The adult head of representatives of different adephagan families – aquatic, semiaquatic and terrestrial – were examined and compared. External and internal structures were described and documented in detail for the genera Trachypachus (Trachypachidae), Haliplus (Haliplidae), Amphizoa (Amphizoidae) and the recently discovered Aspidytes (Aspidytidae). A list of characters of potential phylogenetic relevance was compiled and the data matrix combined with the large data set of thoracic and abdominal features for different life stages. The cladistic analysis of this comprehensive data matrix of 138 characters for 16 taxa covering all adephagan families led to one most parsimonous tree. The monophyly of the Geadephaga (Trachypachidae + Carabidae) is strongly supported. The Gyrinidae are the sistergroup of all remaining adephagan beetles. The Meruidae are sister to the Dytiscoidea and both together form the sistergroup of the Haliplidae. The sistergroup relationship of Aspidytidae and Amphizoidae is confirmed. The placement of Meruidae is impeded by the lack of larval characters. It may change when information on structural features of immature stages becomes available. The Trachypachidae, a small relict family with its greatest diversity and distribution in the early Mesozoic, probably come close to the last common ancestor of the Adephaga in the structural features of the adult head. They share structural similarities with the aquatic Dytiscoidea and the terrestrial Carabidae. It is hypothesized that the common ancestor of Adephaga had a relatively unspecialised head morphology and was a predator, possibly with a preference for a riparian habitat. Adaptations for an aquatic environment evolved at least two times and possibly even three times independently. Within these lineages a great diversity of different life styles developed such as the highly specialised surface gliding Gyrinidae, the hygropetric Aspidytidae, the strongly miniaturised Meruidae or the algophagous Haliplidae.

Adephaga, Trachypachidae, Haliplidae, Aspidytidae, Amphizoidae, adephagan ground plan, cladistic analysis, head morphology, Geadephaga, Hydradephaga