The chemical strategies by which parasites manage to break into the social fortresses of ants offer a fascinating theme in chemical ecology. Semiochemicals used for interindividual nestmate recognition are also involved in the mechanisms of tolerance and association between the species, and social parasites exploit these mechanisms. The obligate parasites are odorless ("chemical insignificance") at the time of usurpation, like all other callow ants, and this "invisibility" enables their entry into the host colony. By chemical mimicry (sensu lato), they later integrate the gestalt odor of this colony ("chemical integration"). We hypothesize that host and parasite are likely to be related chemically, thereby facilitating the necessary mimicry to permit bypassing the colony odor barrier. We also review the plethora of chemical weapons used by social parasites (propaganda, appeasement, and/or repellent substances), particularly during the usurpation period, when the young mated parasite queen synthesizes these chemicals before usurpation and ceases such biosynthesis afterwards. We discuss evolutionary trends that may have led to social parasitism, focusing on the question of whether slave-making ants and their host species are expected to engage in a coevolutionary arms race.
