EVOLUTION OF CRANIAL STRUCTURE IN ADULT COLEOPTERA

Abstract

The variety in the structure of the cranium is due largely to the disproportionate growth of its sclerites. The epifrons may grow anteriorly at the expense of the frons until the latter is almost or completely suppressed and the antennal sockets lie near or against the clypeus. The parietals may also grow laterally and invade the ventral side where they may form a considerable portion of the ventral wall at the expense of the postgenae.The clypeus may either contract or expand. Its lateral lobes, the anterior condyles of the mandibles, are usually separated from the body of the clypeus by sutures. The anterior tentorial pits retain their primitive position against the clypeus at the level of the mouth.New evidence is given for the origin of the gula by sclerotization of the neck membrane between ventral extensions of the postoccipital ridge which now form the gular ridges. The gula may be partially or completely suppressed by the mesal growth of the postgenae.The basal limit of the submentum may be defined by a line drawn through the ends of the postoccipital or gular ridges but, rarely, the mesal growth of the postgenae may crowd out its proximal region and thus a postgenal bridge is interposed between the gula and the definitive submentum.