The phylogeny of Ptiliidae (Coleoptera: Staphylinoidea) – the smallest beetles and their evolutionary transformations. Ptiliidae phylogeny

The smallest beetles and the smallest non-parasitic insects belong to the staphylinoid family Ptiliidae. Their adult body length can be as small as 0.325 mm and is generally smaller than 1 mm. Here we address the phylogenetic relationships within the family using formal analyses of adult morphological characters and molecular data, and also a combination of both for the first time. Strongly supported clades are Ptiliidae + Hydraenidae, Ptiliidae, Ptiliidae excl. Nossidium, Motschulskium and Sindosium, Nanosellini, and a clade compris- ing Acrotrichis, Smicrus, Nephanes and Baeocrara. A group comprising Actidium, Oligella and Micridium + Ptilium is also likely mono- phyletic. Limulodes + Cephaloplectus, strongly supported as a clade (Cephaloplectus included only in morphological trees), are deeply nested within Ptiliidae in the morphology-only non-weighted and in the molecular analyses, but placed as sister to the remaining Ptiliidae (excl. Nossidium, Motschulskium and Sindosium) after successive reweighting. We propose that Ptiliidae should be taxonomically divided into two subfamilies: the most recently established Nossidiinae and Ptiliinae, the latter currently composed of seven tribes: Acrotrichini, Cephaloplectini, Discheramocephalini, Nanosellini, Ptenidiini, Ptiliini and Ptinellini, although the monophyly and taxonomic status is still uncertain in some cases. Important morphological innovations evolved in the stem group of Hydraenidae and Ptiliidae, including partly internalized mouthparts suitable for saprophagy and sporophagy, a labral-mandibular locking device, a specific elytral locking mechanism with elongated alacristae, wings with fringes of hairs, and a very unusual spermathecal pump. A complex feature of Ptiliidae linked with miniaturization is the transformation of the alae into “feather wings”, with a highly efficient flight mechanism unique in beetles. Nano- sellini include the smallest ptiliid species and display features correlated with extremely small body size, such as simplification of the endoskeleton of the head and thorax, far-reaching reduction of the wing venation, and a reduced number of antennomeres.