The head morphology of Clambidae and its implications for the phylogeny of Scirtoidea (Coleoptera: Polyphaga).

External and internal structures of the head of adults of Clambus are described and illustrated in detail. The results are compared with structural features found in the clambid genus Calyptomerus, in representatives of other scirtoid families, and also in species of other coleopteran suborders, notably Myxophaga. The results tentatively support the monophyly of Scirtoidea and a close relationship between Clambidae and Eucinetidae is suggested by one shared derived feature of the mandible, a long and slender apical tooth with a serrate edge. The monophyly of Clambidae is very strongly supported and Acalyptomerus is probably the sistergroup of a clade Calyptomerus + Clambinae. Potential scirtoid autapomorphies are the loss of the dorsal tentorial arms, a bulging gula, a strongly transverse labrum, and a ridge separating the mediostipes from the lacinia. However, all these features are homoplasious. The monophyly of Clambidae is supported by modifications of the head capsule which is strongly flattened and broadened, by a deep clypeofrontal incision enabling vertical antennal movements, and a series of antennal features. Synapomorphies of Clambinae + Calyptomerus (Clambidae excluding Acalyptomerus) are the conglobate body form with the ventral side of the head capsule in contact with the mesocoxae, and compound eyes integrated in the contour of the head. The completely subdivided eye is an autapomorphy of Clambus. An entire series of features is shared by Clambidae (or Scirtoidea) and Myxophaga. Most of them are apomorphies that apparently evolved independently in both groups. However, the presence of well-developed maxillary and labial glands is arguably a retained groundplan feature of Coleoptera, with parallel loss in Archostemata, Adephaga and various groups of Polyphaga.