The puzzling role of immature-stage morphology in the taxonomy of Merodon (Diptera: Syrphidae) Andrijana Andrić1, Andrea Aracil2, Celeste Pérez-Bañón2, Snežana Radenković3, Grigory Popov4, Ruslan Mishustin5 & Ante Vujić3 RESULTS AND CONCLUSIONS: Merodon aureus (Fig.1:1,2) and M. aff. cinereus (Fig.1:3,4) (aureus lineage): Nearly barrel-shaped prp in dorsal view, with annular grooves at the base and below the spiracular plate (the network resembling ornamentation much more pronounced in M. aureus); the outline of the spiracular plate with indentations at the locations of inter-spiracular setae and with the curving outward edges just beneath. The surface of M. aff. cinereus pupal spiracle with a polygonal pattern around tubercles, smoother at the base and more granular at the apex, the letter also characteristic of M. aureus, but with unornamented surface between tubercles (Fig.2:1,2). INTRODUCTION: The immature stages of the phytophagous genus Merodon Meigen are understudied, with the descriptions of larvae and/or puparia published for roughly just 5% of species (Heiss 1938; Stuckenberg 1956; Ricarte et al. 2008, 2017; Andrić et al. 2014; Preradović et al. 2018), and the identification key to Merodon early stages published only recently (Ricarte et al. 2017). The main reason for this information gap is difficulty of finding specimens in the field, since larval food-plants and the breeding and oviposition sites are not recorded for most Merodon species (Speight 2018). Known larvae feed on underground storage organs of geophytes (i.e. Asparagaceae, Amaryllidaceae and Iridaceae). The associations with host-plants are proven in several cases only (Ricarte et al. 2017; Speight 2018), such as a recent study confirming monophagy within M. constans species group in relation to Galanthus L. bulbs (Popov & Mishustin 2019, in press). Another problem is the determination, since rearing could be time-consuming, and the identification using sequencing analysis (DNA COI barcode libraries) could not be possible in the cases of rotten or parasitoidised specimens (Preradović et al. 2018). The aim here is to review some diagnostic morphological characters of Merodon preimaginal stages and to assess the importance of these traits as taxonomic tools for delimitation within the genus. Certain characters previously considered as taxonomically important, e.g. presence of accessory teeth on mandibular hooks or spiracular setae on posterior respiratory process, turned out to be rather ambiguous (Preradović et al. 2018). For other differentiating traits, e.g. related to cephalopharyngeal skeleton or spiracular openings of the anterior respiratory process, there is a lack of available data for some species. The sclerotized structures of the respiratory organs, i.e. the posterior respiratory process (prp) and pupal spiracles, are mostly addressed in the available descriptions of Merodon early stages. Eleven species are analysed here (5 previously described with published photographs of these structures, and 6 for which descriptions are in preparation), with representatives from all 4 well-supported lineages (putative subgenera) within the genus Merodon (aureus, avidus-nigritarsis, natans and albifrons+desuturinus) defined by adult morphological and molecular characters. Figure 1. Posterior respiratory process in Merodon preimaginal stages: 1, 2 M. aureus Fabricius figs 3A,B in Preradović et al. 2018; 3, 4 M. aff. cinereus (Fabricius) (in prep.); 5 M. aff. serrulatus Wiedemann in Meigen (in prep.); 6, 7 M. avidus (Rossi) fig3D in Preradović et al. 2018, fig4A in Andrić et al. 2014; 8, 9 M. aff. pulveris Vujić & Radenković (in prep.) (500μm); 10 M. pulveris (500μm); 11 M. dzhalitae Paramonov sensu Vujic et al. (in prep.); 12 M. luteihumerus Marcos- García, Vujić & Mengual fig8b in Ricarte et al. 2008: 136; 13 M. geniculatus Strobl fig6D (250μm) in Ricarte et al. 2017: 10/22; 14, 15; M. rufus Meigen figs 3E,F in Preradović et al. 2018; 16, 17 M. aff. planifac