The pterothoracic skeletomuscular system of Scirtoidea (Coleoptera: Polyphaga) and its implications for the high-level phylogeny of beetles

Abstract

External and internal structures of the pterothorax of six adult representatives of three families of Scirtoidea were examined. A detailed description of the skeleton and musculature is presented for the scirtid species Elodes pseudominuta. Phylogenetically relevant characters are discussed and a comprehensive table of thoracic muscles of adult beetles is presented. The monophyly of Scirtoidea is supported by derived features of the skeleton, the musculature and the hind wing. Thoracic characters suggest a branching pattern {Decliniidae + [Eucinetidae + (Clambidae + Scirtidae)]}. However, the interrelationships of the scirtoid families were difficult to reconstruct, as internal characters of Decliniidae are unknown and Clambidae are characterized by many autapomorphic features related to miniaturization. The largest family, Scirtidae, is well supported as a clade. The position of the superfamily remains ambiguous. A basal position within Polyphaga is only suggested by one character state, the complete absence of the bending zone of the hind wing in all other lineages of the suborder. A position of Scirtoidea within Elateriformia is tentatively supported by the elongated anepisternum 3 (polarity unclear), a triangular radial cell, and the reduction of the apical branches of the radius posterior in the hind wing. The thoracic features of Scirtoidea – like those of all polyphagan lineages – are clearly derived compared with the conditions in Adephaga, and especially Archostemata. A basal position of Archostemata and a sister group relationship between Polyphaga and Myxophaga is supported by the results of this study. Characters in support of a basal position of Polyphaga were not found.