The male postabdomen of the “ancestral” archostematan beetle Tetraphalerus bruchi Heller, 1913 (Ommatidae) and its phylogenetic significance

Abstract

External and internal features of the male postabdomen of Tetraphalerus bruchi were examined with a broad spectrum of morphological techniques and are described in detail. The conditions found in males of Tetraphalerus are compared to those in other archostematan beetles and members of other coleopteran suborders. The far-reaching reduction of the sternite I, structural modifications of sternite II, the retracted condition of the terminal segments, and ventromedially fused apodemes arising from the anterior margin of tergite IX are likely autapomorphies of Coleoptera. The male postabdomen of Tetraphalerus is less derived than in most other groups of Coleoptera. The sclerotized elements are symmetrical. In contrast to earlier statements on the archostematan male genital apparatus a distinctly developed, sclerotized basal piece is present. The aedeagus is trilobed and all elements of the copulatory apparatus are distinct. The muscular equipment is simple and moderately developed. All muscles (except the transverse muscles 61 and 62) occur pairwise and symmetrically. The distinct increase of the number of postabdominal muscles in representatives of the higher lineages of Coleoptera is likely linked with a torsion of the copulatory apparatus, which also results in asymmetries of the sclerotised parts. The testes of Tetraphalerus are long, multi-coiled tubes like in other archostematans, Myxophaga (Torridincola) and Adephaga. The presence of a deep notch on the parameres is a synapomorphy of Tetraphalerus and Omma. Curved parameres, a shortened distal portion, and a distinctly shortented penis are potential synapomorphies of Omma rutherfordi and Omma mastersi. The large size of the sclerotized part of the phallobase (‘basal piece’) and the division of the sclerotization of sternum IX are potential groundplan autapomorphies of Archostemata, with secondary modification of the latter feature in Cupedidae. The reduced condition of the sclerotization of sternum VIII is an apomorphic condition which has likely evolved independently in Tetraphalerus and Paracupes. Further anatomical investigation of the male genital apparatus of Coleoptera and holometabolous insects in general is required for a reliable morphological and phylogenetic interpretation. Concerning the presence or absence of particular sclerotizations (e.g., ‘basal piece’ of phallobase) histological section series and Confocal Laser Scanning Microscopy can add more precise information to what can be observed using permanent preparations of macerated specimens.