Morphology of Hydrozetes confervae ( Schrank, 1781) and H. parisiensis Grandjean, 1948 (Acari: Oribatida: Hydrozetidae), and keys to European species of Hydrozetes Berlese, 1902

Hydrozetes confervae [Schrank, F.P., 1781. Enumeratio insectorum Austriae indigenorum. Augustae Vindeliciorum, 1–552] and Hydrozetes parisiensis [Grandjean, F., 1948. Sur les Hydrozetes (Acariens) de l’Europe occidentale. Bull. Mus. Nat. Hist. nat., Paris, ser. 2(20), 328–335] are common European species of the aquatic oribatid mite family Hydrozetidae, but their external morphology is incompletely known. We describe and illustrate their larvae for the first time, and the nymphs of both species are redescribed and illustrated. Juveniles of the two species differ mainly by the position of gastronotal seta lm in relation to the opisthonotal gland opening (gla) and by the number of extremely long posterior hysterosomal setae in nymphs. In H. confervae seta lm inserts anteromedial to gla, but posterior to gla in H. parisiensis; nymphs of H. confervae have three pairs of long hysterosomal setae, while H. parisiensis has a larger, but variable number. Separate identification keys for the larvae, nymphs and adults of all known European species of Hydrozetes Berlese, 1902, are presented.