Leg morphology and muscle systems in millipedes (Myriapoda, Diplopoda) investigated utilizing synchrotron micro-CT

In this study the leg anatomy of representatives of 15 of the 16 millipede orders is described and analyzed. Therefore, scans obtained by utilizing Micro-computed tomography (μCT) and Synchrotron-radiation Micro-computed tomography (SR-μCT) of 21 millipede specimens, including representatives of Siphonocryptida and Stemmiulida, which were never described for their leg anatomy before, as well as photographs of 15 of the 21 specimens are used to describe 79 characters concerning the leg anatomy. A phylogenetic analysis of the described characters is performed and used for the evaluation of leg characters and their relevance for phylogenetic hypotheses. Relevant descriptions in the literature based on dissection and histology are compared to the findings.
The examined species strongly differ in the shapes and sizes of the podomeres and in the attachment points of homologous muscle strands. Several unique muscles of certain species are described. A character-mapped strict consensus tree based on 2 same-length trees with the length of 197 steps, as well as a character-mapped tree based on the most recent Diplopoda phylogeny are described. In comparison to former descriptions of the leg anatomy of representatives of certain millipede orders, many similarities and differences are found in the μCT-reconstructions of this study. Some descriptions in the literature, as well as some findings of this study are suspected to be misinterpreted due to preparation artifacts.
An adjusted terminology for the podomeres of Polyxenida is suggested. The homology of podomeres traditionally denoted as coxa, trochanter, prefemur, femur, postfemur, tibia, tarsus 1, and tarsus 2 is in doubt. Recent publications and the observation of a muscle inserting at most distal podomere provide evidence for the Polyxenida to possess less podomeres than formerly described and for the podomeres to be denoted as coxa, prefemur, femur, postfemur, tibia, and tarsus. The legs of Siphonocryptida possess a typical character set for “rock-climbing” and “fast-running” millipedes, and Stemmiulida possess features which cannot be assigned to a certain type of locomotion.
Overall, no phylogenetic trend is seen in the evolution of specific modifications of leg characters. The here produced strict consensus tree does not recover any phylogenetic hypotheses of the past. The findings of this study can show that the leg anatomy of millipedes strongly depends on environmental rather than evolutionary factors.