Model for tergite – sternite/leg pair correlation in millipedes based on embryonic evidence from basal non-ring-forming millipedes. The model is based on the way the dorsal and ventral metameric units form in the embryo. Tergites are denoted with roman numbers (I–VI), sternites and leg pairs are denoted with Arabic numbers (1–8). This model assumes that the tergites are directly derived from the dorsal embryonic segments, and accordingly correlates the tergites with the sternites and leg pairs (panel A). Panel B: hypothetical extrapolated model for ring-forming millipedes. The situation in ring-forming millipedes can only be explained by assuming a shift in the fusion of dorsal and ventral exoskeletal elements (indicated as blue shades in panel B). For instance, the first ring in the ring-forming millipedes forms via the fusion of tergite II to the leg pair 1, the second ring via the fusion of tergite III with leg pair 2, and so on; the different shades of blue in panel B represent the rings in adult specimen. The first tergite is the collum that is not fused to any leg pair. This model leads to a different tergite – sternite/leg pair correlation as in the 'adult morphological' model in Fig. 3. For details, see text.