Comparative study of thoracic structures of adults of Hydrophiloidea and Histeroidea with phylogenetic implications (Coleoptera, Polyphaga)

External and internal structures of adults of Hydrophiloidea and Histeroidea were examined. Skeletal and muscular features of Helophorus aquaticus and Margarinotus brunneus are described in detail. Morphological data are presented as a list of characters and data matrix, and analysed together with other characters of adults, characters of larvae, and characters related to reproduction, habitats and feeding habits. The results of the analysis (characters unweighted) of the full dataset unambiguously support the monophyly of the following clades: [Scarabaeoidea (represented by a genus of Lucanidae and a genus of Scarabaeidae) + Hydrophiloidea + Histeroidea], [Hydrophiloidea + Histeroidea], Histeroidea, [Sphaeritidae + Histeridae], Hydrophiloidea, Hydrophiloidea (excluding Helophoridae), Hydrophiloidea (excluding Helophoridae and Hydrochidae), [Epimetopidae + Georissidae], and [Spercheidae + Hydrophilidae]. The monophyly of all histeroid and hydrophiloid families and of Hydrophilidae (represented by hydrophilines and sphaeridiines) excluding Berosus is also supported. The placement of Scarabaeoidea is in contrast to a taxonomic treatment as a lineage with the same rank as Staphyliniformia. Hydraenidae are not closely related to Hydrophiloidea. The clade comprising Hydrophiloidea and Histeroidea is well supported, but mainly by larval features correlated with predacious habits. The position of Spercheidae implies that a considerable number of seemingly plesiomorphic features of the head are due to reversal and specialized feeding habits (filter feeding). Histeridae show a highly derived pattern of thoracic features with unusual muscular modifications, a long horizontal, dorsal part of the mesopleuron, widely separated metacoxae and a strongly simplified metafurca. Hydrophiloidea are well supported by character transformations of the thorax and other body parts, which are probably related to the invasion of the aquatic habitat in the adult stage, e.g. surface modifications related to the ventral plastron. Georissidae + Epimetopidae are characterized by derived features, which may be the result of a secondarily terrestrial or semiterrestrial life style (partly reduced ventral pubescence), and by a weakly sclerotized mesonotum, mesofurca and metafurca. Hydrochidae, Georissidae and Epimetopidae show a considerable number of autapomorphies, whereas Helophoridae are probably close to the groundplan of Hydrophiloidea. The adaptations to an aquatic life style and the specific habits are very different in Hyrophiloidea and the aquatic groups of Adephaga. (C) 2004 Elsevier GmbH. All rights reserved.