  Condensed active matter is exemplary for its capacity to morph and exhibit
internal flows despite remaining cohered. To facilitate understanding of this
ability, we investigate the cause of finger-like protrusions that emerge from
super-organismal, aggregated rafts of fire ants (Solenopsis invicta). While
these features are easily observed, what permits their recuring initiation,
growth, and recession is not immediately clear. Ants rafts are comprised of a
floating, structural network of interconnected ants on which a layer of freely
active ants walks. We show here that sustained shape evolution is permitted by
treadmilling defined by the competition between perpetual raft contraction due
to displacement of bulk structural ants into the active layer, and outwards
raft expansion due to deposition of free ants into the structural network at
the edges. Furthermore, we see that protrusions emerge due to asymmetries in
the edge deposition rate of surface ants, and we provide strong evidence that
these asymmetries occur stochastically due to wall accumulation effects and
local alignment interactions. Together these effects permit the cooperative,
yet spontaneous formation of protrusions that fire ants utilize for functional
exploration and to escape flooded environments. Ant raft dynamics mirror the
treadmilling that facilitates morphogenesis and motility of cytoskeletons, thus
providing another example in which reshaping of condensed active matter is
explained by free constituent transport, yet whose continuance relies on
perpetual phase transition between structural and free members.
