Challenging the definition of the subfamily Usiinae: Phthiraxini, an unusual new tribe of bee flies (Diptera: Bombyliidae), based on Phthiraxia bowdeni gen. and sp. nov. from Western Australia

A new genus Phthiraxia gen. nov. (Usiinae: Phthiriini) is described from a remote region of Western Australia based on a male and a female specimen collected 35 years ago. The pair represent a single new species Phthiraxia bowdeni sp. nov. Two important characteristics of this new genus, however, challenge the definition of the subfamily Usiinae: the clypeus does not reach the base of antennae, and the palp is two-segmented. Moreover, Phthiraxia gen. nov. has two other features: the wing vein M₂ present, shared by all members of Phthiriini, and flagellum with subapical shallow sulcus containing a style, differentiating it from Phthiriini with a deep apical sulcus, and Usiini with a deep subapical sulcus. Results of a morphological phylogenetic analysis show that the new genus belongs to a lineage that is sister to the remaining members of the subfamily. Therefore, a monotypic new tribe, Phthiraxini tribus nov., is described to accommodate Phthiraxia gen. nov. A key to the Australian genera of Usiinae is provided. We reevaluate the syapomorphies of the Usiinae in light of the new tribe.