Ontogeny of the famulus in selected members of Damaeidae (Acari: Oribatida) and its suitability as a phylogenetic marker

Abstract

The famulus is a specialised seta on the dorsal side of the tarsus of the first pair of legs in acariform mites. It has various shapes, but a stable location within the whole group. In the oribatid family Damaeidae, two states of famulus are known: emergent and sunken. The emergent famulus is a simple short seta with fully emergent insertion. It is present in the adult of all known Damaeidae and in immature stases of many damaeid genera. The sunken famulus is reduced in size and submerged in a fovea with an elevated rim, so that only its tip is visible. In this study, the ontogenetic development of the famulus in selected Central European damaeid species, namely Damaeus (Adamaeus) onustus, D. (Paradamaeus) clavipes, Epidamaeus tatricus, Spatiodamaeus verticillipes, Kunstidamaeus lengersdorfi, and Belba compta, was studied with light and scanning electron microscopy and compared with that of Gymnodamaeus bicostatus (Gymnodamaeidae). Literature data on the ontogeny of the famulus in Damaeidae are summarized and the significance of the famulus as a phylogenetic marker is discussed. In agreement with previous studies, adults of all studied members of Damaeidae had an emergent famulus. The immatures of all studied members of Damaeus sensu lato, except for E. tatricus, had a sunken famulus, whereas the immatures of B. compta had an emergent famulus. Immatures as well as adults of G. bicostatus had a sunken famulus. In contrast to Norton’s phylogenetic hypotheses, all immature stases of E. tatricus possessed an emergent famulus, similar to B. compta. Therefore, either the monophyly of Epidamaeus is questionable, or, more likely, reversal to a plesiomorphic state occurred in E. tatricus.