A unique yet technically simple type of joint allows for the high mobility of scorpion tails

Abstract

Although being one of the most well-known animal groups, functional and constructional aspects of scorpions and especially of their tail (metasoma) have so far been overlooked. This tail represents a special construction, as it consists of five tube-shaped segments made up of strong cuticle, which are movable against each other and thus manoeuvre the notorious stinger both quickly and very precisely in space. This high mobility of an exoskeletal structure can be attributed to the connection between the segments described here for the first time. This joint allows for the twisting and bending at the same time in a single, simple construction: adjoining metasomal segments each possess an almost circular opening posteriorly, where the next segment is lodged. Anteriorly, these segments possess two saddle-like protrusions laterally, which are able to rotate in two directions on the rim of the posterior circular opening of the previous segment allowing for twisting and bending. The metasomal joint is particularly noteworthy since its mechanism can be compared to that of arthropod appendages. The scorpion metasoma is actually the only known case in Chelicerata, in which an entire body section has been modified to perform tasks similar to that of an appendage while containing digestive organs. The joint mechanism can also inspire technical applications, for instance in robotics.