1	Postpronotum, lateral sides of scutum and face yellowish (Suppl. material 1: Fig. S31)	Merodon luteihumerus Marcos-García, Vujić & Mengual, 2007
–	Postpronotum, lateral sides of scutum and face black or dark	2
2	Pro- and mesolegs strongly modified (Suppl. material 1: Fig. S32A, C)	Merodon mixtum Vujić, Radenković & Likov, 2019
–	Pro- and mesolegs with usual shape	3
3	Pilosity on the posterior part of abdomen (at least tergum 4) denser and strikingly golden to reddish-yellow (as in Suppl. material 1: Fig. S33A) contrasting with the colour of the pilosity on the rest of the abdomen	constans species group (Vujić et al. 2020a)
–	Pilosity on the posterior part of abdomen not denser compared to the anterior part (Suppl. material 3: Fig. S2B)	4
4	Large (11–17 mm) bumble bee-like species with dense and long body pilosity (Suppl. material 3: Fig. S2A, B); males with strong apical dorsal calcar on metatibia (Suppl. material 1: Fig. S34A)	equestris species group (Marcos-García et al. 2011)
–	Species with different characters	5
5	Medium to large sized species (9–13 mm) with black, bronze lustre terga (lack pollinose fasciate maculae), except a pair of small, orange, lateral markings on tergum 2 (Suppl. material 1: Fig. S33B); scutum and terga covered with erect, mostly yellowish to reddish pilosity, except few black pile medially on terga 3 and 4 (Suppl. material 1: Fig. 33C). Male: metatrochanter can have a calcar, but metatibia always without calcar (Suppl. material 1: Fig. S34B)	rufus species group (Radenković et al. 2020)
–	Species with different characters; usually with pollinose fasciate maculae on terga	6
6	Female (genitalia not visible)	9
–	Male (genitalia visible externally)	7
7	Metaleg with some modifications on metatrochanter, metafemur and/or metatibia; male genitalia often with hook-like posterior surstyle lobe or cercus with prominence(s)	8
–	Metaleg usually without modifications (exception is Merodon trochantericus Costa, 1884, see in Suppl. material 1: Fig. S34C); male genitalia with rounded posterior surstyle lobe, biramous anterior surstyle lobe pliers-like (with thorn-like interior accessory lobe process), and cercus without prominences (as in Suppl. material 2: Fig. S2A)	albifrons species group
8	Metatrochanter with blunt calcar apically covered with long pile (Suppl. material 1: Fig. S34D); metatibia with apicomedial carina (Suppl. material 1: Fig. S34E); male genitalia with rounded or acute posterior surstyle lobe, biramous anterior surstyle lobe with apical extension more developed, and cercus with prominence(s) (as in Suppl. material 2: Fig. S2G: marked with arrow)	geniculatus species group (Vujić et al. 2018a)
–	Metatrochanter with sharp apical calcar (Suppl. material 1: Fig. S34F); metafemur usually with ventral tubercle or calcar (as on Suppl. material 1: Fig. S34F: marked with arrow); metatibia usually with apicolateral process (as on Suppl. material 1: Fig. S34F: marked with arrow); male genitalia usually with hook-like posterior surstyle lobe, biramous anterior surstyle lobe with moderately developed interior accessory lobe and apical extension, and cercus without prominences (as in Suppl. material 2: Fig. S3A: c)	ruficornis species group (Vujić et al. 2012)
9	Metatibia narrow, not swollen apically (Suppl. material 1: Fig. S34G); terga 3–5 black	ruficornis species group (Vujić et al. 2012)
–	Metatibia incrassate apically (Suppl. material 1: Fig. S34H); terga 3–5 usually partly reddish	10
10	Scutum usually with five distinct pollinose vittae (Suppl. material 1: Fig. S35A); terga 2–4 with well-defined pollinose fasciate maculae (Suppl. material 1: Fig. S35B); metatibia with concave ventral margin in apical half (Suppl. material 1: Fig. S34H)	geniculatus species group (Vujić et al. 2018a)
–	Scutum with indistinct pollinose vittae; terga 2–4 without or with narrower pollinose fasciate maculae (Suppl. material 1: Fig. S35C); metatibia with straight ventral margin in apical half (Suppl. material 1: Fig. S33D)	albifrons species group