taken into account – space-time correlations show the same behavior in all swarms, so they obey dynamic “scaling laws”: this is a fundamental property to be able to describe every swarm in a universal way, and develop models of group behavior.

“Our study is a breakthrough in the application of statistical physics methods to biological systems and provides robust experimental results, which every theory of swarming and collective movement in active matter must confront with”, Irene Giardina says.

CoBBS is a joint group of CNR Institute for Complex Systems and of the Physics Department. CoBBS studies flocking and swarming phenomena starting from experiments on real systems and develops theories and models based on the data (http://www.cobbs.it/).

References:
Dynamic scaling in natural swarms Nature Physics 13, 914–918 (2017) – A. Cavagna, D. Conti, C. Creato, L. Del Castello, I. Giardina, T.S. Grigera, S. Melillo, L. Parisi, M. Viale – doi:10.1038/nphys4153