Morphology and evolution of the larval head of hydrophiloidea and histeroidea (Coleoptera: Staphyliniformia)

Abstract

Internal and external features of the larval head of Spercheus emarginatus, Hydrochus sp., Helophorus sp., and of Hister sp. are described in detail. Character transformations of these structures apparently play an important role in the evolution of Hydrophiloidea and Histeroidea. A basal position of Spercheidae within this lineage of Staphyliniformia is suggested by a considerable number of plesiomorphic features: head subprognathous, adnasalia and nasale absent, gula broad and short, posterior tentorial arms arise close to hind margin of head capsule, tentorial bridge fairly broad and straight, maxillary groove deep, with well developed articulating membrane, unidentate mandibular retinaculum, cardo undivided, moveability between cardo and stipes fully retained, stipes not tube-like, maxillary palp inserted laterally, lacinia strongly developed and hook-like, prepharynx short, anatomical mouth dilatable, brain located within head capsule, glands present in labial region. Some of these character states are also found in larvae of Hydrochus but not in other larvae of Hydrophiloidea and Histeroidea. A cladistic analysis, which is exclusively based on larval characters, results in the following branching pattern (strict consensus tree): Scarabaeidae + (Agyrtidae + Leiodidae + Hydraenidae + (Spercheidae + (Hydrochidae + (Histeridae + Synteliidae + Sphaeritidae) + (Helophoridae + ((Georissidae + Epimetopidae) + Hydrophilidae)))))). A monophyletic unit which comprises Histeroidea and subgroups of Hydrophiloidea is in contrast to other phylogenetic hypotheses. It cannot be fully excluded that derived character states which are related with predacious habits have independently evolved in both superfamilies.