Head morphology of the smallest beetles (Coleoptera: Ptiliidae) and the evolution of sporophagy within Staphyliniformia

摘要

Ptiliidae include the smallest known beetles. External and internal head structures of species with different body sizes and feeding preferences were examined and described in detail. Saprophagous and sporophagous species are compared. The observed features are evaluatedwith respect to their phylogenetic and functional signifcance, and their correlation with extreme size reduction. A putative autapomorphyof Staphyliniformia is an unusual extrinsic maxillary muscle, which among ptiliids is only present in the saprophagous species. Synapomorphies of Ptiliidae and their sister group Hydraenidae are a lateral mandibular process forming a unique locking device with a lateralgroove of the labrum, and mandibles divided into a main body and a mesal molar part, both connected by a membrane. Extreme body sizereduction is a presumptive autapomorphy of Ptiliidae that probably resulted in the following derived features: the loss of cephalic suturesand ridges, a simplifed tentorium, and a brain modifed in shape and very large in relation to the head size. The ptiliid species with saprophagous and sporophagous feeding habits show only subtle differences in their cephalic structures, notably in details of the epipharynxand galeae and in the confguration of maxillary muscles. Two alternative scenarios are suggested for the evolution of feeding habits, basedon the morphological results and presently available information on phylogenetic relationships. One option is to assign saprophagy to thegroundplan of the family, with two switches to sporophagy; frst in the basal Nossidium and then a second time in the extremely small Nanosellini, which are characterized by feeding habits that we address as microsporophagy. An alternative scenario is that feeding on sporesis ancestral for Ptiliidae, with reversals to saprophagy in several branches of the family, and a specialization on very small spores in thestrongly miniaturized nanoselline species. A well-founded species level phylogeny of Ptiliidae with a dense taxon sampling will help toclarify this issue.