Wearing a raincoat: exocrine secretions contain anti-wetting agents in the oribatid mite, Liacarus subterraneus (Acari: Oribatida)

Abstract

Liacarus subterraneus is a large, soil-dwelling oribatid mite species that possesses a conspicuously shiny, clean and not wettable cuticular surface. The exocrine cuticular chemistry of this species was investigated by means of gas chromatography-mass spectrometry. Besides a fraction of hydrocarbons and a terpene, hexane extracts of whole mite bodies exhibited free carboxylic acids and their glycerides as main components. The compounds were arranged in three distinct extract profiles. Based on data from individual extracts, (1) the majority (more than 3/4) of specimens showed large amounts of 1,2-dioctanoyl-glycerol (and three other related esters) but no (or only traces of) free carboxylic acids. (2) In about 1/8 of extracts, free acids (mainly octanoic (caprylic) acid) and glycerides were detected. This second type of profile highly varied with respect to the relative abundance of acids and esters. (3) The third profile (in about 7% of specimens) exclusively exhibited free acids and no (or only traces of) glycerides. In addition, a few extracts exhibited no components at all. The extract compounds most likely originate from the lipid layer of the cerotegument of L. subterraneus. The cuticle of individuals that possessed extractable cerotegumental compounds (profile I, II, III) exhibited strong water repellent properties, while the cuticle of individuals that possessed no components in their extract did not. After hexane extraction, water repellent properties got lost. The distinct extract profiles detected most likely portray the stepwise generation of an anti-wetting, exocrine surface lipid layer of glycerides: If this layer is lost, fatty acids may be discharged again (profile III) and may subsequently esterify (profile II) to larger and more stable esters (diacyl-glycerols), eventually building up the "raincoat" (mainly profile I) of L. subterraneus.