Camponotine ants rely on a bacterial mutualist that lives exclusively within host cells. Top panel: Wood-nesting Camponotus pennsylvanicus, like other members of the diverse tribe Camponotini, possess an obligate bacterial endosymbiont, Blochmannia. © Adam B. Lazarus. Bottom panel: A single bacteriocyte (specialized host cell) from the ant C. pennsylvanicus, with numerous rod-shaped Blochmannia filling the cytoplasm. Blochmannia has lived exclusively within an intracellular niche for tens of millions of years. Sample was prepared from homogenized ant larvae, fixed, stained with DAPI, and visualized with a fluorescent microscope. Image shows host nucleus at the center. © Erika del Castillo.