Substructure in Insect Swarms

Among collectively behaving animal aggregates swarming insects stand out with respect to bird flocks or fish schools because they seemingly lack global order; insects appear to fly around randomly through the swarm with little alignment. Nonetheless, this out-of-equilibrium system exhibits various emergent properties, giving investigators a strong incentive to uncover underlying order or structure in an effort to explain the emergent behavior. Here we address this question and reveal that laboratory swarms of flying midges display spatially heterogeneous substructure. In previous lab and field studies, individual midges were assumed to explore the whole volume of the swarm as no evidence suggested otherwise. But by making use of improved tracking algorithms we were able to follow individuals in laboratory swarms for longer time scales and show that individual midges limit themselves to vertical subvolumes. This effect leads to a vertical stratification of trajectories, which may have biological consequences.