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The Tydeoidea as a whole (Tydeidae, Iolinidae and Ereynetidae) are analysed cladistically for the first time, based on a critical reappraisal of morphological characters. In addition to the chaetotaxy, solenidiotaxy and poroidotaxy, the following characters ale considered: form of dehiscence line; number of eyes; presence of a posterior trichobothrium; number of discs on genital acetabula; breadth of cis-acetabulal area; sexual dimorphism (indicative of true mating segmentation of legs and palps; presence and structure of ereynetal organ; shape of chelicerae and tarsus II and number of calyptostases. Special attention is paid to a comparative study of the segmentation and chaetotaxy of the palp within the superfamily. as well as to the presence of prodorsal eye-spots, variations of the posterior sensilla and the segmentation of femur IV during ontogeny. Three types of phylogenetic analyses are employed: phenetic, cladistic and ontogenetic. The phenetic approach reveals that the current classification relies heavily on overall similarity between taxa, especially in adults, supplemented by ontogenetic peculiarities, such as the calyptostatic nymphs of Speleognathinae. The cladistic analyses lead to a reorganization of the Tydeoidea into four families. The Meyerellidae, characterized by the presence of three prodorsal eye-spots, include the Meyerellinae and Triophytydeinae. while the Tydeidae are restricted to Australotydeinae, Pretydeinae and Tydeinae. The remaining two families, Iolinidae and Ereynetidae, form the informal group Procurvata, characterized by the procurved dehiscence line. The family Iolinidae is enlarged to encompass the subfamilies Tydaeolinae, Pronematinae and Iolininae. The Ereynetidae, characterized by the ereynetal organ and double genital discs, include the Ereynrtinae :senior synonym of Pseudotydeinae, transferred from the Tydeidae), Lawrencarinae and Speleognathinae. Minor discrepancies were found between the results for immatures and adults. These can be explained by ontogenetic trajectories that are not parallel and undergo a spectacular expansion into the character space as they extend. Within the Tydeoidea, diversification and adaptation have occurred through acceleration, with adult adaptations extending into earlier stases. Heterostasy is only expressed in the Speleognathinae, in which the nymphs are all calyptostatic. The monophyly of the Tydeoidea remains questionable, since the Meyerellidae might constitute a separate group, more closely related to the Eupodoidea. The Meyerellidae aside, the tydeoid mites seem to have originated from a group of free-living forms that colonized the soil and related habitats and underwent an early radiation, giving rise to three major lineages: the Tydeidae, Iolinidae and Ereynetidae. The Tydeidae are characterized by a low evolutionary rate combined with a high diversification indicative of a secondary adaptive radiation within the Tydeoidea. In contrast, the Iolinidae are characterized by a high evolutionary rate combined to a low diversification. The third lineage, the Ereynetidae, is highly diverse, showing high rates of evolution and speciation, linked to the adoption of endoparasitic habits. Different hypotheses to explain the success and diversification in Tydeidae and Ereynetidae are examined. (C) 2000 The Linnean Society of London

Phylogeny, ontogeny and adaptive radiation in the superfamily Tydeoidea (Acari: Actinedida), with a reappraisal of morphological characters