etween the metameric units and (4) the dorsal metameric units differ in number from the ventral ones. (5) But most significantly, the genetic interactions must be different as wingless expression is missing in dorsal tissue. Dorsal and ventral segmentation thus are not coupled in Glomeris.
Our data show that the tergite borders are defined independently from the embryonic segment borders (both ventral and dorsal) and are established by a distinct genetic mechanism that uses engrailed but not wingless signalling. Thus, in terms of genetic mechanisms involved, the segmentation of the dorsal exoskeleton appears to be different from the segmentation processes leading to the formation of the ventral and dorsal embryonic segments. On the other hand, the formation of the tergites seems to build on the prior establishment of the dorsal embryonic segments. Tergite borders invaginate roughly in the middle of each dorsal embryonic segment and the tergites themselves are formed by the posterior half of one dorsal embryonic segment and the anterior half of the following one. This process bears some resemblance to resegmentation in vertebrates where the primary metameric units, the somites, are divided in the middle and the final metameric units, the vertebral bones, are formed from the posterior half of one somite and the anterior half of the next one (reviewed in ). The two processes in Glomeris and vertebrates certainly are not homologous, but might nevertheless share some developmental mechanisms by convergent evolution. These new insights now allow a modified model for tergite – sternite/leg pair correlations in basal milipedes with non-fused exoskeletal elements, like Glomeris (see Fig. 9A). But our data also have ramifications to the ring-forming process in derived millipedes. According to our results, the tergites are shifted in relation to the sternites on the ventral side of the animals. In order to fuse into a ring, a tergite therefore has to "choose" between two alternatives: for example tergite III may fuse either with the sternite a bit in front of it (sternite of ventral embryonic trunk segment 2), or with the sternite a bit behind of it (sternite of ventral embryonic trunk segment 3). The former alternative where the tergites fuse with the sternites shifted slightly in front of them would lead to the situation found in the large majority of all ring-forming species (Fig. 9B): tergites II, III, and IV fuse with sternites 1, 2, and 3 respectively, whereas all following tergites fuse with two consecutive sternites, leading to three rings with a single leg pair followed by rings with two leg pairs each. However, the second alternative also seems to be realized in nature, because there is a group of juliform millipedes, the Spirobolida, that have four single leg bearing rings . Assuming that in these animals the tergites fuse with the sternites of the slightly posterior ventral embryonic segments, the result would be four rings with a single leg pair followed by rings with two leg pairs each (Fig. 9C).
Although our results suggest a solution to the problem of tergite – sternite/leg pair correlations in diplopods, they do not provide an immediate answer as to the origin and evolution of diplosegments in this myriapod group. Our results do not support the existence of single segments with two pairs of legs each. Rather, the expression of engrailed, wingless and other segmentation genes  suggests that the ventral embryonic segments correspond to the body segments of other arthropods and these ventral embryonic segments bear only a single pair of legs each. The term "segment" is problematic when used for metameric units in millipedes, because as we have shown during embryogenesis there are at least three different series of repeated units (ventral embryonic segments, dorsal embryonic segments, tergites) all of which are established independent from each other. In the past the term "segment" has been used quite indiscriminately for all these repeated units and also for composite units like the fused armour rings in ring-forming 