Evolution of a hyper-complex intromittent organ in rove beetles – the endophallus of Xantholinini (Staphylinidae: Coleoptera)

Abstract

Abstract Studies on the functional morphology and evolution of genitalia have been crucial to understanding sexual traits in speciation, reproductive isolation and sexual selection in Coleoptera and insects in general. However, the focus of investigation of the intromittent organ of beetles was largely confined to the sclerotized elements of the aedeagus, whereas the membranous structures of the endophallus (=internal sac) have often not been adequately considered. Using a micro-operating technique, we observed living male rove beetles and found five different types of endophallus eversion and related morphological modifications. Analysing genital data of a larger sample of Xantholinini, we could demonstrate that endophallus complexity and modifications tend to vary inversely with the median lobe (penis: intromittent organ). Our comparative morphological study, combined with a molecular phylogenetic analysis, suggests that endophallus spiralling occurring after endophallus eversion is an innovation in beetle evolution.