Systematics, biogeography and host plant associations of the Pseudomyrmex viduus group (Hymenoptera: Formicidae), Triplaris - and Tachigali -inhabiting ants

Abstract

The Pseudomyrmex viduus group is a Neotropical clade of arboreal ants containing 13 species, of which three are newly described here P. insuavis, P. ultrix, and P. vitabilis. Most species inhabit the domatia of specialized ant-plants. The ants keep brood and scale insects (Coccoidea) in the plant cavities, and defend their nest sites much more aggressively than do generalist species of Pseudomyrmex nesting in dead twigs. Five species are obligate associates of trees in the genus Triplaris (Polygonaceae) and five taxa are restricted to Tachigali (Fabaceae: Caesalpinioideae). One species, P. viduus (F. Smith), is much less host-specific, inhabiting Cordia, Coussapoa, Ocotea, Pseudobombax, Pterocarpus, Sapium, Triplaris and other myrmecophytes. Nothing is known about the biology of P. vitabilis, a close relative of P. viduus. The last member of the species group, P. kuenckeli (Emery), is a non-specialist but aggressive species that nests in dead branches of various plants. A cladistic analysis indicates that this aggressive behaviour evolved before obligate associations with specialized ant-plants, and that the Triplaris andTachigali inhabitants each form their own clade. P. viduus, which is nested within the Triplaris -associated clade, suggests a possible model for host plant evolution in these ants wherein shifts from one ant-plant to another involve an intermediate phase of expanded host plant use. At least nine other Pseudomyrmex species, from two different species groups (not closely related to the P. viduus group), have evolved specialized associations with Triplaris or Tachigali including five new species P. crudelis, P. deminutus, P. eculeus, P. ferox and P. hospitalis. Although the P. viduus group is centred in the Amazon basin, the geographic ranges of most species do not coincide with the Pleistocene forest refugia proposed by Haffer and others. A consideration of the phylogenetic relationships, distribution patterns, and host plant specificity of the ants indicates that much of the diversification of the P. viduus group occurred before the Pleistocene, and that the interactions with Triplaris and Tachigali plants are also of Tertiary origin.