Shallow subterranean environments have recently received attention as a habitat for Carabidae beetles, and many new species have been discovered using collection techniques devised for this habitat. We report the discovery of two new species of the macrogenys species group of the Pterostichus subgenus Nialoe Tanaka, 1958, collected by subterranean baited traps in northern Honshu, Japan. Pterostichus shinbodakensis Sasakawa & Itô, sp. n. is described from Mt. Shinbodake, Niigata Prefecture, and P. tateishiyamanus Sasakawa & Itô, sp. n. is described from the southeastern foot of Mt. Tateishiyama, Fukushima Prefecture. Comparative male genital morphology shows that among the known species, the two new species are most closely related to P. falcispinus Sasakawa, 2005 and P. chokaisanus Sasakawa, 2009, respectively. In addition, sympatric occurrence of P. shinbodakensis with a smaller, unidentified species of the species group was also confirmed. The implications of these results for future studies of the macrogenys species group, as well as those of Nialoe, are discussed.

Trephionus Bates, 1883, a Japanese endemic genus in the subtribe Synuchina (Coleoptera, Carabidae, Sphodrini), is revised taxonomically based mainly on the shape of the endophallus, a membranous inner sac everted from the aedeagus of the male genitalia. Three known species from central Honshu, T. kinoshitai Habu, 1954; T. shibataianus Habu, 1978; and T. babai Habu, 1978, are re-defined based on this genital character, and five new species are described from the region: T. cylindriphallus Sasakawa, sp. n., T. niumontanus Sasakawa, sp. n., T. inexpectatus Sasakawa & Itô, sp. n., T. abiba Sasakawa & Itô, sp. n., and T. bifidilobatus Sasakawa & Itô, sp. n. The observed interspecies differences in endophallus morphology are discussed in terms of the species-level phylogeny and genus-level taxonomy of Trephionus.

An endemic subterranean Japanese carabid beetle lineage, the Pterostichus macrogenys species group, was recently revealed to have marked regional differentiation. Studies of such features reveal insect species diversity and provide insight into the mechanisms driving species diversity. We examined specimens of this species group collected from the southern Tohoku District of Honshu, Japan, where its diversity has not yet been fully elucidated, using fine-scale field sampling and detailed comparative morphological analysis of male genitalia. In total, 103 specimens from 13 localities were classified into one new (P. monolineatus sp. n.) and eight known species. In four of the known species, we observed disjunct distributions, which have not previously been reported in this species group and may be more common than previously recognized. Species coexistence was observed at four sites, with two species of different body sizes coexisting at three sites and three species coexisting at the remaining site. The three coexisting species included one large and two small species, the latter of which have male genitalia of a different size. This newly discovered coexistence pattern implies separate effects of differential body and genital size in species coexistence, which has rarely been reported in insects.

The Pterostichus macrogenys species group is an endemic subterranean Japanese carabid lineage that shows marked regional differentiation, but unresolved taxonomic issues remain, even at the species level. Based on morphological examinations of the genital structures of newly collected specimens and an external morphometric analysis of all the species concerned, P. falcispinus Sasakawa, 2005 syn. nov. is synonymized with P. asahinus Habu & Baba, 1960, and P. awashimaensissp. nov. is described from Awashima Island, a small island off the coast of Honshu. Based on external morphometrics, P. awashimaensis was determined to be most similar to P. yahikosanus Sasakawa, 2009, but its external and genital features differ distinctly from the latter species and it is thought to be more ancestral.