These legs were made for walking... - Evolutionary morphology of the walking appendages in arachnids (Chelicerata; Arthropoda)

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Arachnids possess four pairs of appendages primarily used for locomotion. From a phylogenetic point of view arachnids constitute a monophyletic group, meaning that their common ancestor already possessed these four pairs of locomotory appendages. Nonetheless, when the appendages are compared in recent taxa, a broad morphological disparity is apparent, with structural differences observable at a number of different levels. Variation within taxa mainly occurs in legs belonging to different segments, and between taxa in homologous appendages, i.e. belonging to the same body segment. The study of chelicerate walking legs has a long history and a number of different approaches have been explored with regard to homologizations, including i) types of joints, ii) attachment of tendons, and iii) insertion of muscles. Most previous studies, however, have focused on either the number of podomeres, the arrangement of joints or their musculature, and much less on the innervation of the legs. An analysis of all these features across Arachnida has yet to be performed.Although the way in which arachnid locomotory limbs are used, i.e. for walking, has not changed significantly during arachnid evolution, two different methods of leg extension are found. While some taxa extend major joints hydraulically and lack the muscles to do so otherwise, others extend their major joints using muscles. The structural basis of neither mechanism is well understood. It is not clear for all arachnid taxa whether and where muscles for leg extension are present, and in the case of hydraulic extension the structures responsible for generating pressure are still the matter of some debate. What is clear, however, is that since hydraulic pressure is generated in the prosoma, comparative morphological studies of arachnid legs must also take the prosoma into account. This study sets out to three-dimensionally analyze the complete set of prosomata and locomotory appendages of representatives of all major chelicerate lineages (~ 20 species). These features will be studied with regard to the exoskeleton, muscles and tendons using µCT. The innervation of legs will be explored using immunolabelling combined with confocal laser-scanning-microscopy. In selected species, backfilling techniques will complement these data. For the purposes of data communication, all studied structures will be documented using interactive virtual 3D formats in combination with linguistic description based on ontologically structured vocabularies.On the basis of these descriptions, combined hypotheses for the homologization of the listed leg features will be formulated, permitting a detailed analysis of the structural evolution of walking legs and prosomata in arachnids, the group of chelicerates that most probably conquered land as early as around 420 Ma ago.