Hydrobius, Leach, 1815

Key to the Eurasian species of Hydrobius View in CoL

1. Mesoventral elevation very small and blunt, not high, acute and dentiform ( Fig. 15 in Fossen et al., 2016). Male genitalia with parameres robust, inturned apically ( Fig. 16). 5.8–6.4 mm. Subarctic Eurasia............................. H. arcticus View in CoL

– Mesoventral elevation prominent, acute and dentiform ( Fig. 14 in Fossen et al., 2016; Figs 3–25 in Ryndevich, 2016)............................. 2

2. Mesoventral elevation low (Figs 6, 7). Large punctures (with trichobothria) in anterior half of elytra situated within or close to the 3rd and 5th elytral striae (Fig. 5)................................. 3

– Mesoventral elevation high ( Figs 11–25 in Ryndevich, 2016). Large punctures (with trichobothria) in anterior half of elytra situated in the intervals between the 2nd and 3rd, and between the 4th and 5th striae ( Fig. 1 in Ryndevich, 2016).......... 5

3. Pubescent part of metafemora occupying about half (55–60%) of femoral length ( Figs 8, 9). Aedeagus with parameres slender and scarcely inturned apically ( Fig.11). 6.5–7.1 mm. Japan....... H. pauper View in CoL

– Pubescent part of metafemora occupying more than 65% of femoral length ( Fig. 10). Aedeagus with parameres robust, inturned apically ( Figs 17, 18) .............................................. 4

4. Lateral margins of elytra in middle portion almost parallel ( Figs 28, 29). 5.9–6.7 mm. China: Qinghai, Sichuan................................. H. pui View in CoL

– Lateral margins of elytra in middle portion slightly rounded ( Fig. 27). 6.2–7.8 mm. Europe............................................ H. rottenbergii View in CoL

5. Aedeagus larger ( Fig. 12–14), more than 0.2 times as long as elytron................................ 6

– Aedeagus smaller ( Fig. 15), less than 0.18 times as long as elytron. 7.8–8.5 mm. China: Heilongjiang............................... H. punctistriatus View in CoL

6. Aedeagus in lateral view with parameres only slightly curved towards apex ( Fig. 10 C in Fossen et al., 2016). Legs of most specimens pale, yellowish, but occasionally darker. Body usually more elongate ( Figs 22, 23). 6.5–7.7 mm. Eurasia and North America............................. H. fuscipes View in CoL

– Parameres in lateral view more curved towards apex ( Fig. 10 D in Fossen et al., 2016). Legs dark. Body generally more robust ( Fig. 21). 6.7–7.5 mm. Europe............................ H. subrotundus View in CoL