The socially parasitic ant genus Myrmoxenus varies strongly in fundamental life history traits, such as queen-worker ratio, the timing of sexual production, and mating behavior. Myrmoxenus queens generally take over nests of Temnothorax ants, kill the resident queen by throttling, and force the workers to take care of the social parasite's brood. Young queens of M. ravouxi and other species produce large numbers of workers, which during "slave-raids" pillage host pupae from neighboring Temnothorax colonies to increase the workforce in their own nests. Other species, such as M. corsicus, have lost caste polyphenism and rear only male and female sexual offspring. Using sequences of the genes CO I/CO II and wingless we reconstruct the phylogeny of Myrmoxenus and document that the worker caste was lost convergently at least three times. Furthermore, mating in the nest and inbreeding obviously also evolved in parallel from ancestors whose sexuals presumably mated during nuptial flights. Myrmoxenus might thus provide a suitable model to investigate caste differentiation and the plasticity of mating behavior in Hymenoptera.