The gnathosoma is a bad character rather than evidence for mite monophyly.

Abstract

In recent years, the case for the monophyly of mites or Acari (Parasitiformes + Acariformes) has looked increasingly weak. Much of the remaining doubt about the artificiality of this taxon stems from the importance long attributed to the gnathosoma, widely considered the most convincing morphological character supporting monophyly. The gnathosoma has long been interpreted as originating via the fusion together of the palpal coxae, which is thought to have contributed to the consolidation of the mouthparts into a compact feeding apparatus that articulates as a single unit. However, an investigation of the mouthparts of Acariformes, reported herein, revealed that fusion together of the palpal coxae is an uncommon state that convergently evolved in multiple acariform taxa rather than evolving only once, as a synapomorphy uniting Acariformes and Parasitiformes. Moreover, other defining features of the gnathosoma involve either very different modifications or structures that are not homologous between both main lineages of mites. Therefore, the gnathosoma is a bad character-poorly defined and based on a series of misinterpretations-that should not be treated as evidence for mite monophyly.