Muscular anatomy of the millipede Phyllogonostreptus nigrolabiatus (Diplopoda: Spirostreptida) and its bearing on the millipede "thorax"

The muscular anatomy of the millipede Phyllogonostreptus nigrolabiatus (Newport, 1844) (Diplopoda; Spirostreptida; Harpagophoridae) is comprehensively surveyed. The musculature of the first three postcollum pleurotergites, the "thorax," and their associated appendages was found to be more complex than that of the postthoracic rings. It is hypothesized that the musculature of the postthoracic segments is derived relative to that of the thoracic segments, which retain primitively free sternites and are not diplosegments. This hypothesis is discussed relative to previous hypotheses positing that the anteriormost three leg-bearing rings in millipedes are diplosegments. The musculature of spirostreptid gonopods is described in detail for the first time. Comparison of the cephalic musculature is made with previously described musculature in Julida showing that, while many aspects of the musculature are conserved, there exist interordinal differences, documenting the potential utility of comparative anatomical studies for resolving millipede phylogeny.