On corniculi, rutella and pseudorutella - some ultrastructural details of key-characters in Acari [Arachnida]

The gnathosoma is considered by most authors as the main constitutive character of a monophylum Acari. However, this has been questioned due to fundamental differences regarding its morphology in Anactinotrichida (=Parasitiformes s.l.) and Actinotrichida (=Acariformes). A key character which might indicate homology is represented by stout cuticular structures called corniculi, rutella, and/or pseudorutella present in most of the main groups of mites. However, the homology of these structures has also been doubted. In the present paper a first comparative ultrastructural study on these elements is presented taking representatives of all main groups of Acari in consideration (except of ticks lacking these structures). It was shown that the structures under concern are quite different even in closely related taxa. Hence the question on homology could not be solved definitely. Evidently more studies are needed to elucidate this problem.