Bugs Playing Ball: Anomalous Diffusion of a Passive Tracer in a Swarm of Locusts

In the wild, animal groups such as fish, birds, or locusts exhibit giant-scale synchronization and collective motion. Remarkably, similar collective behaviors are reproduced by models of interacting active particles, offering a minimal description in which biological and behavioral aspects do not seem to play a role. However, it remains unclear how far this analogy extends, i.e., when is modeling living creatures as active particles not enough? To explore these questions, we investigate the dynamics of passive tracer particles in a swarm of marching locusts and compare our results to tracer particles embedded in an active liquid. We reveal that the tracers, lightweight transparent balls, perform anomalous diffusion with heavy tailed movement and stopping time distributions, in stark contrast to tracers in active liquids. We show that this originates from highly complex interactions with the locusts: instead of colliding with the balls, locusts climb on top of them, topple and drag them across the arena. Thus, the tracers’ motion reflects deep behavioral aspects of the surrounding locusts, providing new insights to modeling living swarms.