Whodunnit? The case of midge swarms

TL;DR

This paper investigates the complex collective behavior of midge swarms, comparing laboratory and field observations, and introduces an extended Vicsek model to better explain their scale-free correlations and shape dynamics.

Contribution

It extends the harmonically confined Vicsek model to anisotropic confinement, providing a better match to observed swarm properties and critical exponents.

Findings

Swarm correlation length exhibits scale-free behavior in the field.

Extended Vicsek model produces elongated swarm shapes.

Model results align with natural swarm measurements.

Abstract

As collective states of animal groups go, swarms of midge insects pose a number of puzzling questions. Their ordering polarization parameter is quite small and the insects are weakly coupled among themselves but strongly coupled to the swarm. In laboratory studies (free of external perturbations), the correlation length is small, whereas midge swarms exhibit strong correlations, scale free behavior and power laws for correlation length, susceptibility and correlation time in field studies. Data for the dynamic correlation function versus time collapse to a single curve only for small values of time scaled with the correlation time. Is there a theory that explains these disparate observations? Among the existing theories, whodunnit? Here we review and discuss several models proposed in the literature and extend our own one, the harmonically confined Vicsek model, to anisotropic confinement. Numerical simulations of the latter produce elongated swarm shapes and values of the static critical exponents between those of the two dimensional and isotropic three dimensional models. The new values agree better with those measured in natural swarms.