Journal article Open Access

Morphology and biology of the flower-visiting water scavenger beetle genus Rygmodus (Coleoptera: Hydrophilidae)

Yusuke Minoshima; Matthias Seidel; Jamie R. Wood; Richard A. B. Leschen; Nicole L. Gunter; Martin Fikáček

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 Rygmodus 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 demonstrate that Rygmodus 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 Rygmodus beetles are 
generalists feeding on pollen of at least 13 plant families. Rygmodus 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
specimens was identified using DNA barcoding of two molecular markers,
mitochondrial cytochrome oxidase 1 (cox1) and nuclear histone 3 (H3). Larvae of two 
species, Rygmodus modestus and Rygmodus sp., are described in detail and illustrated; 
they closely resemble ambush-type predatory larvae of the hydrophilid tribe 
Hydrophilini in the head morphology. Rygmodus is the only known hydrophilid beetle 
with adults and larvae inhabiting different environments.

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