Arthropod segments and segmentation – lessons from myriapods, and open questions

The current understanding of the segmental organization of myriapods is dramatically different from the traditional views, due both to fresh research on a number of myriapod species and to progress in comparative developmental biology at large. In the late eighties, the emerging paradigm of ecdysozoan affinities of arthropods prompted a revisitation of the concept of the segment as an archetypical body unit. Fresh approaches to myriapod comparative morphology and pioneering studies on the developmental genetics of segmentation in Strigamia maritima, Lithobius atkinsoni and Glomeris marginata contributed to the definitive abandonment of the Articulata hypothesis and suggested a reformulation of the segment concept as the product of a functional integration between a number of distinctly repeated serial units, e.g. neuromeres, tergites, sternites. In the light of this refreshed interpretation of myriapod segmental architecture, we must reject the naïve views that (a) segment production is necessarily prior to segment patterning, (b) serial features evolve from polymerous and poorly patterned to oligomerous and strongly patterned series (Williston’s law), and (c) the divide between embryonic and post-embryonic development is necessarily a major turning point in morphogenetic processes. Open questions remain, e.g. to which extent the production of serial structures along the main body axis of myriapods is multiplicative rather than sequential.