Protection of hydrofuge respiratory structures against detrimental microbiotic growth by terrestrial grooming in water beetles (Coleoptera: Hydrophilidae, Hydraenidae, Dryopidae, Elmidae, Curculionidae)

Terrestrial grooming, i.e, grooming outside of water, was investigated in 37 species of polyphagous water beetles belonging to 26 genera and 5 families (Hydrophilidae, Hydraenidae, Dryopidae, Elmidae and Curculionidae). During terrestrial grooming, body regions covered with water repellent pubescence were groomed by preference. When submerged, this pubescence is enveloped by an air sheath which serves as oxygen reservoir and physical gill. If the beetles were prevented from climbing on land and grooming, their air bubbles decreased in size and their mortality rose. Terrestrial grooming could be released by increasing water temperature and light intensity and in some cases by wetting the hydrofuge pubescence when submerged. It is likely that terrestrial grooming helps with the drying out of microorganisms in the sun and their mechanical removal, The epizoic microorganisms and other contaminants would interfere with the hydrophobous properties of the pubescence and lead to drowning of the beetles. All species examined, except Elmis aenea (Muller 1806), groomed on land. Elmidae and some other hydrophagous water beetles have cuticular structures which are particularly resistant to wetting and do not need to be cleaned despite steadily increasing contamination. The possible evolution of the terrestrial grooming behaviour in polyphagous water beetles is discussed and compared to the secretion-grooming behaviour in Hydradephaga.