A rudimentary sheath for the smallest of “biting” chelicerae: the mouthparts of Cunliffea (Nematalycidae) and a new hypothesis on the origin of the stylet sheath of Eriophyoidea (Acariformes)

The cheliceral digits of soil inhabiting Nematalycidae are among the shortest of any chelicerate (≈5 µm long). But Eriophyoidea, which may be nested within Nematalycidae, have relatively long (10–70 µm), styliform cheliceral digits that are enveloped within a subcapitular sheath. Scanning electron microscopy observations on the mouthparts of a nematalycid, Cunliffea cf. strenzkei (Cunliffe), show it has a rudimentary sheath for chelate (“biting”) chelicerae. This discovery – presented herein – is the basis for a new hypothesis, which proposes that the stylet sheath of Eriophyoidea could have originated when the chelicerae were still chelate. Accordingly, the sheath would have originally covered the space between the fixed and movable digits, extending the preoral cavity anteriorly. As the digits became increasingly styliform, the sheath would have expanded around them, otherwise the hermetic seal of the preoral cavity would have been compromised. The sheath would have expanded for this reason unti...