In the twilight zone-The head morphology of Bergrothia saulcyi (Pselaphinae, Staphylinidae, Coleoptera), a beetle with adaptations to endogean life but living in leaf litter

The thoracic anatomy of Pselaphus heisei (Pselaphinae, Staphylinidae, Coleoptera)

We document external and internal thoracic structures of the free-living pselaphine beetle Pselaphus heisei (Pselaphitae) using a set of traditional and modern techniques. Like in the specialized myrmecophile Claviger testaceus (Clavigeritae), the skeletal elements of the pro- and pterothorax are highly compact, with largely reduced inter- and intrasegmental sutures. Features previously listed as synapomorphies of staphylinid subgroups, to which Pselaphinae belong, are confirmed for P. heisei. The only previously proposed thoracic synapomorphy of Pselaphinae, the mesoventral foveae, is likely transformed in P. heisei: we assume that the concavities are directed mesad and internally fused, thus forming a broad channel ("perforation") extending through the keel-like median region of the mesoventrite. The prothoracic foveal system is strongly reduced, with only one pair of pits present in front of the procoxae. Their internal invaginations form a transverse ventral endoskeletal bar that stabilizes the prothorax. The condition observed in the free-living P. heisei is in contrast with previous hypotheses linking the reduction of the foveal system with myrmecophily. Moreover, traces of the mesoventral foveae are even preserved in the highly specialized inquiline C. testaceus. Gland cells are associated with areas of hyaline squamose setae on different body regions, suggesting release of secretions on the ventral side of the head, pro- and mesothorax, and abdominal base. Similar specialized setal patches are common in Pselaphini and related groups within Pselaphitae. The prothoracic musculature in P. heisei is more complex than that in the myrmecophilous C. testaceus and the free-living, unspecialized Creophilus maxillosus (Staphylininae). The metathoracic muscle system is strongly simplified, demonstrating that P. heisei cannot fly, even though wings, some skeletal elements of the flight apparatus, and some small direct flight muscles are preserved. It cannot be fully excluded that indirect flight muscles and thus a functional flight apparatus is preserved in a certain percentage of individuals.

Evolution of cephalic structures in extreme myrmecophiles: a lesson from Clavigeritae (Coleoptera: Staphylinidae: Pselaphinae)

Pselaphinae is a large subfamily, comprising over 10 000 species of the megadiverse Staphylinidae (rove beetles). A remarkable feature of this group is the extreme structural diversity of different body regions, especially the head and its appendages. Within Pselaphinae, Clavigeritae stand out as a clade of highly specialized myrmecophiles. We examined internal and external head structures of the clavigerite species Diartiger kubotai Nomura, using state-of-the-art techniques. The cephalic morphology indicates in a phylogenetic context that the loss of eyes in some Clavigeritae was the latest of major evolutionary changes. We compiled the largest set of morphological data ever scored for the subfamily, comprising 155 characters of the head. Parsimony analyses and Bayesian inference yielded a similar phylogenetic pattern, largely congruent with results published previously. We retrieved Pselaphinae as a clade, and Faronitae as sister to all remaining groups of the subfamily. Faronitae are followed by a "Euplectitae grade" and non-monophyletic Goniaceritae, Batrisitae and Pselaphitae. Clavigeritae are monophyletic, but have evolved within the pselaphite grade. The enigmatic Colilodion Besuchet, recently shifted from Clavigeritae to a paraphyletic Pselaphitae, was placed as sister to extant clavigerites based on an array of cephalic synapomorphies. The current classification of Pselaphinae is unstable and deep changes should be made maintaining only monophyletic units, whereas most of the supertribes are paraphyletic. Characters of the head, with a concentration of mouthparts and sensory structures, and essential parts of the digestive tract and the nervous system, are highly informative phylogenetically. Study of internal structures, presently still at a very preliminary stage, obviously is essential for understanding the evolution of Pselaphinae. Future genetic investigations may reveal mechanisms behind the unique structural megadiversity in this exceptional group of rove beetles.