The gnathosoma of Hermannia convexa (C.L. Koch) (Acarida: Oribatina) and comparative remarks on its morphology in other mites

In most groups of mites the gnathosoma is rather easy to study as a whole because it can be completely separated from the idiosoma without much difficulty. In Oribatid mites, however, the membrane attaching the chelicerae to infracapitulum and rostrophragma is very thin; separation of the gnathosoma consequently results in tearing the membrane, either between chelicerae and infracapitulum, or between chelicerae and rostrophragma. In both cases the original position of the chelicerae and their exact relation to camerostome and remaining part of gnathosoma, is difficult to reconstruct. As a result of this, only parts of the Oribatid gnathosoma have been described in literature. An extensive study of the infracapitulum (especially of its ventral surface) was published by Grandjean (1957b). Its dorsal surface was studied separately by the same author (Grandjean, 1957a). Many data are also known on the chelicerae. Until recently, however, no serious, modern attempts had been made to study the Oribatid gnathosoma in its entirety. In view of a comparative morphology of the acarid mouthparts, a better understanding of the condition in Oribatid mites was, however, indispensable. For this reason a species had to be chosen in which especially the cheliceral frame could be studied in its original position. Consequently the species should be large (in order to enable sectioning and dissection), and the prodorsum should not be too dark (in order to enable a study of the membrane by transparency). The species should moreover be present in sufficient numbers. Among the material in our collection, one species especially appeared to fulfil these requirements, viz., Hermannia convexa (C. L. Koch). This large size mite (length about 1.5 mm) had been collected by me in great numbers at the type-locality (Schwaighauser Forst, N. of Regensburg,