The transformation from monocondylous to dicondylous mandibles in the insecta

The evolution of the dicondylous mandible of insects can be reconstructed b ecause the Lepismatidae show an intermediate condition between monocondylou s and dicondylous mandibles. Monocondylous mandibles as in the Entognatha a nd Archaeognatha have only the dorsal (primary) condyle found in all Mandib ulata and are moved around a vertical axis. Dicondylous "biting-type" mandi bles as in the Pterygota have an additional (secondary) articulation anteri or to the primary articulation, so that movement is limited to a transverse adduction around a horizontal axis of swing. Mandibles of the Lepismatidae have been considered intermediate with respect to their elongated shape an d their musculature (BORNER 1909). Video recordings of the feeding movement s of Lepisma saccharina L., 1758 show an unexpected motion pattern of the m andibles. The anterior articulation of L. saccharina comprises a clypeal an d a tentorial condyle forming a guide for two sclerotized ridges on the dor sal surface of the mandible. Contrary to orthopteroid mandibles, which poss ess ball and socket type articulations, the movement of lepismatid mandible s is mainly a pro- and retraction. The secondary articulation acts as a gui de. The same components of the secondary articulation as in the Lepismatida e can be found in larval Ephemeroptera, where the tentorial condyle is atta ched to the inner ridge of the mandible. In Odonata and Neoptera the second ary articulation is a ball joint, formed between the clypeal condyle and th e ridges. These results show that the mandibles in the Lepismatidae have an intermediate condition in regard to shape, musculature and function of the secondary articulation. The further transformation of the insect mandible took different paths in Ephemeroptera and Metapterygota.