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During the summer of 1957 the writer became interested in water mites while collecting insects in an unnamed pond (see map in appendix) near Tomales, California. It was noted during that period that specimens of a giant water-bug, Lethocerus americanus Leidy (1847) had many larval mites attached to their ventral sides, and the lack of information concerning these animals and their life histories stimulated interest in investigating the problem detailed in this thesis. This work was begun in the summer of 1957 and has continued through the summer of 1959 and 1960. Due to the lack of information and lack of agreement by other investigators concerning the morphology of the adult Hydrachna miliaria Berlese (1888) [bilunata Marshall (1931)], the problem of this thesis is to redescribe it and to describe the stages of its life history. Stages of the life history of H. miliaria have not been described previously by other investigators.

This study is part of the Swedish Taxonomy Initiative (STI) - one of the most ambitious all taxa biodiversity inventories in the world. One of the pillars in STI is to support taxonomic research on the most neglected taxonomic groups with the aim to lift the level of knowledge of biodiversity in the country. There is still a lot to be discovered, especially in the microscopic world, and this includes mites. Many aspects of mite biology and diversity are poorly known, such as species richness, abundance, distribution, lifestyle and behavior of species. Mites inhabits all sorts of aquatic, terrestrial, arboreal and parasitic habitats, nevertheless even in well-studied systems mites are often overlooked. Despite being among the smallest of arthropods, they are of medical and economical importance and may be very abundant in the ecosystems they inhabit. This thesis focuses on Parasitengona (Acariformes: Prostigmata), one of the most diverse taxa among the arachnids. It includes the aquatic Hydrachnidia (water mites) and the terrestrial Trombidia (e.g. velvet mites, chiggers). A unifying characteristic of Parasitengona is their complex life cycle of active and inactive stages, parasitic larvae and predatory deutonymphs and adults. They typically parasitize and prey on arthropods, except the chiggers which have vertebrates as hosts. The aim of this thesis is to shed light on the phylogeny and taxonomy of Parasitengona with emphasis on the Swedish fauna. To achieve this, mites were collected from different localities throughout the country between the years 2007-2016. Water mites were sampled with a hand net. Larvae of terrestrial Parasitengona were collected with sweeping nets and sorted out from malaise trap samples from the Swedish Malaise Trap Project. To collect the adults Berlese-Tullgren extractor and pitfall traps were used as well as hand collecting and sifting with litter reducer. The material collected abroad was kindly provided through collaboration.  Methods used in the papers included morphometrics, multivariate analyses, experimental rearing, DNA extraction and sequencing, Bayesian phylogenetic analyses and molecular species delimitation. In paper I and II, we combine molecular species delimitation models and morphological data to resolve taxonomical issues. This integrative taxonomic approach of combining data resulted in Piona dispersa Sokolow, 1926 as a valid species and redescriptions, new synonyms and neotypes provided for Erythraeus phalangoides (De Geer, 1778),  E. cinereus (Dugès, 1834) and E. regalis (C.L. Koch, 1837). Based on the new inventories we produce an updated and annotated checklist of 105 terrestrial Parasitengona species for Fennoscandia in paper III, and use metadata to increase the knowledge on distribution, habitat preferences, life stages and abundance. Out of these, 20 species are new findings for the region and five are potential new species for science. In paper IV, we provide a molecular phylogeny of Parasitengona based on the genes 18S, 28S and COI for 80 taxa with a sampling focus on the terrestrial lineages. Based on the results we offer a revised higher-level classification of the group. In particular the analyses supported Tanaupodoidea Thor, 1935 as a separate superfamily, but Trombiculoidea Ewing, 1929 was not monophyletic and was synonymized, along with Chyzerioidea Womersley, 1954, with Trombidioidea Leach, 1815. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p><p> </p>