(Limnaeoperyphus) Nakane 1963

Subgenus Limnaeoperyphus Nakane, 1963

Limnaeoperyphus Nakane, 1963: 23. Type species: Lymnaeum quadriimpressum Motschulsky, 1860, original designation. Lymnaeoperyphus auct.

Diagnosis. Small (length 3.5–5.0 mm), slightly convex beetles, with distinct microsculpture over entire dorsal surface; colour of upper parts with metallic sheen over entire dorsal surface or elytra brown. Head rather large, not punctured; eyes bulging; posterior supra-orbital pore located at posterior edge of eye; frontal grooves wide, parallel, not bifurcated; mandibles rather narrow, with apices slightly curved inward; antennae with light basal segments. Pronotum small, distinctly cordiform, slightly wider than head; bases near the basal foveae is slanted forward; posterior angles rectangular, with pointed apices; midline very narrow; basal foveae small, not punctate; laterobasal carina weak. Elytra small, oval, moderately convex; shoulders strongly rounded, not protruding; basal border very short, smoothly passing into lateral margin, reaching of 5 th stria; striae complete up to apex; third interval with two discal pores lying on 3 rd stria. Hind wings reduced. Legs slender and relatively short. Underside: not punctate; metasternal process with a thin, often indistinct border; metepisternum short; abdominal sternites simple, with neither pubescence nor additional setae.

Aedeagus elongate (5.0–5.6 times longer than wide), penis tube almost straight, lamella short, wide, rounded and rather strongly bent ventrally. Complex of central sclerites with a developed flagellum, ribbon brush small, slightly larger than basal sclerite. Lateral sclerite small, without processes.

Endophallus straight, with two lateral tubercles at base and two small tubercles at apex.

Tergite IX of female weakly sclerotized, transverse, with sparse setae at apical margin. Basal gonocoxite subtriangular, with a convex outer margin and 1–4 ventral subapical setae. Apical gonocoxite falcate, 1.8–2.1 times as long as wide at base, with a pair of thin subapical setae. In addition to the dorsal spine, apical gonocoxite bearing two or three lateral ones.

Spermatheca sclerotized, spermathecal duct moderately long, not coiled, and a vaginal sclerite (annulus receptaculus) absent. Spermathecal gland developed, its subbasal part thickened and bearing ring folds, weakly sclerotized distally.

Species composition. The subgenus presently includes two species, Bembidion quadriimpressum (Motschulsky, 1860) and B. petromarinum sp. nov., both distributed on the coastal areas of the northwestern Pacific.

Notes. For more than a century, B. quadriimpressum was considered within the genus/subgenus Lymnaeum Stephens, 1828 [= Limnaeum auct.] (Motschulsky 1860; Netolitzky 1935, 1943; Jedlička 1965; Kryzhanovskij et al. 1975; etc.), although doubts were expressed already by F. Netolitzky (1943: 100 /72). In 1963, T. Nakane established the monotypic subgenus Limnaeoperyphus Nakane, 1963 for this particular species, pointing out in a short diagnosis the combination of a habitus characteristic of Cillenus and the structure of the genitals typical of Peryphus (Nakane 1963: 23). The new subgenus was not immediately accepted by specialists. For example, A. Jedlička (1965: 160), in his monograph of the Bembidiini of East Asia, considered Limnaeoperyphus as a junior synonym of Lymnaeum.

From species of the Western Palaearctic subgenus Lymnaeum, which for a long time included B. quadriimpressum, Limnaeoperyphus species differ in having large eyes, very short temples, the location of the posterior supra-orbital pore at the level of the posterior edge of the eye, relatively short oval elytra and completely rounded shoulders.

The structure of the genitalia allows Limnaeoperyphus to be attributed to the Ocydromus Series, within which it shows some similarities to the Nearctic Ocydromus Clade (sensu Maddison 2012). The relative size and shape of the aedeagus sclerites resemble those of the Nearctic species of the subgenus Ocydromus, members of the ‘ striola’ group (sensu Lindroth 1963). This is consistent with the results of a phylogenetic analysis using seven genes (Maddison & Maruyama 2019).