Genus Isadelphus Förster, 1869

Isadelphus Förster, 1869: 177 . Type: Hemiteles inimicus Gravenhorst, 1829 . Designated by Viereck (1914).

Perosis Förster, 1869: 169 . Type: Echthrus armatus Gravenhorst, 1829 . Designated by Viereck (1914).

Cecidonomus Bridgman, 1880: 264 . Type: Cecidonomus gallicola Bridgman, 1880 .Designated by Viereck (1914). In this study, I record a new species, I. nigrus sp. nov., from Japan. This is the first record of this genus from Japan. Key to Eastern Palearctic species can be distinguished by the following key.

Key to Eastern Palearctic species of the genus Isadelphus (♀)

1. Hind coxa red to reddish yellow. Anterior part of clypeus with a tongue-like protrusion, its apex more or less truncate. Metasomal tergites at least partly red. .......................................... I. inimicus (Gravenhorst, 1829)

-. Hind coxa brown to black (Fig. 46A). Anterior part of clypeus with two weak or indistinct teeth (Fig. 62I). Metasomal tergites at most tinged with brown. ............................................................................................ 2

2. Hind coxa and T II brown. Ovipositor sheath 1.8 times as long as hind tibia. .................................................. I. compressus Sheng, 2001

-. Hind coxa and T II black (Figs 46A, C, E). Ovipositor sheath 1.35 times as long as hind tibia. ................................................................... I. nigrus sp. nov.