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Redescription of the mite Glaesacarus rhombeus (Koch & Berendt, 1854) from Baltic amber (Upper Eocene): evidence for female-controlled mating

Glaesacarus rhombeus (= Acarus rhombeus Koch & Berendt, 1854) from the Upper Eocene Baltic amber is the oldest known fossil of Astigmata, and therefore is important for calibrating the molecular clock for mite phylogenies and inferring the direction of character state changes in phylogenetic analyses. Based on the imprecise original description, it has been included in five different families of astigmatan and non-astigmatan mites. Using a novel method of sample preparation, we provide a detailed taxonomic characterization of G. rhombeus based on the type material and many additional specimens. We show that G. rhombeus belongs to a unique, previously unknown lineage of astigmatan mites (Glaesacarus gen. nov. and Glaesacaridae fam. nov.). The family Glaesacaridae does not have any close recent relatives, although some apomorphies or presumed apomorphies indicate that it may be distantly related to the extant family Lemanniellidae. Morphological evidence found in Glaesacaridae further challenges the current concept of the superfamily ‘Canestrinioidea’, an assemblage including Canestriniidae, Heterocoptidae, and Lemanniellidae, and calls for the exclusion of the two latter taxa from this superfamily. Furthermore, we show that the authorship of Sarcoptidae, one of the families in which G. rhombeus was placed historically, should be Berendt (1845). The terminal hysterosoma of G. rhombeus females has a unique, pad-like organ, resembling several structures found in males of many Astigmata that are used to hold the female during copulation. An exceptionally well-preserved copulating pair of G. rhombeus provides insights into the functional morphology of this organ, which apparently was used by the female to cling to the male during mating, suggesting that females assumed the active role during mating and have partial or complete control over copulation. This reversal of the stereotypic sex roles is a very rare example among animals performing direct insemination.