The Japanese species of the genus TherionCurtis, 1829 (Ichneumonidae: Anomaloninae) were revised based on examination of 447 Japanese specimens. Four species are recognized based on morphology and sequence data (mtCOI). Two of them are identified as previously known species: T. circumflexum (Linnaeus, 1758) and T. giganteum (Gravenhorst, 1829). The other two species are described as new: T. carinatum Shimizu & Bennett, sp. nov. and T. nigrigasterum Shimizu, Bennett & Ito, sp. nov. In addition, T. rufomaculatum (Uchida, 1928) stat. rev. is once again synonymized with T. circumflexum. The Japanese species of Therion are easily distinguished from each other by a newly provided key. The lasiocampid moth, Dendrolimus spectabilis (Butler, 1877), is newly recorded as a host of T. giganteum.
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