Phylogenetic reconstruction of hexapod basal clades : Comparative embryological analyses of entognathy formations of all three entognathan constituents

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Recently, Entognatha-Ectognatha System has been challenged, and doubt has been cast on the interordinal relationships within and monophyly of Entognatha from multiple sources. Entognathy has been regarded as a most reliable autapomorphy of Entognatha. I compare the entognathy formations of Protura, Collembola and Diplura, examine the homology of their entognathies, and discuss the interordinal relationships and monophyly of Entognatha.Protura and Collembola are revealed to closely resemble each other in major points of entognathy formation, such as concerning : 1) the origin of mouth folds, which are formed by the extension and fusion of terga of intercalary and three gnathal segments, and 2) origin of the postgnathal region of entognathy, which is derived from labial terga medially migrated and united. However, the entognathy of Diplura is constructed in a different manner, which is characterized by : 1) the mouth fold is formed by the fusion of extended terga of intercalary and three gnathal segments and labial coxa, 2) the postgnathal region of entognathy is derived from labial subcoxae which medially migrate and unite into a form of postmentum, and 3) the rotation of labial appendages is involved in the entognathy formation. Thus, the homology of entognathy is well supported between Protura and Collembola, hence the monophyly of Ellipura may be supported. While the manner of entognathy formation in Diplura is revealed to be much different from in Protura and Collembola either, hence the homology of entognathy between Protura-Collembola and Diplura is not always verified, and the monophyly of Entognatha could not be substantiated.