Hide-and-seek with hoverflies: Merodon aureus - a species, a complex or a subgroup?

In order to disentangle the currently confused interpretations and nomenclature of Merodon aureus and M. aeneus, we have reviewed all existing type material and species names known to us as assigned synonyms of these taxa. We resolve M. aeneus as being a junior synonym of M. aureus. We designate a lectotype for M. aureus and a neotype for M. aeneus. Additionally, we provide evidence that M. aureus, together with two newly discovered taxa (M. calidus sp. nov. and M. ortus sp. nov.), represent a complex of cryptic species named the M. aureus species complex. This complex, together with the M. unicolor species complex and the species M. pumilus, is part of the M. aureus subgroup. The M. unicolor species complex comprises two cryptic species: M. unicolor and M. albidus sp. nov. The new species are described by applying an integrative taxonomic approach using several data types (COI and 28S rRNA genes, geometric morphometry of the wings, ecological and distributional data). Based on the COI gene sequence analysis and distributional data, the pupa previously described as an immature stage of the species M. aureus is redefined as an immature stage of the new species M. calidus. Speciation within the M. aureus subgroup is discussed in the context of the phylogeographic history in the studied region.