Treadmilling and Protrusion Dynamics in Fire Ant Rafts

Collective living systems often owe their complex emergent behaviors to the actions of their individual constituents. Despite the inherent complexity of emergent structures, individual agents are generally governed by simple rules. This presentation will discuss our recent work on the collective behaviors of the red imported fire ant that forms a buoyant raft, entirely out of their own bodies, to escape flooding. Like cellular systems, these aggregates are highly dynamic and active. As such, they are not only capable of adapting their behavior to load (mechanosensitivity) to avoid premature failure, but also display collective morphogenesis and the stochastic emergence of long protrusions from the raft's edge. One exemplary feature observed experimentally in rafts is treadmilling: a process that drives raft morphogenesis through a combination of contraction-induced agent ejection (removal) and agent deposition (addition), similar to actin treadmilling found in the cytoskeletal network of many cells. Employing experimental characterization and a dynamic network model, we unveil a set of local rules – developed from our observations of treadmilling – that reproduce the emergence of these instabilities in the absence of external factors. Finally, we show initial findings on the formation of bridges that emerge in response to an applied perturbation, thereby demonstrating aggregate mechanosensitivity.