  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 $N=50,100$.
