Lamingtonacarus posidonis, an inhabitant of water-filled treeholes in Queensland, Australia, exhibits an interesting polymorphism in regard to dorsal setae. Although setae se, c3, and e2 are hair-like, the morphologies of other dorsal setae vary between stases with none being hair-like in immature instars. Comparing dorsal setae between immature instars reveals a progression from long, bipectinate setae in larvae to shorter, biserrate/lanceolate setae of tritonymphs. In adults, setae si, c1, c2, c3, cp, d1, d2, and e1 are short and hair-like. The few adult setae that retain a biserrate condition with short barbs (f2, h1, h2, ps1 and ps2), are much shorter relative to body length than in early immature instars. It is postulated that the long, stout dorsal setae of immatures function as a deterrent to predators. The algophagid mites share treeholes with predatory arrhenurid mites, ascid mites, and tanypodine midge larvae. Although such setal morphology could offer protection to adults as well as immatures, it would also have a negative effect during copulation. Males of algophagid species mount dorsally during copulation, facing the same direction as females and overlapping them to a great extent. Enlarged, bipectinate dorsal setae would undoubtedly hold males away from females and interfere with copulation. It is interesting that Lamingtonacarus oreillyorum, a second species of algophagid also found in Queensland treeholes, retains greatly enlarged pectinate setae in adults, however it has evolved a different strategy to prevent interference during copulation. The enlarged dorsal setae are lost early in the adult stage through abscission.
Species 1: Acari Algophagidae Lamingtonacarus posidonis
Species 2: Acari Algophagidae Lamingtonacarus oreillyorum
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