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The systematic status of Fuscozetes Sellnick, 1928, is not clear in the literature. Therefore, the morphological ontogeny of F. fuscipes (C.L. Koch, 1844), the type species of this genus, was investigated and compared with its congeners in this study, and a new diagnosis of Fuscozetes is given. The juveniles of F. fuscipes are light brown, with a brown prodorsum, sclerites, epimeres, and legs. In all juveniles, a humeral organ and a humeral macrosclerite are present. The gastronotum of the larva has 12 pairs of setae (h3 is present), whereas the nymphs have 15 pairs. In the larva, the gastronotal shield is weakly developed, and most gastronotal setae are short except for a slightly longer h2. Most of the gastronotal setae are inserted on the microsclerites except for h3, and several other macrosclerites and many microsclerites are present on the hysterosoma. In the nymphs, the gastronotal shield is well developed, with 10 pairs of setae (d-, l-, and h-series, and p1), and setae p2 and p3 are located on a large posteroventral macrosclerite. In all the instars, femora I and II are oval in cross-section, without a large ventral carina. Mitochondrial COI sequence data revealed a deep split between the Nearctic and Palearctic populations of F. fuscipes, and a less, but significant, divergence within each continent. These strong geographical barriers were contrasted with multiple cases of shared haplotypes over long distances in the Palearctic, indicating high migration rates in modern times.

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Nanhermannia coronata Berlese, 1913, is a common and abundant oribatid species in peatlands but can be easily mistaken for N. sellnicki Forsslund, 1958, as an adult. Therefore, the identity of adults of N. coronata from several sites in Norway and Ireland was supported by the COI sequence data, and based on this material, the morphological ontogeny of this species is described and illustrated to highlight the differences between N. coronata and N. sellnicki. In all juvenile stages of N. coronata, the bothridial seta is absent, but two pairs of exobothridial setae are present, including short exp and exa reduced to its alveolus. In the larva, seta f1 is setiform, but in the nymphs, it is reduced to its alveolus. Most prodorsal and gastronotal setae of larva are short, and of nymphs they are long. In all instars, the leg segments are oval in cross section and relatively thick, and many setae on tarsi are relatively short, thick and conical, except for longer apical setae. Seta d accompanies solenidion σ on all genua, φ1 on tibia I and φ on other tibiae. We found some morphological characters that clearly differentiate N. coronata from N. sellnicki, like the number of setae on femora of adults and tritonymphs, the shape of insertions of prodorsal seta in and all gastronotal and adanal setae of juveniles; in N. sellnicki, these setae are inserted in small individual depressions, whereas in N. coronata, these depressions are absent. Our ecological observations confirm a common occurrence of N. coronata in raised bogs, a high percentage of juvenile stages in its populations and a preference of this species for humid microhabitats, whereas N. sellnicki is less common than N. coronata and occurs in drier habitats.

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Arachnid orders, Mesostigmata, Trombidiformes, and Sarcoptiformes, commonly known as 'mites', are abundant in mires, both as adults and as juveniles. However, due to the challenges of identification, the juvenile forms are often excluded from analyses. This is the first study in mires that included all three mite orders identified to the species level, including juvenile instars. We aimed to compare how diversity and the response to ecological variables differed if only the adults (ad) vs. the total number of specimens (ad+juv) are considered. Samples of 20 Sphagnum species (five subgenera) were collected and mites were extracted using Berlese funnels. Overall, nearly 60,000 mites were analyzed; of these Mesostigmata made up 1.87% of the total, Trombidiformes -0.27%, and Sarcoptiformes -97.86%. The study revealed 154 species (33 Mesostigmata, 24 Trombidiformes, and 97 Sarcoptiformes), the highest diversity of mites ever reported from mires. The inclusion of juveniles increased observed species richness by 6%, with 10 species (one Mesostigmata, six Trombidiformes, and three Sarcoptiformes) represented only by juvenile forms. Seventeen species are new to Norway (four Mesostigmata, one Sarcoptiformes, and 12 Trombidiformes, including five undescribed species of Stigmaeidae and Cunaxidae). Four of these were represented in the samples only by juveniles. Including the juveniles explained a greater amount of the variability of Trombidiformes (explanatory variables account for 23.60% for ad, and 73.74% for ad+juv) and Mesostigmata (29.23% - ad, 52.91% - ad+juv), but had less of an impact for Sarcoptiformes (38.48% - ad, 39.26% - ad+juv). Locality, Sphagnum subgenus and species, wetness, and trophic state significantly affected the mite communities and should be taken into consideration when studying mires. Since juvenile stages contribute significantly to mite diversity in mires, they should also be included in mite studies in other habitats.

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Vegetation is widely used in the assessment of the quality of peatlands, while the invertebrate fauna of peatlands is relatively poorly studied. We compared the bioindicator values of vegetation with two arthropod groups widespread in peatlands, saprophagous Oribatida (Acariformes) and predatory Mesostigmata (Parasitiformes) mites. Samples were collected from ecotones at the edges of peatland ponds in Poland, including four in near-natural condition (i.e., peatlands unaffected by human activity) and three in previously disturbed but now recovering peatlands. A set of abiotic parameters was measured at each site: pond area, mean annual temperature, annual precipitation, and water parameters (pH, conductivity, colour, total nitrogen, phosphorus, calcium, and organic carbon). Overall, 63,635 specimens of Oribatida and 448 of Mesostigmata were recovered in the sampling. Species richness of Oribatida (56 species) was higher than that of flora (46) and Mesostigmata (15). Vegetation was significantly associated with annual precipitation in the years 1998-2007 which accounted for 29.1 % of the variation in vegetation communities. Oribatida variability was significantly associated with the content of organic carbon in water accounting for 32.4 % of variation. In contrast, variation in the Mesostigmata was not significantly associated with any of the abiotic parameters. Vegetation at ponds in previously disturbed and now recovering peatlands had higher bush cover than at near-natural ponds and the pond in the cutaway peat had lowest moss cover and the highest number of associate species (i.e., species with wide tolerance not characteristic of the certain community). Mite communities did not differ consistently between near-natural and recovering peatlands. Sphagnum divinum Flatberg et Hassel was recorded from Poland for the first time.

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The morphological ontogeny of Punctoribates ghilarovi Shaldybina, 1969 is described and illustrated. This species is most similar to P. mundus Shaldybina, 1973, but is larger than the latter species. The juveniles of P. ghilarovi are light brown, with most prodorsal setae of medium size, except for long seta ro in the larva, and short and smooth seta ex in all instars. The larva has 12 pairs of gastronotal setae, including h3, the nymphs have 15 pairs. Most of them are medium sized and barbed, except for slightly longer dm, dp, lm, lp and h1, and short and smooth h3 in the larva, and short and smooth da and dm, slightly longer dp, and clearly longer h1, h3 and lp in the nymphs. In all juveniles, a humeral organ is present.

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The morphological ontogeny of Pilogalumna tenuiclava (Berlese, 1908) is redescribed and illustrated. The prodorsal setae of juveniles are of medium size or long and barbed, and bothridial seta is fusiform, with thick, barbed head. Larva has 11 pairs of gastronotal setae, including h2, most are short and smooth, except for longer and barbed c2, c3, dp, h1 and h2. The nymphs have 15 pairs of these setae, most are short and smooth, except for longer and barbed c3. Gastronotal shield of juveniles is poorly developed, with setae of d-, l-series, and h1 in the larva, and d-, l-, h-series and p1 in the nymphs, setae of c-series, p2 and p3 are inserted on unsclerotized integument. In all juveniles, a humeral organ is present.

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The morphological ontogeny of Pilogalumna tenuiclava (Berlese, 1908) is redescribed and illustrated. The prodorsal setae of juveniles are of medium size or long and barbed, and bothridial seta is fusiform, with thick, barbed head. Larva has 11 pairs of gastronotal setae, including h2, most are short and smooth, except for longer and barbed c2, c3, dp, h1 and h2. The nymphs have 15 pairs of these setae, most are short and smooth, except for longer and barbed c3. Gastronotal shield of juveniles is poorly developed, with setae of d-, l-series, and h1 in the larva, and d-, l-, h-series and p1 in the nymphs, setae of c-series, p2 and p3 are inserted on unsclerotized integument. In all juveniles, a humeral organ is present.

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The morphological ontogeny of Phauloppia nemoralis (Berlese, 1916) is described and illustrated. The adult of this species is most similar to Ph. incomperta (Pérez-Íñigo et Peña, 1996), but differs from it mainly by the shape of bothridial seta, porose areas Aa and A1, and location of porose area A1 and seta ad3. In the juveniles of Ph. nemoralis, the prodorsal setae ro, le and in are of medium size or long, ex is short, and the bothridial seta is of medium size and clavate, with barbed head. The larva has 12 pairs of gastronotal setae, the nymphs have 14 pairs; all are of medium size and barbed. In this species, the basal excentrosclerites occur, at setae c2, h1 and l-series in the larva, and c2, p1 and l- and h-series in the nymphs. In all nymphs, solenidion ω2 on tarsus I is longer and thinner than ω1, and seta ft'' is relatively long and located posterior to solenidion ω2.

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Studies on Oribatida from Svalbard have nearly a 150-year long history. This paper reviews species diversity of Oribatida in Svalbard from a historical aspect, summarizes how often species have been found and detects erroneous reports. A list of 93 oribatid species (including Astigmata) from the Svalbard archipelago is presented. The species represent 30 families, of which Brachychthoniidae (14 spp.) and Crotoniidae (12 spp.) are particularly species-rich. The most often occurring oribatid species is Diapterobates notatus (Thorell, 1871), mentioned in 50% of publications, followed by Ameronothrus lineatus (Thorell, 1871) and Hermannia reticulata Thorell, 1871, mentioned in 30% papers each. About one third of the species have been found in Svalbard only once, and half of them (i.e. 15 species) were reported only in the last century, including five very old records. Acarological studies in Svalbard are heavily biased since they have concentrated on the island of Spitsbergen with limited sampling of other islands / island groups: for example, Barentsøya, Bjørnøya, Danskøya, Edgeøya, Hopen, Lågøya, Kong Karls Land, Prins Karls Forland and Sofiaøya.

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The morphological ontogeny of Zachvatkinibates latilamellatus Bayartogtokh Aoki, 1998 is described and illustrated. The juveniles of this species are light brown, with several pairs of small, sclerotized depressions on the gastronotum, which are unique in Zachvatkinibates Shaldybina, 1973. The larva has 12 pairs of gastronotal setae, including h3, all are inserted on unsclerotized integument; nymphs have 15 pairs, of which c-series is inserted on individual sclerites, d-, l-, h-series and p1 are on sclerotized integument, and p2 and p3 on unsclerotized integument. In all juveniles, a porose humeral organ is present.

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The morphological ontogeny of Liebstadia mongolica Bayartogtokh, 2001 is described and illustrated. The juveniles of this species are elongated and unpigmented. The larva has 12 pairs of gastronotal setae, and three of them are with excentrosclerites (c2, la, lp), the nymphs have 15 pairs, and five of them are with excentrosclerites (c2, la, lp, h2, h3). In all instars, solenidion ω1 on tarsus I is curved laterally, and is shorter than ω2.

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The morphological ontogeny of Oribatella hungarica Balogh, 1943 is described and illustrated. The nymphs of this species are apopheredermous, i.e. they carry the exuvial scalps of previous instars away from the dorsal integument, using modified setal pair da. The larva has 12 pairs of gastronotal setae, most of them have long barbs, except for c3 with short barbs and minute h3, whereas the nymphs have 13 pairs with short barbs (dm and dp are lacking). In the tritonymph, leg seta d on all femora and genu IV, l" on femora I and II and pair l on genua and tibiae I and II, l' on femur III and genua and tibiae III and IV, and ft on tarsi II-IV are thicker than other setae on these segments. In all juveniles, a humeral organ is present. In the adult, epimeral seta 4c and leg seta l" on genua and tibiae I and II, and l' on genu III and genu and tibia IV are thickened.

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The morphological ontogeny of Ctenobelba brevipilosa Mahunka, 1964 is described and illustrated for the first time; this species was investigated since its description mainly in ecological aspect. The adult instar is of medium size, with setiform bothridial seta, bearing 11-17 anterior branches, and with short notogastral setae. Seta d is absent from genua I-III and all tibia. The bothridial seta of juveniles is setiform and barbed, and prodorsal and gastronotal setae are short, except of medium sized ro. Nymphs are quadrideficient and eupheredermous, i.e. they carry the exuvial scalps of previous instars. Anal valves of protonymph and deutonymph have two pairs of alveolar setae, which is rare in Brachypylina. In the juvenile instars, seta d on all genua and tibiae is present.

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The morphological ontogeny of Punctoribates astrachanicus Shaldybina, 1973 is described and illustrated. This species was rarely investigated since its description, including its ecological and biological aspects. The adult has fusiform bothridial seta, straight anterior tectum of notogaster, four pairs of saccules, and notogastral setae mostly alveolar, except for short p-series. Tarsus I has a short dorsal projection. Juveniles of this species are light brown, with short and thin prodorsal seta ex, and relatively large humeral organ. The gastronotal setae of larva are of medium size and pointed, in nymphs they are short.

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The morphological ontogeny of Achipteria nitens (Nicolet, 1855) from Crimea peninsula is redescribed and illustrated, and ecology and some biological parameters of this species are investigated. The larva of this species has short, thick and barbed setae in and of c-series, and anterior part of prodorsum reticulate, whereas the nymphs have most gastronotal setae of medium size and barbed, except for shorter c3, and shorter, thick and smooth p2 and p3. A key to the larva and nymphs of eight species of Achipteria Berlese, 1885 is provided.

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The morphological ontogeny of Trichoribates naltschicki (Shaldybina, 1971) is described and illustrated, based on juvenile and adult specimens from a horse pasture in the steppe of northern Mongolia (Bulgan Province). The juveniles of T. naltschicki lack a humeral organ and a dark pigmented cuticle around the opisthonotal gland opening, which occur in some species of Trichoribatinae. In the adult, seta l" on genua I and II and all tibiae is thick, spine-like and barbed.

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The morphological ontogeny of Achipteria punctata (Nicolet, 1855) is redescribed and illustrated. The adult of this species has genu IV approximately as long as tibia IV, seta bv'' on femur II is as long as seta d, with short seta l' on femur III and smooth claws of all legs. The juveniles of A. punctata are stocky, with most prodorsal setae short and smooth, except for slightly longer and barbed seta ro, and gentle reticulation of cuticle between the bothridia and in the posterior part of prodorsum. In the larva, most gastronotal setae are short, thick and barbed, except for smooth c2, c3 and h2; in the nymphs the gastronotal setae are short, thin and finely barbed. In the juveniles of A. punctata, setae of l-series on legs are not thickened.

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The morphological ontogeny of Minunthozetes semirufus (C.L. Koch, 1841) is redescribed and illustrated. The juveniles of this species have a humeral organ and a gastronotal shield, which bears four pairs of setae (dm, dp, lp, h1) in the larva, and 10 pairs (d-, l-, h-series, p1) in the nymphs. In the larva, the anterior part of gastronotal shield (pygidium) reaches the insertions of setal pair dm. In the juveniles, the prodorsal and gastronotal setae are short, and in the nymphs setae of c-series are inserted on separate microsclerites. In the larva, 12 pairs of gastronotal setae are present, including h3, in the nymphs there are 15 pairs. In the nymphs of M. semirufus, solenidion ω2 on tarsus I is located anterolateral to solenidion ω1, whereas in the adult it is located posterior to ω1. In the tritonymph, seta l' is added on femora I and II, whereas seta l" on these femora is added in the adult, which is unique in Punctoribatidae.

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This study compared the oribatid mites in two natural and four industrially exploited bogs. One natural bog (Zakret, Z) was located in northeastern Poland and the other one (Toporowy Staw Nizni, TSN), in southern Poland. The four exploited bogs were also located in southern Poland and can be ranked from least to most degraded as follows: Lysa Puscizna (LP), Baligówka (B), Puscizna Mala (PM) and Kaczmarka (K). In the natural bogs, the water pH was higher than in the degraded ones, but other parameters were lower (conductivity, colour value, oxygen demand, and concentration of chlorides). In the natural bogs, the Oribatida were highly abundant (average density was 169,100 ind./m(2)), but with low species diversity and one dominating species. In bog Z the most abundant was Limnozetes foveolatus that had dominance of 75 % and in bog TSN, located at higher altitude, Trimalaconothrus maior dominated (73 %). In two degraded bogs that had still good water conditions (LP and B) the oribatid communities resembled those from the natural bogs; in LP the most abundant species was Hydrozetes lacustris and in bog B, L. foveolatus. In contrast, in two more degraded bogs (PM and K) the abundance of mites was lower (average density was 17,850 ind./m(2)), species diversity of the Oribatida was higher, and no species achieved a high dominance like in the natural bogs. Additionally, in more degraded bogs the abundance of parthenogenetic species was lower than in the natural bogs.

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Oribatida are one of the main groups of Acari comprising mostly important decomposers in soils. Most species are particle feeders, an exceptional mode of nutrition in Arachnida. Hence, their feeding organs, the gnathosoma, are of special functional interest. We studied nearly all components using scanning and transmission electron microscopies as well as reconstructions based on synchrotron X-ray microtomography from the model oribatid Archegozetes longisetosus. Besides cuticular structures, we describe the full set of muscles and confirm the presence of a trochanter remnant at the base of the chelicera. Setae on the prodorsum and the anterior and posterior infracapitular setae are mechanoreceptors innervated by two dendrites ending with tubular bodies. Dendrites of adoral setae, anterior setae of the chelicerae, and the supracoxal setae show neither obvious tubular bodies nor wall or terminal pores. Thus their function remains obscure. For the first time, a muscular proprioreceptor has been found in Arachnida. It likely monitors the actions of muscles moving the movable digit of the chelicera. Glandular structures within and associated with the gnathosoma are described. Dermal glands represented by secretory porose areas are found within the infracapitulum. More complex associated glands comprise the podocephalic glands and the infracapitular glands, the ducts of which were traced completely for the first time. The components described are mostly fundamental for the gnathosoma of Actinotrichida (Acariformes), one of the two lineages of Acari, to which Oribatida belong. The gnathosoma is generally considered the most relevant putative synapomorphy of Acari. Since the monophyly of Acari has become more and more questionable during the last decades, a thorough reinvestigation of this body part is necessary for a comprehensive understanding of acarine and even arachnid phylogeny and evolution. This article provides a starting point of such a re-evaluation of the gnathosoma.

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