AbstractPhylogenetic, morphological, and developmental data concerning the Arthropoda are reviewed and discussed with the aim of reconstructing the ancestral body plan of the mandibulate arthropods (Myriapoda, Hexapoda, Crustacea). Comparative morphology as well as embryology of malacostracans and hexapods (cell‐lineages, patterns of mitotic domains, patterns of en‐stripe formation, expression zones of pair‐rule, homeotic, and gap‐like genes) suggest that (a) the basic boundary subdividing the mandibulate body into the primary embryonic regions, anterior protocephalon and posterior protocorm, runs anteriorly to parasegment PS1 (=within the mandibular segment); (b) protocephalon (pregnathal region) probably is not a unitary body region; (c) maxillary segments are closely related to the postgnathal trunk segments; (d) the “typical” mandibulate head (pregnathal‐mandibular‐maxillary) is not developed in all Mandibulata and has evolved several times in parallel; and (e) postcephalic tagmosis is much less conserved, and probably more recent, than tagmosis of more anterior areas. The arachnomorphan anterior tagma, the prosoma, is compared with the hypothesized ancestral mandibulate head.
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