Systematics and biogeography of the bee genus Caenonomada Ashmead, 1899 (Hymenoptera: Apidae: Tapinotaspidini)

The species of Caenonomada, a bee genus characteristic of the xeric habitats of eastern South America, are revised. Three species are recognized, including a new one, C. labrata sp. nov., from central Brazil. A lectotype is designated for C. tertia Cockerell, 1912, which is considered a synonym of C. unicalcarata (Ducke, 1908). This species has its main area of occurrence in the semi-arid Caatinga of northeastern Brazil and two isolated records, here interpreted as disjunct populations, in central Brazil and in Tucuman, Argentina. C. bruneri Ashmead, 1899 occurs in northern Argentina, Paraguay, and southern Brazil. Tetrapedia pluricincta Vachal, 1909, previously considered a species of Caenonomada, is transferred to the genus Monoeca Lepeletier & Serville, 1828. An identification key and illustrations are provided. Phylogenetic analysis based on morphological characters of adult specimens supported only one hypothesis of relationship among the species: ((C. bruneri + C. labrata sp. nov) C. unicalcarata). The resulting area cladogram implies two vicariance events, followed by dispersal of C. unicalcarata from northeastern Brazil to the arid regions of northern Argentina. The historical relationship between the areas, as indicated by the present study, is different from that obtained by other authors who studied forest associated organisms. It is postulated that this divergence is due to the fact that the historical hypotheses refer to different ancestral biotas, one associated with xeric habitats and the other associated with forest habitats.