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 ∼ ρ^−ν with ν ≈ 0.4 over two orders of magnitude in density (ii) an inertial delay for velocity alignment of approximately 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 an motility-induced phase separation (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.

我们研究了在水面上自由游动的旋转甲虫Dineutus discolor (Coleoptera: Gyrinidae)群体的集体动力学。我们为每只甲虫提取单独的轨迹，包括位置和方向，并使用它来发现 (i) 与密度相关的速度缩放，如v ∼ ρ ^{- ν}和ν≈ 0.4 超过两个数量级的密度 (ii) 大约 13 ms 的速度对齐的惯性延迟和 (iii) 共存的高密度和低密度相，与运动诱导相分离 (MIPS) 一致。我们将标准的活性布朗粒子 (ABP) 模型修改为在开放空间中发挥作用的封闭 ABP (CABP) 模型，该模型通过结合甲虫的密度依赖性重新定向，朝向集群。我们使用我们的新模型来检验我们的假设，即运动诱导相分离 (MIPS)（或类似 MIPS 的效应）可以解释我们在数据中看到的高密度相和低密度相的共存。然后，拟合模型成功地恢复了N = 200 的类似 MIPS 的凝聚相，而对于较小的组大小N = 50、100 ，则没有这种相。