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Several recent studies have detected and described complexes of cryptic and sibling species in the genus Merodon (Diptera, Syrphidae). One representative of these complexes is the Merodon avidus complex that contains four sibling species, which have proven difficult to distinguish using traditional morphological characters. In the present study, we use two geometric morphometric approaches, as well as molecular characters of the 5’-end of the mtDNA COI gene, to delimit sibling taxa. Analyses based on these data were used to strengthen species boundaries within the complex, and to validate the status of a previously-recognized cryptic taxon from Lesvos Island (Greece), here described as Merodon megavidus Vujić & Radenković sp. nov. Geometric morphometric results of both wing and surstylus shape confirm the present classification for three sibling species-M. avidus (Rossi, 1790), M. moenium Wiedemann in Meigen, 1822 and M. ibericus Vujić, 2015-and, importantly, clearly discriminate the newly-described taxon Merodon megavidus sp. nov. In addition to our geometric morphometric results, supporting characters were obtained from molecular analyses of mtDNA COI sequences, which clearly differentiated M. megavidus sp. nov. from the other members of the M. avidus complex. Molecular analyses revealed that the earliest divergence of M. ibericus occurred around 800 ky BP, while the most recent separation happened between M. avidus and M. moenium around 87 ky BP.
Detection of morphologically indistinguishable cryptic species implies using an integrative taxonomic approach with a combination of molecular, contemporary morphological, ecological and other relevant analyses. Within a contemporary morphological analysis, two approaches are commonly used in hoverfly taxonomy: a geometric morphometric analysis of the wing and surstyle shape. Here, the importance of the R4+5 vein shape is tested in cryptic species delimitation within four Merodon species groups using linear and semilandmark geometric morphometric analyses. As expected, geometric morphometrics showed a stronger resolution compared to linear morphometrics. Linear morphometrics failed to detect differences related to sexual dimorphism or differences among the species M. pruni and M. obscurus. However, all cryptic species and sexes were separated with high significance based on the R4+5 vein shape. Moreover, obtained results concurred with the landmark-defined wing shape and molecular results published in previous studies. Additionally, combining two characters, the semilandmark R4+5 vein shape and the landmark-defined wing shape, provided more detailed and precise insights into the shape differences. Our results showed that the R4+5 vein shape stands out as an important character in species delimitation of hoverflies where the sinuation of this vein is present. Therefore, it can be beneficial as a single character or in combination with a landmark-based wing shape analysis.