Whirligig Beetles as Corralled Active Brownian Particles

TL;DR

This study investigates the collective behavior of whirligig beetles on water, revealing density-dependent speed, inertial delay in velocity alignment, and phase separation, and introduces a modified active Brownian particle model to explain these phenomena.

Contribution

The paper develops a Corralled Active Brownian Particle (CABP) model incorporating density-dependent reorientation to explain observed phase separation in beetle groups.

Findings

Beetle speed scales inversely with density as v∼ρ^(-0.4).

An inertial delay of approximately 13 ms affects velocity alignment.

High and low density phases coexist, consistent with motility induced phase separation.

Abstract

We study the collective dynamics of groups of whirligig beetles Dineutus discolor (Coleoptera: Gyrinidae) swimming freely on the surface of water. We extract individual trajectories for each beetle, including positions and orientations, and use this to discover (i) a density dependent speed scaling like $v\sim\rho^{-\nu}$ with $\nu\approx0.4$ over two orders of magnitude in density (ii) an inertial delay for velocity alignment of $\sim 13$ ms and (iii) coexisting high and low density phases, consistent with motility induced phase separation (MIPS). We modify a standard active brownian particle (ABP) model to a Corralled ABP (CABP) model that functions in open space by incorporating a density-dependent reorientation of the beetles, towards the cluster. We use our new model to test our hypothesis that a MIPS (or a MIPS like effect) can explain the co-occurrence of high and low density phases we see in our data. The fitted model then successfully recovers a MIPS-like condensed phase for $N=200$ and the absence of such a phase for smaller group sizes $N=50,100$.