Head structures of Priacma serrata Leconte (Coleptera, Archostemata) inferred from X-ray tomography

Internal and external features of the head of Priacma serrata were studied with X-ray microtomography and with histological methods. The comparison of both techniques shows that X-ray tomography is a promising new technique for the investigation of insect anatomy. The still somewhat coarse resolution of the X-ray data is compensated for by advantages like the nondestructive and artifact-free data acquisition. The head of P. serrata and other adults of Archostemata is characterized by many derived features. Muscular features of Priacma, especially muscles of the labium and pharynx, differ strongly from what is found in other groups of Coleoptera. Several character states are considered as autapomorphies of Archostemata: scale-like surface structures, constricted neck, strongly reduced tentorium, and the plate-like, enlarged prementum. The scales provide a protecting surface pattern and may have evolved with a more exposed lifestyle. The enlarged prementum forms a lid, which closes the mouth and covers the ligula when it is pulled back by contraction of the unusually strong submento-premental muscle. The presence of four cone-shaped protuberances on the dorsal side of the head is considered an autapomorphy of Cupedidae. The galea with a narrow stalk and a round and pubescent distal galeomere is another autapomorphy of this family. It has probably evolved as an adaptation to pollen-feeding. The shape of the mandible of Cupedidae is plesiomorphic compared to what is found in adults of Ommatidae. The vertical arrangement of apical teeth is an autapomorphy of the latter family. The lateral insertion of the antenna in Priacma is a groundplan feature of Cupedidae. The dorsal shift is a synapomorphy of all other cupedid genera. A cladistic analysis of characters of the head and additional data resulted in the following branching pattern: ((Crowsoniella + (Omma + Tetraphalerus)) + (Micromalthus + (Priacma + (Paracupes + (Cupes + Tenomerga + Prolixocupes + Rhipsideigma + Distocupes + (Adinolepis + Ascioplaga)))))).