A new workerless social parasite in the ant genus Pseudomyrmex (Hymenoptera: Formicidae), with a discussion of the origin of social parasitism in ants

The New World ant genus Pseudomyrmex (subfamily Pseudomyrmecinae) contains about 180 species, of which only one workerless social parasite, P . leptosus , from Florida, has been previously recorded. A new species discovered recently in northern Argentina, P . inquilinus sp. nov., is more derived morphologically and behaviourally than P . leptosus and has convergently developed features characteristic of the workerless inquilines known in other ant subfamilies. These features include diminutive size, reduced mouthparts, a broadened petiole and postpetiole, well‐developed subpetiolar and subpostpetiolar processes, and the habit of straddling the gaster of the host queen. A cladistic analysis confirms that the new species is not closely related to its host nor to P . leptosus ; it belongs in fact to a different species complex within the genus Pseudomyrmex . Two widespread Neotropical species, P.filiformis and P.subater , to which P.inquilinus appears to be related, show indications of being temporary social parasites on other, unrelated species of Pseudomyrmex . Thus, there is evidence from comparative natural history that the extreme social parasitism seen in the new species from Argentina was preceded by a period of temporary social parasitism, but ‘Emery’s Rule’– the claim that social parasites are close relatives of, and evolve from, their hosts – is not supported. Rather, parasitism in Pseudomyrmex seems to have arisen through interspecific colonization and exploitation of congeners. A review of the evidence suggests that Emery’s Rule has received too uncritical an acceptance in the literature on social parasitism. Uncertainties about the generality of Emery’s Rule and about the plausibility of achieving prezygotic isolation under sympatric conditions undermine the theory that social parasites evolve from within populations of their host species.