The water scavenger beetle subtribe Hydrobiusina (Coleoptera: Hydrophilidae: Hydrophilini) comprises 17 valid species that collectively are widely distributed, though highly localized, in all biogeographic regions. Five additional new species have recently been discovered in Hawaii. Adult morphological data was analyzed in combination with DNA sequence data from COI and 18s rDNA to develop a phylogenetic hypothesis for the lineage and to examine patterns of character and habitat evolution. A complex stridulatory file found in four of the six genera is derived within the subtribe, evolving independently from stridulatory organs previously described for the family. All Hawaiian taxa constitute a monophyletic lineage that has adaptively radiated, with both larval and adult stages evolving from occupation of traditional aquatic habitats to semi-aquatic and even fully terrestrial habitats. This extreme ecological shift, the first of its kind reported for the aquatic Hydrophilinae, was accompanied by morphological specializations, including loss of swimming setae and flight wings, and the gain of tarsal pads characteristic of terrestrial beetle lineages. Hygropetric environments are hypothesized to serve as transitional habitats, providing one means for hydrophilids to shift from aquatic to arboreal and other terrestrial lifestyles.