The mandibular gnathal edges: Homologous structures throughout Mandibulata
Increasing evidence, in particular from gene expression data, indicates that mandibles throughout the Mandibulata are gnathobasic and that myriapod and hexapod mandibles do not represent 'whole limbs'. These observations do not necessarily imply that the gnathal edges of the mandibles are also homologous, though this homology finds some support from gene expression. The gnathal edges of mandibles commonly consist of three parts : an incisor process (pars incisivus), a molar process (pars molaris) and a lacinia mobilis (or similar structures with different names), situated between the two processes. A comparative SEM study shows that, in contrast to the lacinia mobilis, a broad homology of the pars incisivus and the pars molaris in Myriapoda, Crustacea and Hexapoda can potentially be defended. In Chilopoda, the proximal part of the gnathal edge is represented by a bristled pad, the 'Haarpolster', that has been identified as equivalent to a pars molaris. By comparison to other myriapods, notably Symphyla, Scutigeromorpha is assumed to be plesiomorphic in having the Haarpolster on a separate sclerite and in having a molar plate formed by rows of confluent spines amidst the Haarpolster bristles. In many crustacean and hexapod mandibles, the surface of the pars molaris is covered by a pattern of scaly transverse ridges built by rows of spines, often confluent with each other and with projections of the gnathal edge. This implies that the grinding surface of the pars molaris is not the original surface of the gnathal edge, but is built on a more distal second level. Rows of spines and a spinose marginal fringe with gaps between the rows are common features of the pars molaris in myriapods, hexapods and crustaceans, providing evidence for homology by special structure, in addition to the criterion of positional correspondence.