The puzzling role of immature-stage morphology in the taxonomy of Merodon (Diptera: Syrphidae)

The genus Merodon Meigen (Diptera: Syrphidae) is distributed throughout the Palaearctic and Ethiopian regions, and is characterized by a large number of species groups comprising taxa with subtle morphological differences. The taxonomic status and identification of many species still requires clarification. The adults of Merodon species are well studied, especially since various recent publications successfully used an integrative taxonomic approach, combining morphological and molecular information. The immature stages of this phytophagous genus, however, remain understudied, with the descriptions of larvae and/or puparia published for roughly just 5% of species. Considering both published and new data, this study aims to review and to re-evaluate the main diagnostic morphological characters of the preimaginal stages and to assess the importance of these traits as taxonomic tools for delimitation within the genus. Features distinctive among different species are mostly associated with the posterior respiratory process, pupal spiracles and cephalopharyngeal skeleton. Potentially diagnostic features are recognized for four well-supported lineages (putative subgenera) within the genus Merodon (avidus-nigritarsis, albifrons+desuturinus, aureus and natans) defined by adult morphological and molecular characters; however, no clear pattern is achieved for all studied taxa at this level. Furthermore, certain characters previously considered as species-specific turned out to be rather ambiguous. This study confirms the need for both more species and more specimens of Merodon immature stages to be examined and compared in order to obtain better supported conclusions.