Taxonomic complexity in the genus Merodon Meigen, 1803 (Diptera, Syrphidae)

Abstract The genus Merodon Meigen, 1803 is distributed across the Palaearctic and Afrotropical Regions. The present work summarizes the knowledge from recent taxonomic and systematic revisions and includes an identification key for the five monophyletic lineages (namely albifrons, aureus, avidus-nigritarsis, desuturinus and natans), 24 species groups, two species subgroups and 10 unplaced species, along with diagnosis and illustrations. A list of 234 taxa, including 194 described and 40 undescribed species, is appended. Most of the species are distributed in the Palaearctic (209 taxa, 181 described), while 27 species (14 described) are known from the Afrotropical Region. Three lineages (aureus, desuturinus and natans) are present in the Afrotropical Region, as well as in the Palaearctic. The Afrotropical melanocerus species group of the desuturinus lineage and the bombiformis species group of the aureus lineage are endemic to the Afrotropical Region, and all other species groups belong to the Palaearctic fauna. The albifrons lineage contains six species groups (albifrons, constans, equestris, geniculatus, ruficornis and rufus) and two unplaced taxa. The aureus lineage includes five species groups (aureus, bombiformis, funestus, nanus and spinitarsis). The avidus-nigritarsis lineage is divided into 10 species groups (aberrans, aurifer, avidus, clavipes, fulcratus, italicus, nigritarsis, pruni, serrulatus and tarsatus) and eight unplaced taxa. The desuturinus lineage contains two species groups: the Afrotropical melanocerus group, with the melanocerus and planifacies subgroups plus the species M. cuthbertsoni Curran, 1939, and the Palaearctic murorum species group. The natans lineage consists of the natans species group plus the species M. segetum Fabricius, 1794.

The genus Merodon was described by Meigen (1803) based on the type species Syrphus clavipes Fabricius, 1781. Until now, two synonyms of Merodon are recognized: Lampetia Meigen, 1800, originally described without included species, was suppressed by ICZN (1963, Opinion 678: 339); and Exmerodon, created by Becker (1913) as a subgenus of Merodon based on the type species Exmerodon fulcratus Becker, 1913, was listed as synonym by Peck (1988). Currently, the genus Merodon contains 194 described species and 40 undescribed species listed here. The genus is restricted to the Palaearctic and Afrotropical Regions Šašić et al. 2016), except for M. equestris (Fabricius, 1794) that has been introduced into the Nearctic Region and New Zealand (Speight 2020). The genus is divided in lineages, species groups, species subgroups and species complexes following the system proposed by Šašić et al. (2016) based on different levels of morphological differentiation. Šašić et al. (2016) proposed a system of four levels (ranks) for classification of the genus Merodon: (1) the broadest (first) level consists of large monophyletic lineages where each contains multiple morphologically different species groups; (2) the second broadest level involves taxa that constitute morphologically defined species groups within lineages; (3) the third level represents subgroups that include species with very similar morphologies, but exhibiting small, consistent interspecific character variations that facilitate their distinction; (4) the narrowest (fourth) level are species complexes that comprise morphologically inseparable taxa based on classical taxonomical methods, which can only be resolved by employing integrative taxonomy involving molecular markers, morphometry, and ecology.
In Europe, Merodon is the most speciose genus with 120 described species (152 including Turkey) (Speight 2020;Vujić, unpublished). The highest species diversity is recorded for the Mediterranean Basin (Vujić et al. 2012), which is associated with a high diversity of bulb plant species that serve as larval host plants (Ricarte et al. 2008(Ricarte et al. , 2017Andrić et al. 2014;Preradović et al. 2018). Asia Minor and Eastern Europe (especially the Balkan Peninsula) are considered hot spots and regions with high endemism for the genus Merodon (Kaloveloni et al. 2015), as documented by several studies in the Eastern Mediterranean Basin (Vujić et al. , 2020aStåhls et al. 2009Ståhls et al. , 2016Radenković et al. 2011Radenković et al. , 2020Kaloveloni et al. 2015;Ačanski et al. 2016a, b;Likov et al. 2020). Unlike this area, Afrotropical Region and Eastern Palaearctic are characterized by a low number of species (Vujić, unpublished).
The aim of this work is to summarize the knowledge from recent taxonomic and systematic revisions, to help taxonomists to have a central reference for the recent published literature, and to present an identification key for the identification of lineages, species groups, species subgroups and unplaced species of Merodon.

Material and methods
A total of 255 species belonging to the tribe Merodontini (genera Azpeytia Walker, 1865, Eumerus Meigen, 1822, Megatrigon Johnson, 1898, Merodon and Platynochaetus Weidemann, 1830 from the Palaearctic and Afrotropical Regions were studied. All specimens were identified by Ante Vujić and Snežana Radenković. Representative specimens are deposited in the collections of the Department of Biology and Ecology, Faculty of Sciences, University of Novi Sad, Serbia (FSUNS).
Morphological terminology follows Thompson (1999), except for the male genitalia that follows Marcos-García et al. (2007). We use the terms "fossette", "hypostomal bridge", "postalar" and "occipital foramen" from Doczkal and Pape (2009), and "oral margin" from Radenković et al. (2018a). For the pollinose markings on abdominal terga we used the term fasciate maculae. These markings are elongate and usually separated medially. Sometimes the fasciate maculae may have joined medially forming an entire fascia or band, but we consistently referred to them as fasciate maculae.
Male genitalia were extracted from dry specimens previously relaxed in a humidity chamber. After genitalia were pulled out with a hook-tipped entomological pin, they were cleared by boiling in warm 10% potassium hydroxide (KOH) for 3-5 min. Acetic acid was then used to neutralize the KOH during 5 s, and genitalia were immersed briefly in ethanol to remove the acid. Prepared genitalia were stored in microvials containing glycerol.
Photographs were taken using a Leica DFC 320 digital camera, attached to a Leica MZ16 stereomicroscope and Nikon Coolpix D7100 digital camera attached to a Nikon SMZ 745T stereomicroscope. Digital photographs were stacked using Com-bineZ software (Hadley 2006). A Leica MZ16 binocular microscope was used with an FSA 25 PE drawing tube to make the drawings.

Identification key to the Merodon lineages
In this section and sections below, we provide several identification keys to the 24 species groups, two species subgroups and 10 unplaced species within the genus Merodon. For further species identification inside species groups, species subgroups and species complexes, published revisionary works are cited in brackets.

1
Mesocoxa without long pile posteriorly (Suppl. material 1: Fig. S4B), or if mesocoxa with 1-3 long pile posteriorly then inner side of metafemur with a row of spinae (Suppl. material 1: Fig. S5A   Yellow-grey pilosity on terga more dense and striking laterally, as well as on pollinose fasciate maculae of terga 2 and 3, and on tergum 4 (Suppl. material 1: Pedicel shorter than basoflagellomere (Suppl. material 1: Fig. S29F Key to the species groups and unplaced species of the avidus-nigritarsis lineage 1 Inner side of metafemur with a row of spinae (Suppl. material 1: Fig. S5A). Male with two fossette (small apical one on the inner side, and a large one on the outer side) (Suppl. material 1: Fig. S10A Male genitalia without ctenidium at hypandrium (Suppl. material 2: Fig. S7K: marked with arrow); small sized species (5-9 mm) with metallic shiny body and distinctly dichoptic eyes, separated by distance almost as long as distance between ocelli (Suppl.

Merodon albifrons lineage
Diagnosis. From small to large sized species (7-19 mm) with non-tapering abdomen and a characteristic structure of male genitalia. It is defined by having the mesocoxa pilose posteriorly (> 10 pile) (Suppl. material 1: Fig. S4A), anterior anepisternum with bare area ventral to postpronotum (Suppl. material 1: Fig. S7B), and male genitalia with a biramous anterior surstyle lobe having an apical extension and interior accessory lobe, and a hammer-like lateral sclerite of the aedeagus (except for the rufus species group where the lateral sclerite of the aedeagus is not enlarged apically, but with a slightly curved apex) (Suppl. material 2: Fig. S2C: s).

1) albifrons species group (Suppl
Diversity and distribution. The albifrons species group contains eight described species (Suppl. material 5: Table S1) distributed in the Mediterranean Basin, with its highest diversity in the western part.

2) constans species group (Suppl
Diversity and distribution. Predominantly northern and eastern Mediterranean distribution, with no representatives on the Iberian Peninsula (Marcos-García et al. 2007). Its highest diversity is in the Caucasus Region and on the Balkan Peninsula.
Identification. Vujić et al. (2020a) provided an identification key for 15 species of the constans species group and distribution maps.

5) ruficornis species group (Suppl
Diversity and distribution. A total of 18 species are recognized in the ruficornis species group (Vujić et al. 2012).The group has a predominantly Eastern Mediterranean distribution with a very high level of endemism. Among the 18 taxa (Suppl. material 5: Table S1), 12 are limited-range endemics and are only found in a few mountain areas or in a small part of the total range of the group. Two regions with a high level of endemism are the Anatolian Peninsula and the Caucasus Region.
Identification. Distributional data and an identification key for 18 species are provided by Vujić et al. (2012). . material 3: Fig. S3C, D) Diagnosis. In general appearance similar to the members of the ruficornis species group. This group comprises black species with bronze reflections that are covered with golden-yellow erect pile, shiny terga and sterna without any trace of pollinosity, and tergum 2 with a pair of small lateral orange maculae (Suppl. material 1: Fig. S33B). Males lack the extensions on the metafemur and metatibia (contrary to the species of the ruficornis species group). The male genitalia have biramous anterior surstyle lobe consisting of an interior accessory lobe carrying two spines and protruded apical extension, cercus without prominences (Suppl. material 2: Fig. S3D: c), and with lateral sclerite of the aedeagus curved apically (Suppl. material 2: Fig. S3F: s). In females, the tergum 4 is without a transversal depression (contrary to the female of the ruficornis species group), whereas the frons and vertex are shiny, black, and without any pollinosity, with the exception of a narrow line along the eye margin (Suppl. material 1: Fig. S36B).

6) rufus species group (Suppl
Diversity and distribution. The European rufus species group is composed of four species, three of which belong to recently described species from Mediterranean mountains, namely M. kozufensis , M. olympius Vujić & Radenković, 2020, and M. orjensis . The fourth species is M. rufus Meigen, 1838. Identification. Radenković et al. (2020) recognized this group for the first time and provided a revised identification key.

1) aureus species group (Suppl. material 3: Figs S5A-D, S6A, B)
Diagnosis. Small to medium sized species (8-12 mm) with a short rounded abdomen, a distinct calcar on the metatrochanter in males. Male genitalia have a characteristic posterior surstyle lobe with parallel margins and rounded apex (as in Suppl. material 2: Fig. S4A: pl) and a narrow, elongated, sickle-shaped hypandrium without lateral sclerite of the aedeagus (as in Suppl. material 2: Fig. S4D: marked with arrow).
Diversity and distribution. The aureus species group comprises a large number of previously known and recently discovered taxa distributed mostly in the Mediterranean Region and South Europe with a high number of local endemics. Šašić et al. (2016) defined six species subgroups within the aureus species group: aureus, bessarabicus, cinereus, chalybeus, caerulescens and dobrogensis, and one unplaced species (M. unguicornis Strobl in Czerny and Strobl 1909). Each of these species subgroups comprises at least one species complex of cryptic species (Šašić et al. 2019), although they may contain multiple complexes of species such as the bessarabicus species subgroup (see Veselić et al. 2017). Recent publications (Veselić et al. 2017;Radenković et al. 2018b;Šašić Zorić et al. 2019;Vujić et al. 2020c) increased the number of known species in the aureus species group to 45, including eight undescribed cryptic species of the ambiguus, bessarabicus, and sapphous species complexes (Suppl. material 5: Table S1).
Šašić et al. (2016) defined species complexes as morphologically inseparable species, which can only be resolved by employing an integrative taxonomy approach including different data types such as molecular, geometric morphometry, and ecological niche modelling (ENM). Applying this approach for the aureus species group has led to the discovery of previously unknown species complexes. In the cinereus species subgroup, Šašić et al. (2016)

Diversity and distribution.
The bombiformis species group consists of six related, though clearly morphologically different species distributed in central and southern Africa, of which three have been described (M. bombiformis Hull, 1944, M. multifasciatus Curran, 1939, and M. nasicus Bezzi, 1915 and three remain undescribed (Suppl. material 5: Table S1).
Identification. A taxonomic revision of this species group is in preparation (Vujić, unpublished). . material 3: Fig. S6C) Diagnosis. The funestus and the bombiformis species groups differ from other species and species groups of the aureus lineage by the elongated pedicel, approximately as long, or even longer, than basoflagellomere (Suppl. material 1: Fig. S29A, C) and the small lateral sclerite of the aedeagus (Suppl. material 2: Fig. S5I: s). The funestus species group can be distinguished from the bombiformis species group by the presence of a calcar on the metatrochanter in males (Suppl. material 1: Fig. S28G), which is absent in the males of the bombiformis species group, and a strongly dentate apicoventral triangular lamina on the metafemur in both sexes (Suppl. material 1: Fig. S28F), which is less dentate in the members of the bombiformis species group and usually has a distinct apical dens (Suppl. material 1: Fig. S28E, H).
Identification. A taxonomic revision is currently being prepared (Vujić, unpublished).

4) nanus species group (Suppl. material 3: Fig. S7B)
Diagnosis. Small to medium-sized species (6-12 mm) with a short rounded abdomen. Differs from the members of the aureus species group by the absence of a calcar on the metatrochanter in males and abdominal terga with transverse fasciae of pale pile instead of pollinose fasciate maculae (rarely with indistinct pollinosity). Male genitalia with a broad hypandrium (as in Suppl. material 2: Fig. S4G) with the apical part of the aedeagus large, in a form of biramous pliers (as in Suppl. material 2: Fig. S4H). The studied morphological characters show high morphological similarity in all taxa, with the exception of M. telmateia Hurkmans, 1987, which has completely pale and unicoloured tarsi (this character clearly separates this taxon from all other members of the nanus species group). The five other species can be distinguished by differences in the partly black to brown tarsi and structure of male genitalia (see Vujić et al. 2015;Kočiš Tubić et al. 2018). Diversity and distribution. We recognized six taxa within the nanus species group (Suppl. material 5: Table 1). All species from the nanus species group are widely distributed across the Anatolian Peninsula, which holds the highest diversity for this species group. Besides the Anatolian Peninsula, this species group occurs to the west including Greece, North Macedonia and Serbia, to the north to the Caucasus Region and Crimean Peninsula, and to the east and south to Syria, Lebanon, Israel and Iran.
Identification. Vujić et al. (2015) and Kočiš Tubić et al. (2018) revised the taxonomy of this species group. . material 3: Fig. S7A) Diagnosis. Members of this species group resemble in their overall appearance species of the nanus species group, from which they can be easily distinguished by black tibiae and tarsi (mostly pale in the nanus species group), and the structure of the male genitalia: hypandrium of male genitalia strongly modified, anfractuous in apical half, with subapical ctenidium and stitched theca (Suppl. material 2: Fig. S4K), and posterior surstyle lobe narrow and pointed (Suppl. material 2: Fig. S4I: pl). Additionally, males of the spinitarsis species group have a basoventral lamina on the metatarsus. Diversity and distribution. Only two species are known, M. spinitarsis Paramonov, 1929, and an undescribed species (Suppl. material 5: Table S1). Merodon spinitarsis is distributed in Greece, Romania and Turkey, while the undescribed species is found in Israel and Palestine (Vujić, unpublished).
Identification. A taxonomic revision including an identification key and descriptions for the four new species is in preparation (Vujić, unpublished).
Diversity and distribution. Besides M. aurifer Loew, 1862 distributed in the north Mediterranean and Turkey, the species group consists of at least one additional taxon, an undescribed species from Turkey and Azerbaijan.

3) avidus species group (Suppl
Diversity and distribution. The avidus species group is composed of the avidus species complex with four species, and the species M. femoratus Sack, 1913 andM. rutitarsis Likov, Vujić &Radenković, 2020 (Suppl. material 5: Table S1). This species group is distributed all across Europe, mainly in central and southern zones, and less diverse in the Near and Middle East and in North Africa (Algeria and Libya).

Diversity and distribution.
The clavipes species group contains four species (Suppl. material 5: Table 1) distributed in the Mediterranean Region and up to Iran in the east.
Identification. A taxonomic revision of this species group is under preparation (Vujić, unpublished). . material 3: Fig. S9C) Diagnosis. They are small sized species (5-9 mm) with metallic shiny bodies; scutum and terga strongly punctate, without or with very weak pollinose areas (Suppl. material 1: Fig. S17A, B); metafemur with very small apical triangular lamina apicoventrally (Suppl. material 1: Fig. S12A). Males of this species group are clearly separated from other species groups of the avidus-nigritarsis lineage by distinctly dichoptic eyes and lack of ctenidium at hypandrium.

6) italicus species group (Suppl
Diversity and distribution. Two species share these morphological features and belong to this species group: M. italicus Rondani, 1845 recorded from most of the Mediterranean and M. erivanicus Paramonov, 1925 distributed from Croatia to Azerbaijan and Israel. Identification. A taxonomic revision of this group is in preparation (Vujić, unpublished). . material 3: Fig. S10C) Diagnosis. Species with elongate, narrow and tapering abdomen, tarsi dark brown/ black dorsally and partly orange ventrally. Male genitalia: anterior surstyle lobe more or less rhomboid shape (Suppl. material 2: Fig. S8D: al), except in alagoezicus species subgroup where the anterior surstyle lobe is transformed into a narrow, elongate, strongly curved projection (Suppl. material 2: Fig. S8A: al); hypandrium with a pair of apical thorns on the ventral margin directed backwards but often with a pair of lateral projections near the base and well-developed lingula (Suppl. material 2: Fig. S8F: l).

8) pruni species group (Suppl
Diversity and distribution. Four species belong to this species group: M. cupreus Hurkmans, 1993, M. pallidus Macquart, 1842 andM. pruni Rossi, 1790 and one undescribed taxon from Israel. Merodon pruni is distributed in most of the Mediterranean Basin, but the other two described species are more allocated to the east, from Turkey to Israel and Pakistan. Identification. A taxonomic revision of this group is in preparation (Vujić, unpublished). . material 3: Fig. S9D) Diagnosis. Species with characteristic basolateral protrusion on the posterior surstyle lobe at outer surface (Suppl. material 2: Fig. S9G: marked with arrow); legs mostly black; terga black, tergum 2 usually with a pair of reddish orange lateral maculae; metafemur usually with shorter pilosity ventrally, less than width of metafemur (Suppl. material 1: Fig. S14A); basoflagellomere usually narrow and elongated, dark brown, two times longer as pedicel. They are medium-large (11-15 mm) species with a dark scutum and white pollinose fasciate maculae (at least in females) on the dark olive brown terga 2-4 (Suppl. material 1: Fig. S22D).

9) serrulatus species group (Suppl
Diversity and distribution. This species group includes 13 species (Vujić et al. 2020b). Merodon serrulatus Wiedemann in Meigen, 1822 is the species of the genus Merodon with the largest distributional range being distributed from the Iberian Peninsula in the south-west, along the Mediterranean and Balkan Peninsula, through Turkey and southern Russia to Siberia and Mongolia in the north-east. Other species of the serrulatus species group can be found at the edges of this distributional range, albeit with a much more restricted distribution (see Vujić et al. 2020b).

10) tarsatus species group (Suppl
Diversity and distribution. The tarsatus species group consists of seven described and six undescribed species (Suppl. material 5: Table S1). This group of species is geographically restricted to the Near and Middle East, and Central Asia.
Identification. Vujić et al. (2019) and Likov et al. (2020) mentioned this group of species but did not give diagnostic features. A taxonomic revision of the tarsatus species group is in preparation (Vujić, unpublished).
Merodon clunipes (Suppl. material 3: Fig. S12A, B) is a species with broad metatibiae and dark terga, and has clear apomorphic diagnostic characters, including antennal shape: fossette large, extended from dorsal side to outer, covering half of lateral surface (Suppl. material 1: Fig. S13D), and the characteristic shape of the posterior surstyle lobe with the apical hump directed toward cercus (Suppl. material 2: Fig. S10G: marked with arrow). This species has a North Mediterranean distribution.
Merodon crassifemoris (Suppl. material 3: Fig. S12C, D) is a taxon with tubercle on the face below the antenna (Suppl. material 1: Figs S21A, S25B), and a hook-like posterior surstyle lobe (Suppl. material 2: Fig. S10J: pl) unique among all other taxa of the avidus-nigritarsis lineage. It was recently revised and excluded from M. nigritarsis group . The distribution of M. crassifemoris extends from the eastern Balkans through the Anatolian Peninsula as far as Ukraine and Azerbaijan.
Merodon eumerusi (Suppl. material 3: Fig. S13A) possesses a line of spinae on the inner side of the apical quarter of metafemur (Suppl. material 1: Fig. S5A), representing a unique character that is absent in all other species of the genus; male genitalia (Suppl. material 2: Fig. S11A-C) similar to M. ottomanus (Suppl. material 2: Fig.  S11G-I). Differs from other known species of the M. avidus-nigritarsis lineage in having 1-4 fine pile (usually one) on the posterior side of the mesocoxa. In males, the basoflagellomere is elongated with an angular apex, bearing a very large outer fossette and a second inner fossette (Suppl. material 1: Fig. S10A, B), which are absent in almost all other species of the genus except M. serrulatus (Vujić et al. 2020b). This species is recently described from high mountain ranges in Tajikistan, Uzbekistan and Kyrgyzstan .
Merodon hirtus (Suppl. material 3: Fig. S13B) belongs to species with dark terga, males with posterior surstyle lobe divided in two branches (Suppl. material 2: Fig. S9J: pl); eyes slightly dichoptic, distance between eyes about two facets wide (Suppl. material 1: Fig. S11B); in females terga 2-4 finely punctate; posterior half of tergum 4 with shorter, mostly black and adpressed pile (Suppl. material 1: Fig. S19E). This is an extreme eastern Mediterranean species with a range extending from Turkey to Iran and Israel, as well as Cyprus.

Merodon desuturinus lineage
Diagnosis. The specific shape of the lateral sclerite of the aedeagus (gradually tapered, with the tip curved downwards) is the main synapomorphic character that connects all species from the group (as in Suppl. material 2: Fig. S12I: s). Moreover, the species in this species group have pile on posterior side of mesocoxa; a curved distal prolongation of anterior surstyle lobe (as in Suppl. material 2: Fig. S12E: al); basoflagellomere less than two times as long as wide (Suppl. material 1: Fig. S6B); scutum without pollen or with less distinct pollinose longitudinal vittae (Suppl. material 1: Fig. S6D); wing microtrichose between veins R 1 and RS (Suppl. material 1: Fig. S9A); postpronotum usually brown or yellow-reddish; pilosity on lateral side of tergum 4 in female long, medially short and mostly adpressed (Suppl. material 1: Fig. S8A). The desuturinus lineage is closely related to the albifrons lineage, which was named albifrons+desuturinus clade in Radenković et al. (2018a).
The desuturinus lineage contains the Afrotropical melanocerus species group with two species subgroups (melanocerus and planifacies) and the species M. cuthbertsoni Curran, 1939Djan et al. 2020) (Suppl. material 3: Fig. S14C), and the Palaearctic murorum species group with four species ). The desuturinus lineage comprises 14 described and 10 still undescribed species (Suppl. material 5: Table S1). . material 3: Fig. S14A) Diagnosis. Species with patch of dense yellow pile (dense and strong yellow to red brush of pile) on metatrochanter (Suppl. material 1: Fig. S37D). The melanocerus species subgroup has the oral margin notched, slightly produced forward (Suppl. material 1: Fig. S37B) and the planifacies species subgroup has the oral margin reduced, covered with microtrichia (Suppl. material 1: Fig. S37A). Merodon cuthbertsoni, with an unclear position within the species group, has apical fourth of tibiae and all tarsi bright yellow.

1) melanocerus species group (Suppl
Diversity and distribution. Distribution of the melanocerus species subgroup is limited to South Africa, while the planifacies species subgroup has broader range: western, central and southern Africa. Merodon cuthbertsoni occurs in Zimbabwe. Identification. Recent revision of the melanocerus species subgroup ) resulted in the delimitation of five species: M. capensis Hurkmans, 2018, M. commutabilis Radenković & Vujić, 2018, M. drakonis Vujić & Radenković, 2018, M. flavocerus Hurkmans, 2018and M. melanocerus Bezzi, 1915. Part of the planifacies species subgroup was the subject of a recent molecular analysis, which supported the monophyly of the subgroup ). According to their integrative approach, three species are found within the planifacies species subgroup in South Africa: M. planifacies Bezzi, 1915, and two species of the capi species complex characterized by smooth thecal ridge in male genitalia, namely M. capi M. roni Radenković &. The fourth known species from the species planifacies subgroup, M. stevensoni Curran, 1939, was described based on one female from Zimbabwe, and its taxonomic status remains unclear until the discovery of additional material, especially male specimens . Within the planifacies species subgroup, populations with folded thecal ridge of hypandrium in male genitalia could represent a group of geographically isolated species, which needs additional taxonomic research based on integrative approach ). Ten undescribed species are already recognized (Suppl. material 5: Table S1) and descriptions are in preparation (Vujić, unpublished).
2) murorum species group (Suppl. material 3: Fig. S14B) Diagnosis. Species without patch of dense yellow pile (dense and strong yellow to red brush of pile) on metatrochanter.
Identification. Vujić et al. (2018b) recently revised this species groups and provided an identification key for the desuturinus lineage, including the murorum species group.

1) natans species group (Suppl
Diversity and distribution. The natans species group is distributed around the Mediterranean Basin and there is one isolated record in Kenya (Vujić, unpublished).
Identification. A taxonomic revision of this species group is in preparation (Vujić, unpublished).

Discussion
Out of 194 described species (234 in total including undescribed taxa), 180 (209) species are distributed in the Palaearctic Region and 14 (27) are known from the Afrotropical Region. Three lineages (aureus, desuturinus, and natans) have representatives in both the Afrotropical and the Palaearctic Regions. The Afrotropical melanocerus species group of the desuturinus lineage and the bombiformis species group of the aureus lineage are exclusive to the Afrotropical Region, while all other species groups belong to Palaearctic fauna.
The desuturinus lineage, with 24 species (14 described), contains two species groups: the Afrotropical melanocerus species group with two species subgroups (melanocerus and planifacies) and the unplaced species M. cuthbertsoni; and the Palaearctic murorum species group with four species.
The natans lineage contains the natans species group, with four species (three described), and the unplaced species M. segetum.
At present and based on our results, the regions with the highest species richness are the Mediterranean Peninsulas: Iberian, Balkan and especially Anatolian. Certain areas in the Palaearctic (regions of Pakistan, Central Asia and eastern part of the Middle East) and Afrotropical Regions (Central and Eastern Africa) have been under-sampled and they need additional collecting efforts. Central Asia and Pakistan are characterised by numerous endemics with potential significance to understand the evolutionary scenario of the genus Merodon. Finally, the genetic diversity is extremely high in the aureus species group and more taxonomic research still needs to be done in this species group and some others, like the ruficornis, avidus and equestris species groups.  Figure S1.   Figure S31. Merodon luteihumerus, male. A head, anterior view B thorax, dorsolateral view. Scale bar: 2 mm. Figure S32. Merodon mixtum, legs, lateral view. A proleg, male B proleg, female C mesoleg, male D mesoleg, female E metaleg, male F metaleg, female. Scale bar: 1 mm. Figure S33.  (Sack, 1931). Scale bar: 1 mm. Figure S8.