Interactions between species have been central to life's diversification. We are interested in the mechanisms underlying evolutionary change, particularly in the context of symbiosis. Our focus is on the behaviorally complex interspecies relationships that have evolved within the Metazoa. We use rove beetles (Staphylinidae) as our exploratory system, a hugely species-rich clade that has repeatedly evolved highly intimate and phenotypically elaborate symbioses with ants. Such species embody evolution in the extreme, with dramatic behavioral, anatomical and chemical adaptations for life as social parasites inside ant colonies. The widespread evolution of this symbiosis in staphylinids provides a unique paradigm for understanding how obligate interspecies interactions can evolve between free-living organisms. We are interested in the core molecular and neurobiological circuitry by which reciprocal signals are exchanged between ant and beetle, fostering their interaction. Our work is integrative, combining genomics and developmental biology with chemical ecology and behavioural neuroscience to explore all facets of the ant-beetle interaction and its evolutionary basis. We have found that some of the most remarkable symbiotic phenotypes have evolved convergently many times in Staphylinidae, often in distantly related lineages. The system illuminates the enduring question of how complex phenotypic changes can arise repeatedly and predictably over a deep evolutionary timescale.