Key to the genera of the tribe Hydrophilini from Japan (all instars)

1. Frontal lines U-shaped (Fig. 58A); mandibles strongly asymmetrical, left mandible with one inner tooth, right mandible with two closely aggregated inner teeth (Figs. 57 B–C). .............................................................................................. Hydrophilus Geoffroy, 1762

– Frontal lines V-shaped (Fig. 54A); mandibles almost symmetrical, both mandibles similar in shape and with the same number of inner teeth (Figs. 53 B–C)...................................2

2. Nasale with five distinct teeth (Figs. 60C, 62B); each mandible with three inner teeth, distal two large, basal one small (Figs. 61 B–C, 63C–D); abdominal segments with short lateral projections (Fig. 51B). .................................................... Sternolophus (Fabricius, 1792)

– Nasale without distinct teeth, almost straight or slightly serrate (Figs. 52C, 55A); mandibles with two inner teeth, distal one large, basal one small (Figs. 53 B–C, 55D–E); abdominal segments with long lateral projections (Fig. 51A). .............. Hydrochara Berthold, 1827