Diversification of myrmecophilous Clavigeritae beetles (Coleoptera: Staphylinidae: Pselaphinae) and their radiation in New Caledonia

Clavigeritae (Coleoptera: Staphylinidae: Pselaphinae) comprise a supertribe of specialized myrmecophile rove beetles that display numerous behavioural, chemical and anatomical modifications associated with a socially parasitic lifestyle. Due to the extreme morphological diversity of clavigerites, their systematic relationships, patterns of character evolution and host ant use have been challenging to infer. Here, we resolve deep divergences within Clavigeritae by assembling a molecular dataset encompassing the breadth of tribal diversity. We classify Clavigeritae into six tribes: extinct Protoclavigerini, and recent Tiracerini, Mastigerini sensu nov., Clavigerini sensu nov., Lunillini sensu nov. and Disarthricerini stat. nov. The previously recognized subtribes Clavigerodina, Apoderigerina, Dimerometopina, Hoplitoxenina, Miroclavigerina, Theocerina and Thysdarina are demonstrated as polyphyletic or highly derived internal clades of Clavigerini and are hereby synonymized with the latter. Colilodion – a highly enigmatic taxon proposed to be the earliest-branching lineage of recent Clavigeritae – is revealed to be unrelated to Clavigeritae and is transferred to Pselaphitae. We provide a systematic treatment of newly-discovered endemic genera from New Caledonia and infer their phylogenetic affinity to the Australian tribe Tiracerini. The zoogeographic distribution of early-branching Clavigeritae lineages in India and the Australian region indicates a possible Gondwanan origin of the supertribe. Extant clades diversified from the Eocene onwards, correlated with the rise of modern ants. We present evidence for island radiations of Clavigeritae in both Madagascar and New Caledonia during the Oligocene-Miocene. Using a newly-created morphological dataset, we analyse patterns of character evolution and demonstrate widespread convergence in morphology that extends to virtually all traits. Counterintuitively, however, characters postulated to be involved in beetle-ant communication are amongst the most invariant, and least convergent. The host ant spectrum of many Clavigeritae genera and species is broad, contradicting co-cladogenesis with hosts and instead implying widespread host switching. We speculate whether morphological variability in Clavigeritae is truly adaptive, as opposed to the product of ‘morphological drift’ of body parts under weak selection inside ant colonies, in species with potentially very small effective population sizes.