Opisthonotal gland chemistry of early-derivative oribatid mites (Acari) and its relevance to systematic relationships of Astigmata

Abstract

Mites of the suborder Astigmata produce a variety of chemicals – monoterpenes, aromatics and straight-chain hydrocarbons - in a pair of exocrine opisthonotal glands. The more common chemicals also are known from oribatid mites in Desmonomata. We used GC-MS to characterize hexane extracts of four earlier-derivative oribatid mite species. 1) Parhypochthonius aphidinus (in Parhyposomata, the most basal group with the gland): the major component is the aromatic 3-ethylphenol, with the hydrocarbons undecane and tridecene showing moderate peaks in GC profiles. 2) Gehypochthonius urticinus (also Parhyposomata): the major component appears to be an isomer of 1-methy1-2-naphthol, with heptadecadiene and 1-tridecene present in moderate abundance. 3) Nehypochthonius porosus (in Mixonomata): tridecane is usually the major component, with somewhat lesser amounts of an unidentified nonhydrocarbon component of molecular weight 150. 4) Perlohmannia sp. (a more derived mixonomatan): extracts contain pentadecene, the monoterpenes neral and geranial, and the aromatic γ -acaridial (2-formyl-3-hydroxybenzaldehyde). Hexane extracts from four outgroup taxa without opisthonotal glands – oribatid mites Hypochthonius rufulus, Eniochthonius sp., and endeostigmatic mites Lordalycus sp. and Bimichaelia sp. – have no detectable compounds. Considering nonhydrocarbons, all Perlohmannia gland components seem widely distributed in Astigmata, but those of earlier-derivative glandulate mites - P. aphidinus, G. urticinus and N. porosus - are not known from Astigmata. This chemical distribution is evidence that Astigmata evolved from within the oribatid mites, at some level above the earliest Mixonomata; it is therefore consistent with the hypothesis that Astigmata evolved within Desmonomata. At least in P. aphidinus, the gland probably has a role in predator defense; it seems quickly exhausted when mites are disturbed and its components are known irritants.