species (for an extended discussion of arthropod segments and their homology, see Ref.  and ). Our data reject the idea that the rings in adult ring-forming species are homologous to the body segments of other arthropods. Rather, these rings seem to be a specific innovation of these diplopod species and are a composite of parts from different metameric units. In order to avoid further confusion in the future, we suggest not to use the term "segment" for the body rings and to denote the rings with a single leg pair as "haplorings" instead of "haplosegments" and the rings with two pairs of legs as "diplorings" instead of "diplosegments".
It is clear now, that the origin of diplosegmentation cannot be understood on the basis of the tergites alone, but must also take into account the origin and development of the dorsal embryonic segments, especially those that correlate with two ventral embryonic segments. Unfortunately, so far we were not able to identify genes that are expressed at the boundaries between the dorsal embryonic segments; thus at present the establishment of this boundary is unclear. Research into the developmental genetics of the dorsal embryonic segments in Glomeris is thus an exciting topic and will lead to further insights into the peculiar mode of diplosegmentation in the future.
Apart from the still unclear mode of diplosegmentation, however, our finding of the independent segmentation of the dorsal exoskeleton (tergites) in Glomeris, may explain dorsoventral peculiarities in other arthropods. As mentioned in the introduction, taxa like e.g. notostracan crustaceans, pauropods and symphylans show a mismatch of tergite numbers with leg pairs. Independent dorsal and ventral segmentation in a similar manner as in Glomeris, and the independent establishment of the tergites, in theory can lead to any combination of tergites with any number of leg pairs. If tergite formation and the formation of segments also are not coupled in notostracans, pauropods, and symphylans, this could be the cause for the dorsoventral discrepancies in these arthropods.
We would like to thank Wolfgang Dohle for stimulating discussions on Glomeris segmentation. The work of W.D. was supported in part by the DFG via SFB 572 of the University of Cologne and by the European Union via the Marie Curie Research and Training Network ZOONET (MRTN-CT-2004-005624), and the work of N.-M.P. was supported by a DFG grant (TA99/19-2).
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