Structure of male genitalia in a lineage of minute endogean ground beetles: how four new species of Microcharidius Coiffait, 1969 (Coleoptera: Carabidae: Anillini) reveal gradual change and convergent evolution

Typhlocharina is a diverse lineage of minute endogean Anillini (Coleoptera: Carabidae: Trechinae) formed by three genera endemic to the Mediterranean region. Four new species of Microcharidius Coiffait, 1969 from Extremadura (Spain) are described in this work, all of them sharing unusual features of the male genitalia: M. andujari sp.n., M. lencinai sp.n., M. serranoi sp.n., and M. aguiari sp.n. Male genitalia are very important in insect systematics, often bearing diagnostic features involved in prezygotic isolation processes. In Anillini, male genitalia comprise the aedeagus (median lobe and parameres) and ring sclerite, and the aedeagus has been used as a major taxonomic tool to discriminate species. Here we provide an in-depth revision of these structures in Typhlocharina bringing a comparative approach to evaluate their taxonomic potential. The morphological diversity of male genitalia is described in detail, including poorly studied or overlooked characters like ring sclerites, parameres, endophallic sclerites, and apical laminae of the median lobe. The results show phylogenetic patterns and diagnostic differences in male genitalia between Lusotyphlus Pérez-González, Andújar & Zaballos, 2017, Typhlocharis Dieck, 1869, and Microcharidius Coiffait, 1969. Male genitalic anatomy is found to be an efficient taxonomic tool for genus or clade-level recognition, but not for species-specific discrimination, except in a few cases of clear autapomorphies. Also, observed diversity in male genitalia does not have any obvious morphological correlates with the known diversity in female genitalia, but the prevalence of some types of aedeagi in species with unguiform gonocoxites suggests certain parallelisms. The affinities and novel features of the new species are discussed, highlighting the first cases of gradual transition and convergent evolution in acquisition of distinct male genital traits, probably associated with sexual isolation processes. Finally, we use the available data to explore the effect of size reduction in the proportions of male genitalia for first time in a whole lineage of endogean beetles.