Senckenberg Publications

R. Olomski

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The median eye of freshwater mites (Acari: Parasitengonae, Hydrachnidia) and its fate in the stem lineage of the Euhydrachnidia, Witte & Olomski 1991


Abstract

The stem species of freshwater mites (Acari: Hydrachnidia) is generally regarded to be provided with a single median eye which is located below the dorsal cuticle of the prosoma. This character state has been reported from members of the superfamilies Hydrachnoidea, ‘Hydryphantoidea’ and ‘Lebertioidea’. There have been no reports of a median eye from the other five superfamilies until now. In this study, it is shown by histological methods and observations on living specimens:
1.the presence of a single median eye in two other superfamilies, in Stygothrombioidea(Stygothrombium chappuisi)
and Eylaoidea (Limnochares aquatica),
2. the presence of a pair of ocelli situated anteromedially between the lateral eyes in those species of the euhydrachnidian
superfamilies that lack an unpaired median eye, in ‘Hydryphantoidea’(Hydrodroma despiciens), ‘Lebertioidea’
(Lebertia inaequalis), ‘Hygrobatoidea’ (Limnesia maculata, Hygrobates longipalpis, Unionicola crassipes, Piona carnea)
and ‘Arrenuroidea’ (Arrenurus globator, A. cuspidator).
The detection of the median eye in the early derived Stygothrombium and Limnochares confirms the hypothesis that the stem species of the Hydrachnidia was provided with a single unpaired median eye situated dorsally on its prosoma. Additionally, it is assumed that the pair of ocelli in later derived species of the Euhydrachnidia evolved from the median eye by division and separation.

Keywords

evolution, frontal eye, frontal organ, histology, water mites


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