The fungus-farming ants Sericomyrmex amabilis of Central and South America are often plagued by the parasitic Megalomyrmex symmetochus (left), which live among them for years, living off their food and mutilating their virgin queens. But new observations put an odd twist in this parasitic tale. A large population of nonfertile Megalomyrmex workers, whose function had been unclear, patrols their host’s territory to defend against an even more menacing invader: the predatory Gnamptogenys hartmani (right). Scientists staged a series of face-offs among the three ant species in the lab, as they report online today in the Proceedings of the National Academy of Sciences. In Sericomyrmex nests with no parasites, two introduced Gnamptogenys ants obliterated roughly 70% of the colony. But in parasitized nests, the Megalomyrmex ants dispensed an alkaloid venom that not only killed the Gnamptogenys raiders, but also turned them against one another. The scientists compare Megalomyrmex to the mercenaries who protected a medieval city during conflicts but drained its resources in times of peace.