Ontogenetic stages of Gymnodamaeus irregularis, with remarks on morphology of the juveniles of Gymnodamaeidae (Acari: Oribatida: Plateremaeoidea)

The juvenile stages of oribatid mites of the family Gymnodamaeidae are insufficiently known, and little information is available on the morphology of ontogenetic stages. Comparative characteristics of juvenile stages of species belonging to family Gymnodamaeidae are given based on our own data and available literature sources. Herein, the morphology of ontogenetic instars of Gymnodamaeus irregularis Bayartogtokh & Schatz, 2009 is investigated. The larva and nymphs are generally similar in their habitus. Basic characteristics in juveniles are the less sclerotized and unpigmented body with granulate surface of gastronotum, larva with 11 pairs of gastronotal setae (h3 absent) and nymphs with 10 pairs of gastronotal setae (setae la, lm and d-series absent, but setae lp and p-series present). Nymphs with ridge-like structure on prodorsum, and large excrescence on posterior end of gastronotum, bearing one pair of long (h1 ) and another pair of medium-long (p1 ) setae. All prodorsal, notogastral and adanal setae of larva and nymphs are densely barbed throughout. Conspicuous characters in the larval stage are the very long interlamellar setae, which are reduced in nymphs and adults to minute setae and the subequal lengths of gastronotal setae. All stages of nymphs carry gastronotic exuviae with reticulate granular sculpture of the previous instars. Adults are coated with thick cerotegument on the idiosoma, legs and most body setae. The sensillus is dilated in its distal part and covered with cerotegument. The notogaster has two pairs of irregular longitudinal ridge-like structures (procuticle); five pairs of notogastral setae; and a pair of strongly developed tubercles Sp on epimeral region III. Gymnodamaeus irregularis is a Western Palaearctic species, currently known from Central (East Tyrol, Austria) and Eastern Europe (Nizhny Novgorod region, Russia) and appears to be primarily an inhabitant of mosses and litter, and it prefers the xeric habitats.