Unveiling the smallest-systematics, classification and a new subfamily of featherwing beetles based on larval morphology (Coleoptera: Ptiliidae)

The higher-level systematics of featherwing beetles (Coleoptera: Ptiliidae), the smallest non-parasitic insects and abundant in the world's forests, is poorly known. Based on larval morphology, the systematic relationships and internal classification are revised and analysed from a comparative, evolutionary and phylogenetic perspective. We introduce 37 morphological characters of all instars and of strong phylogenetic signal in a first-ever maximum parsimony phylogenetic family analysis, providing support for the erection of Nossidiinae, subfam. nov., keeping Cephaloplectinae within Ptiliidae and sinking Acrotrichini as a tribe into a revised subfamily Ptiliinae. Full homology of characters between Ptiliidae and its sister taxon Hydraenidae is demonstrated and miniaturisation effects on the larval exoskeleton reviewed. Keeping generalised mouthparts adapted to sapro-mycophagy through geological time made three evolutionarily independent invasion events of marine littorals possible in Ptiliidae. Keys to larval instars, subfamilies, tribes and species are presented for the first time, supplemented by model descriptions of multiple hierarchical levels (species, genus, tribe, subfamily, family), introducing many phylogenetically informative characters. The conservative larval morphology/chaetotaxy proves particularly useful for analysis on the family-group taxon level, as exemplified by caudal urogomphi, a key character in ptiliid systematics. The revised nomenclature and classification improves systematic predictability and better reflects natural relationships within Ptiliidae.