Mushroom bodies (MBs) are insect brain centers involved in multimodal sensory integration and memory formation. Advanced Hymenoptera, such as ants and bees, have particularly large and elaborately organized MBs, which are repeatedly implicated in complex behaviors. In this study, to address the developmental aspects of their MBs, gliogenesis of mushroom body neuroblasts (MB Nbs) was examined in the carpenter ant Camponotus japonicus. Reversed Polarity (REPO) is a paired-like homeodomain protein located exclusively in the nucleus of differentiating glial cells in the fruit fly Drosophila melanogaster. First, the molecular aspects of C. japonicus REPO (CjREPO) were identified. Then, the antibody (CjREPO-antibody) was raised against a peptide of CjREPO. Immunohistochemistry revealed that the strong labeling was located in the nucleus of glial cells in the developing brains, whereas no immunoreactivity was detectable in progeny derived from MB Nbs. These findings suggest that MB Nb in the ant is a neuronal precursor that does not produce glial cells.