Independent evolution of a living bridge in the old world army ant lineage

Living bridges formed by army ants are among the most striking examples of collective behavior in social insects, previously known only from New World Eciton species. Here, we report the field observation of living bridge formation in an Old World army ant, Aenictus glabrinotum. We artificially created a small gap in the foraging trail, formed on a twig, to disrupt traffic flow. Ants accumulated near the opening, explored the air space, and interlinked their bodies to form a living bridge, allowing traffic to resume. Bridges were maintained while traffic was high but spontaneously disassembled as flow declined, suggesting a similar behavioral process to that of Eciton behavior. Although it is limited to a single opportunistic observation, this documentation suggests that bridge-formation may have evolved several times in army ants. By motivating further comparative studies across army ant lineages, this study provides new insight into the diversity of self-assembly in social insects.