Several Central American species of Piper Subg. Macrostachys have developed obligate associations with ants (Pheidole bicornis), in which the ant partner nests in and derives food from modified plant structures. In turn, the ants protect the plant against fungal infection and some herbivores. We conducted a study on the comparative morphology and anatomy of ant-associated plant structures of all ant-associated species of Subg. Macrostachys to critically examine the relationship to ant presence. The primary domatia in all obligate myrmecophytes in Piper Subg. Macrostachys are formed by sheathing petioles that close to form a cavity. The ants apparently derive the majority of their nutrients from food bodies that form inside the tube on the adaxial surface of the petiole. In all reported obligate myrmecophytes, the stem becomes hollow and the ant colony occupies the entire plant. Piper calcariformis, a previously unreported obligate myrmecophyte, is an uncommon shrub from Costa Rica and Panama, which is similar to all other known obligate Piper ant plants in Central America except that it lacks caulinary domatia. In addition, variously closed petiolar domatia are found in other Piper species, and an increased degree of closure corresponds to increased density of ant residents. In these facultative ant plants, the association between ants and plants is less specific, with a number of ant species inhabiting these plants. We are currently examining the obligate, facultative, and non-ant plants in Piper Subg. Macrostachys to better understand the evolution of these interactions.

Key words: ant-plant interactions, domatia, Piper