Sharp claws beneath our feet - the diversity of tarsal attachment devices of oribatid mites (Acari, Chelicerata, excluding Astigmata) - a review

The majority of animals possess claws on their legs and they are mainly used for keeping attached to the substrate. Recent studies on claw shapes and function suggest that claws are not just simple attachment devices but highly specialized morphological traits bearing information about lifestyle, ecology and evolution. In that respect, knowledge about claws is extremely scarce, especially in highly diverse groups, as for example the oribatid mites, with more than 11000 named species occurring in every environment all over the world. This review summarizes all information about claws and relevant aspects for this large group of arthropods. There is a huge variety of ambulacral claw morphologies present in oribatid mites and this diversity does not follow a strict phylogenetic or systematic pattern. Lifestyle and ecology apparently play an important role in shaping claw morphologies. Intertidal oribatid mites are mostly characterized by having single large tarsal claws to withstand tidal flooding, a similar tendency can be found in terrestrial limnic species where monodactylous species prevail. Additionally, fresh-water Oribatida often show scaliform and barbed distal tarsal setae which cooperate with the claws helping the mites to stick to water plants. Claws of arboreal mites are often equipped with adhesive pads "pulvilli" allowing them to walk on smooth plant surfaces. Suction pads are also found in a few mites showing epilithic lifestyles and in mites being able to perform sudden evasive jumps. Claws of phoretic oribatid mites show in most cases no apparent adaptations but certain groups are equipped with highly modified claws allowing them to attach to specific structures of their hosts. Finally, this review gives an overview of all claw morphologies present in each larger phylogenetic oribatid mite group.