Current ideas on the phylogeny and the adaptive radiations of Acarida

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Abstract
In this paper, the present classification and ideas on the evolutionary history of the Acarida are examined. Besides the more usual characteristics relative to the body segmentation, tagmata, tracheae and stigmata, setal birefringency, post‐embryonal development, podocephalic canal and sperm morphology, the fossil remains and the geographic distribution of Acarida are also considered. These data do not give a definite answer to the fundamental question of the acarid phylogeny, namely, whether this group is mono‐or polyphyletic. However, an attempt to outline the evolutionary history and detect the starting points for the adaptive radiations of the Acarida is made on the basis of the known elements. Presumably the early actinotrichid lineages passed onto the land with the terrestrial plant invasion, behaving first either as phytophages on plants, fungal mycelia and bacteria (Actinedida and Oribatida) or as saprophages, probably forming intimate relationships with fungi (Actinedida and Oribatida), and later as carnivores (predacious Actinedida), and finally forming parasitic and commensalistic relationships with many other animal groups (Actinedida and Acaridida). Instead, the prevalently carnivorous anactinotrichid lineages seem to owe their evolutionary success to other events, such as the Carboniferous and Mesozoic insect radiations and the Mesozoic and Cenozoic reptile, bird and mammal adaptive radiations. In the former cases, they behaved as predators while in the latter ones the anactinotrichid lineages behaved as commensals and/or parasites. The oribatid lineage is examined in detail and the probable reasons for its adaptive radiations in the soil subsystem and the adaptations consequent to the particular life style of these animals are considered. Lastly, the critical points of the acarid evolutionary success are briefly discussed.