We examined the morphology of the cauline domatia of the semi-myrmecophyte Humboldtia brunonis at different stages of ontogeny. We observed that the hollow chamber in H. brunonis is spontaneously formed by the plant, and suggest this to be a collective effect of both schizogeny and lysogeny, following acropetal lignification of the pith cells as the domatium internode swells up. Unlike some other cauline domatia, there is a self-opening slit that provides access to the domatium chamber. We investigated the micro-structure of the inner wall of the domatia using scanning electron microscopy, and observed that cells that form the inner lining of the domatia cavity have canaliculated, lignified sclerenchyma with numerous plasmodesmata, as reported for the true myrmecophyte Leonardoxa africana, which could explain the observed nutrient flux from domatia-dwelling invertebrates into the host plant. We also observed fungal mycelia in ant-occupied domatia, though the role of fungi in this ant-plant system awaits further investigation.