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Hydroscaphidae is a small family of the beetle suborder Myxophaga comprising miniaturised aquatic beetles resembling some staphylinids. Surprisingly little is known about the taxonomy and biology of these beetles. In this study we present novel data based on freshly collected material of Hydroscapha LeConte, 1874, from Taiwan and Hong Kong. Morphology and DNA barcoding revealed two species: Taiwanese endemic H. takahashii Miwa, 1935, and H. shuihau, a new species, so far known only from Lantau Island of Hong Kong. Both species are (re)described, illustrated, and compared to the types of H. hunanensis Pu, 1948. Both species were found in algal mats in ephemeral hygropetric habitats; in two cases adults massed together with immature stages were observed. Larvae and pupae of H. takahashii were studied and illustrated using SEM and light microscopy. Comparisons of immature stages of Hydroscaphidae based on our new data and a detailed review of the literature reveals that principal differences between genera and species can be found in the modifications of the tracheal system both in larvae and pupae. Four larval instars were found in H. takahashii, corresponding to the number of instars known in the myxophagan family Torridincolidae. Based on our observations, we suggest that the minute body size of Hydroscapha may be one of the adaptations for colonising and exploiting ephemeral habitats. [ABSTRACT FROM AUTHOR]