Pterostichus izurusanus sp. nov., an isolated and specialized species of the macrogenys species group (Coleoptera: Carabidae) discovered on a mountain on the edge of the Kanto Plain, Honshu, Japan

A new species, Pterostichus izurusanus sp. nov., belonging to the macrogenys species group of the subgenus Nialoe Tanaka, 1958 (s. lat.), is described. This species is currently known only from Mt. Izuru-san (Tochigi Prefecture, Japan), which is located on a narrow ridge that protrudes from a mountainous area towards the Kanto Plain; the mountainous area other than the ridge is occupied by P. macrogenys Bates, 1883, another member of the same species group. The origin and differentiation process of this new species are discussed, based on its distribution pattern, morphological characteristics, and morphology-based phylogeny. An updated key to species of the macrogenys species group is provided.