Molecular and morphological evidence reveals a new lepidopteran genus, Micrometasia (Crambidae, Spilomelinae, Steniini), with implications for insular speciation of endemic moths in Japan

Japan, an island nation comprising both continental and oceanic islands, is regarded as a biodiversity hotspot with a large number of endemic insect species. Among these, two spilomeline moth species - Duponchelia naitoi Sasaki, 2008, endemic to mainland Japan, and Metasia bilineatella Inoue, 1996, endemic to the Ogasawara Islands - have attracted our attention due to their uncertain generic placements and limited available information. In this study, we investigate the morphology and phylogenetic position of these two species, and accordingly establish a new genus, Micrometasiagen. nov., to accommodate them, providing redescriptions including the first description of the female of M. bilineatella. Additionally, two new congeners are described: Micrometasia ryukyuensissp. nov. from the Ryukyu Islands, and Micrometasia melanatellasp. nov. from the Ogasawara Islands, Japan. Photographs of the adults and genitalia, along with an identification key, are provided. Maximum likelihood (ML) phylogenetic analysis using one mitochondrial (COI) and three nuclear (CAD, EF1α and RpS5) genes places this genus within Steniini - a predominantly detritivorous tribe - yet specifically within a clade whose larval stages remain almost entirely unknown. In addition, intraspecific COI genetic distances and branching order in a neighbour-joining (NJ) tree, together with the distributional patterns of each species raise several biogeographic and evolutionary implications: (1) a case of small-scale radiation accompanied by remarkable morphological divergence on oceanic islands; and (2) at least two possible colonisation routes into the Japanese archipelago, along with the potential presence of undescribed diversity of this genus beyond Japan. ZooBank: urn:lsid:zoobank.org:pub:F4E35FA1-661F-4AE4-AD4B-4D7DB5CF1569.