Evolution of the male genitalia in the genus L imnebius L each, 1815 ( C oleoptera, H ydraenidae)

We investigated the evolution of the male genitalia of species of Limnebius (Coleoptera, Hydraenidae). This genus is very homogeneous externally but characterized by highly diverse male genitalia, which are in some cases extraordinarily complex. We reconstructed a molecular phylogeny for 70 of the c . 150 known species of the genus with six fragments of mitochondrial and nuclear genes. We found two main lineages of Miocene origin, largely corresponding to the synonymized subgenera Bilimneus and Limnebius , which are here re‐erected. Within the Holarctic Limnebius s.s . we found four well‐supported lineages, although with poorly supported relationships between them: the L .  piceus , L .  gracilipes , L .  parvulus and L .  nitidus species groups. We describe the aedeagus and its different appendages in detail based on the study of 116 species, including serial histological sections to reconstruct the internal structure of seven of them. Using Bayesian methods we reconstructed the ancestral Limnebius as having small males ( c . 1.2 ± 0.5 mm), with a small aedeagus ( c . 0.4 ± 0.3 mm) with a free left paramere, probably an externally fused right paramere, and a possible additional appendage. The species of Bilimneus experienced a reduction in size and a simplification of their genitalia, without free parameres and with a very simple, homogenous structure. Within Limnebius s.s . several independent increases in male body and genital size took place, with a strong correlation as measured with independent contrasts. There was, however, no overall correlation between genital size and number of appendages, even though smaller genitalia tend to be less complex.