Comparative morphology of the head of selected sporophagous and non-sporophagous aleocharinae (Coleoptera: Staphylinidae): musculature and hypopharynx-prementum complex

Abstract

To investigate whether specialization to spore- (or pollen-) feeding in advanced Aleocharinae is mirrored by their head anatomy, we compiled and compared synchrotron X-ray micro-tomography datasets for 11 Aleocharinae in conjunction with previous data for two aleocharine and six outgroup species (two nonstaphylinids, four staphylinids). We describe the presence/absence of head muscles and investigate the variability of points of origin by character mapping analyses. Monophyly of Aleocharinae is supported by the absence of M. 48 (M. tentoriobuccalis anterior), and by changes in the origins of Mm. 1, 2, 17, 18, 28, 29, 30. Within Aleocharinae the origins of the labial muscles (Mm. 28-30) have shifted posteriorly to the gula, which might enhance the movement posterad of the hypopharynx and partly compensate for the loss of M. 48. We also analyzed the general organization of the hypopharynx-prementum complex and the fine structure of the mandibles through SEM studies. In the absence of grinding mandibular molae like those of most mycophagous Coleoptera, seven aleocharine species studied have evolved "pseudomolae" at the ventral side of the mandibles that replace true molae as secondary grinding surfaces. In these species, the hypopharynx is elevated and displaced anteriorly, bearing a bowl-like depression on its surface that functions as a mortar where spores are ground between the hypopharynx and the mandibles. Two of these species are not yet known to feed on spores or pollen. Another species (Oxypoda alternans) is thought to feed on fungus material but bears no pseudomolae on its mandibles.