Suji Kuwagata – Dorcus striatipennis

Introduction

Suji Kuwagata (Dorcus striatipennis) is a small stag beetle of the family Lucanidae, widely distributed in Japan from Hokkaido to Kyushu. It is named for the faint longitudinal grooves that can appear on the wing covers. Although less eye-catching than Japan’s giant stag beetles, it is an important element of oak woodlands and is often confused with Dorcus rectus, so reliable identification points are useful for observers.

Appearance

Suji Kuwagata ranges roughly 14–39 mm in total length; males usually 14–38 mm and females 14–24 mm. The body is dark brown to black, somewhat flattened, with variable gloss.
A key identification feature is the presence of fine longitudinal grooves on the elytra (wing covers), which are most visible in females and small males. Large males may appear smoother.
Male mandibles show a distinct inner tooth that typically remains even in small males, unlike Dorcus rectus. In Suji Kuwagata, this inner tooth is thicker and more squared-off, while in D. rectus it is often more triangular. Females are compact with shorter mandibles and clearly grooved elytra.

Habitat & Distribution

Suji Kuwagata is found in Hokkaido, Honshu, Shikoku, Kyushu, and several surrounding islands including Rishiri, Sado and Tsushima. It inhabits broadleaf and mixed forests from lowlands to around 1000 m elevation.
The species is strongly associated with deciduous hardwoods such as konara oak (Quercus serrata), sawtooth oak (Q. acutissima) and willows, which provide both sap for adults and decayed wood for larvae.

Behavior

Adults are active mainly from May through September, sometimes into October depending on region and altitude. They are primarily nocturnal or crepuscular, though in cooler mountain forests they may also move during the day.
Adults visit tree sap at night and may appear at lights. Some adults overwinter and can be seen again the following year.

Diet

Adults feed mainly on tree sap, especially on oaks and willows.
Larvae develop inside decayed hardwood such as buried logs or stumps. The larval period typically lasts at least one year before pupation.

Reproduction

Breeding occurs from late spring through autumn. Females lay eggs in well-rotted broadleaf wood.
Larvae tunnel and grow within the decayed wood until pupation. New adults may emerge in summer, but those emerging late in the season often overwinter before becoming active.

Conservation

Suji Kuwagata is not nationally threatened in Japan and remains generally common. However, local populations may decline where broadleaf forests are lost or dead wood is excessively removed. Maintaining semi-natural forests with old trees and some dead wood is important for this and many other saproxylic insects.

Author’s Impression

Although small, Suji Kuwagata is a rewarding beetle to look for on summer nights in Japan’s forests. Once you learn the subtle grooves on the wing covers and the difference from Dorcus rectus, you start noticing them more often. Watching how the male mandibles change shape depending on body size adds another level of enjoyment, making each encounter a fun discovery.