Figure 9

Model for tergite – sternite/leg pair correlation in millipedes based on the lack of coupling of dorsal and ventral segmentation. Our model is based on the way the dorsal and ventral metameric units form in the embryo as well as on gene expression data that imply a lack of coupling of dorsal and ventral segmentation. Key in the model is the fact that the tergite boundaries do neither correlate to dorsal embryonic segment boundaries, nor to ventral embryonic segment boundaries. Tergites are denoted with roman numbers (I–VI), sternites and leg pairs are denoted with Arabic numbers (1–8). (A) The tergite boundaries form in approximately the middle of the dorsal metameric units (red line); consequently the tergites are shifted compared to the ventral segmental units in basal non-ring-forming millipedes like Glomeris. (B) The model can be extrapolated to ring-forming millipedes as indicated by blue shading. In most ring-forming millipedes tergite II will fuse with ventral segment 1, tergite III with ventral segment 2, and so on. (C) However, in some ring-forming millipedes (Spirobolida) tergite II will fuse with ventral segment 2, tergite III with ventral segment 3, and so on. For details, see text.