Morphology and biology of the flower‐visiting water scavenger beetle genus <i>Rygmodus</i> (Coleoptera: Hydrophilidae)

<jats:title>Abstract</jats:title><jats:p>Hydrophilidae (water scavenger beetles) is well known as an aquatic beetle family; however, it contains ca. 1,000 secondarily terrestrial species derived from aquatic ancestors. The New Zealand endemic genus <jats:italic>Rygmodus</jats:italic> White is a member of the hydrophilid subfamily Cylominae, which is the early‐diverging taxon of the largest terrestrial lineage (Cylominae + Sphaeridiinae) within the Hydrophilidae. In this paper we report that <jats:italic>Rygmodus</jats:italic> beetles are pollen‐feeding flower visitors as adults, but aquatic predators as larvae. Based on analyses of gut contents and a summary of collecting records reported on museum specimen labels, adult <jats:italic>Rygmodus</jats:italic> beetles are generalists feeding on pollen of at least 13 plant families. <jats:italic>Rygmodus</jats:italic> adult mouthparts differ from those of other (saprophagous) hydrophilid beetles in having the simple scoop‐like apex and mola with roughly denticulate surface, resembling the morphology found in pollen‐feeding staphylinid beetles. Larvae were found along the sides of streams, under stones and in algal mats and water‐soaked moss; one collected larval specimen was identified using DNA barcoding of two molecular markers, mitochondrial cytochrome oxidase 1 (<jats:italic>cox1</jats:italic>) and nuclear histone 3 (H3). Larvae of two species, <jats:italic>Rygmodus modestus</jats:italic> and <jats:italic>Rygmodus</jats:italic> sp., are described in detail and illustrated; they closely resemble ambush‐type predatory larvae of the hydrophilid tribe Hydrophilini in the head morphology. <jats:italic>Rygmodus</jats:italic> is the only known hydrophilid beetle with adults and larvae inhabiting different environments.</jats:p>