Paedomorphosis and sexuality in Eulohmanniidae (Acari, Oribatida): surprising diversity in a relictual family of oribatid mites

The distinctive oribatid mite family Eulohmanniidae has been known almost exclusively from the type species of Eulohmannia, E. ribagai (Berlese, 1910), which is widely distributed in the northern Hemisphere where it inhabits fine humus, typically beneath forest litter and moss. We describe the morphological ontogeny of E. ribagai based primarily on material from New York—supported by specimens from Canada, Europe, and Asia—and correct errors in the literature. Ancillary biological notes relate to phenology, feeding biology, and reproduction; most important is the discovery of apparent sexual populations in the Pacific Northwest in this otherwise thelytokous (‘parthenogenetic’) species. Feeding seems to be preceded by secretion of a thick globule that captures food particles in the ventriculus, but this needs verification. Related nomenclatural actions include the clarification of purported synonyms and the proposal of species-rank for E. ribagai bifurcata Fujikawa. The latter was described as having several traits, including the purported presence of opisthonotal glands, that a study of type specimens show to reflect errors of observation or interpretation. A new diagnosis of Eulohmannia is informed also by preliminary observations on two as-yet undescribed species from Asia, one of which is apparently sexual, and the other paedomorphic in being monodactylous. We propose a new genus, Paedolohmannia n. gen., with P. metzi n. sp. as type, based on type material from Oregon, USA and additional material from California and Washington. The paraprocts of this species are formed by the adanal segment in all post-larval instars; it is only the second unequivocal example in Oribatida of the anal segment failing to appear during anamorphic development. This species also appears to be sexual, so it is unlikely that thelytoky was an ancestral trait of the family. Based on new information, the diagnosis for Eulohmanniidae is adjusted and expanded. Published inferences on the relationships of the family with other members of the paraphyletic infraorder Mixonomata, variously based on morphological and molecular methods, are reviewed and found inconsistent. While properly considered a relictual taxon, Eulohmanniidae is more diverse than previously thought, with the northeast Palaearctic and northwest Nearctic being regions especially deserving of more thorough sampling and genetic analysis.