Revision of the Palaearctic species of the Merodon desuturinus group (Diptera, Syrphidae)

Abstract This revision of material belonging to the Merodon desuturinus group from the Palaearctic Region resulted in the delimitation of four species: Merodon cabanerensis Marcos-García, Vujić & Mengual, 2007; Merodon desuturinus Vujić, Šimić & Radenković, 1995; Merodon neolydicus Vujić, nom. n.; and Merodon murorum Fabricius, 1794. Merodon murorum is redescribed. A neotype for Merodon auripilus Meigen, 1830 is designated, which is a new junior synonym of Merodon murorum. The related Afrotropical species Merodon cuthbertsoni Curran, 1939 is re-evaluated and compared to its sibling Palaearctic taxon Merodon desuturinus. An identification key for the Merodon desuturinus group is provided.


Introduction
The genus Merodon Meigen, 1803 (Diptera, Syrphidae) comprises more than 160 species distributed across the Palaearctic and Afrotropical Regions (Ståhls et al. 2009). The Mediterranean Basin hosts the highest diversity with more than 110 species, most probably due to high variety of bulb plants that are larval hosts of this phytophagous genus (Ricarte et al. 2008, Andrić et al. 2014. Hurkmans (1993) conducted the first published revision of part of this genus, analysing 61 species (only those with tapering abdomens) classified into eleven groups.
Initial research on the phylogeny of the Merodon genus was conducted by Mengual et al. (2006). Based on analysis of COI sequences of the Iberian species, they defined four well-supported groups: desuturinus, albifrons, nigritarsis, and aureus.
One in particular, the so-called "desuturinus group" of range-restricted species, is of special conservation interest as it has members in both the Palaearctic and Afrotropical Regions. Vujić et al. (1995) described an endemic species, Merodon desuturinus, from the high mountains of the Balkan Peninsula. Later, Marcos-García et al. (2007) discovered a related species, M. cabanerensis, from central Spain. Another species was recognized by Hurkmans (unpublished manuscript, cited in Milankov et al., 2008b) as M. lydicus Hurkmans, which was recorded in the Eastern Mediterranean. This latter species is formally described here as M. neolydicus Vujić, nom. n. One additional taxon belonging to the desuturinus group, M. murorum Fabricius, 1794, was uncovered and we redescribed it here. Milankov et al. (2008b) detected low genetic variability in a population of M. desuturinus and demonstrated that this taxon represents an evolutionarily independent lineage among Merodon taxa. Radenković et al. (2018) found new members of the desuturinus group in South Africa, which are related to Merodon melanocerus Bezzi, 1915. Those records represent the first detailed characterisation of Merodon species in the Afrotropical Region using morphological and molecular data.
The aim of this paper is to present a revision of the Palaearctic species of the Merodon desuturinus group in order to clarify its taxonomy with the support of morphological characters, and to present an identification key for the adults of the species within this group.

Materials and methods
This study is based on the examination of all available material of the Merodon desuturinus species group (published and unpublished data), which has been deposited in the museums, universities and private collections listed below. The following acronyms for museums and entomological collections are used in the text: The characters used in the key, descriptions, and drawings follow the terminology established by Thompson (1999), except for the term "pleuron", which follows McAlpine (1981), and those relating to male genitalia are according to Marcos-García et al. (2007). Colour characters are described from dry-mounted specimens. Male genitalia were stored in microvials containing glycerol after clearing in warm 10% potassium hydroxide (KOH) for a few minutes and neutralising in acetic acid for 5-10 seconds.
The following abbreviations are used: f = female, m = male. All information on the specimens (locality, collector, coordinates, etc.) is presented under the description of the respective examined material. The capture locations (geographical coordinates) were entered into the GenGIS (v2.5.1) software to generate the distribution map (Parks et al. 2013).
Diagnoses of species were made according to unique characters attributable to the group, complex, and species considered here, and also to combinations of characters that enabled taxa to be distinguished and recognised. The type material of the included species was examined by Ante Vujić. Drawings were made with an FSA 25 PE drawing tube and digital photographs were taken with a Leica DFC 320 digital camera, both of which were attached to a Leica MZ16 binocular microscope.

Diagnostic characters and diversity of the Merodon desuturinus species group
The M. desuturinus species group sensu Mengual et al. (2006) is characterised by the following adult morphological characters: posterior side of mesocoxa with pile; anterior surstyle lobe with a curved distal prolongation (dp in Figs 1B, 9B, 11B, 12B, 14B); 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 (s in Figs 1C, D, 9C, 11C, 12D, 14C, 16) (Vujić et al. 1995, Milankov et al. 2008b. The M. desuturinus species group is closely related to the albifrons group (Mengual et al. 2006), which has been designated as an albifrons+desuturinus clade in Radenković et al. (2018).
The M. desuturinus group consists of two clearly separate lineages, Palaearctic and Afrotropical based on both adult morphological and molecular data (Radenković et al. 2018). The main morphological diagnostic character that distinguishes these two lineages is the presence of a strong dense yellow to red brush of pile on the metatrochanter of Afrotropical species, which Palaearctic taxa lack. Besides the taxon M. desuturinus, the Palaearctic lineage of this species group includes three additional species, one western Mediterranean endemic (M. cabanerensis) and two species presented here.

General description of the Merodon desuturinus species group
Male. Head (Figs 2, 3, 4): Antenna (Fig. 4) usually dark brown; basoflagellomere generally short, as long as broad (except in M. flavocerus where it is light brown and longer), concave dorsally, with acute apex; arista light brown to dark brown, thickened basally, 1.5-2 times longer than basoflagellomere, covered with dense brown microtrichia. Face covered with long whitish yellow pile, except on median bare vitta that occupies 1/4 width of face. Frons black, often with bronze film and indistinct microtrichia that, at face level, follow a narrow line along the eye margin. Vertical triangle isosceles (Fig. 3), black (brown-red in M. flavocerus) and shiny (except at anterior end covered with microtrichia), predominantly covered with long, black, thick pile, except at posterior end with light yellow pile. Eye pile dense, as long as scape, often pale, but can be  Vujić, nom. n. male genitalia. A Epandrium, lateral view B Epandrium, ventral view C Hypandrium, lateral view D Aedeagous. Abbreviations: al-anterior surstyle lobe, pl-posterior surstyle lobe, c-cercus, e-ejaculatory apodeme, p-phallapodeme, s-lateral sclerite of aedeagus, it-inner thorn on medial part of surstylus, dp-distal prolongation on anterior surstyle lobe, vms-ventral margin of surstylus, vr-ventral ridge of theca, t-theca. Scale bar: 0.2 mm. darker dorsally. Occiput covered with whitish yellow pile, dorsally with metallic, bluish or bronze lustre; white microtrichia from upper eye corner as a narrow line dorsally, becoming dense and wide laterally and ventrally, occupying the lower 2/3 of occiput.
Thorax (Fig. 5): Scutum and scutellum black with bronze lustre (in M. flavocerus postpronotum and posterior rim of scutellum pale yellow), covered with relatively long (as long as or a little longer than basoflagellomere), dense, erect, more or less branched and usually yellow pile (in M. capensis and M. commutabilis mixed with black pile); presence of microtrichia variable (from well-developed in M. drakonis to absent in M. capensis). Pleuron often covered with grey-green microtrichia (lacking in M. flavocerus) and the following parts with long yellow pile: posterior part of anterior anepisternum, posterior anepisternum (except anteroventral part), anepimeron, metasternum, and anterior, posterodorsal and posteroventral parts of katepisternum; katatergum with dense, erect, short, yellowish or light brown pile. Wing hyaline, with dense microtri-   posteroventral spur. Pile on legs predominantly yellow, except for some short black pile on tarsi dorsally. Abdomen (Fig. 6): Black with bronze reflections, slightly tapering, as long as mesonotum. Terga 2-4 black with more or less distinct transverse fasciae of white microtrichia interrupted in the middle (can be connected on tergum 4); tergum 2 in some taxa with pair of antero-lateral orange maculae (lacking in M. capensis, M. cuthbertsoni, M. commutabilis, M. cabanerensis, and M. desuturinus, all of which have dark terga), or areas covered with long, dense, erect, yellow pile; pilosity on lateral sides of terga long, erect and whitish, adpressed on central parts, and white on mictrotichose transversal Female: Similar to the male except for typical sexual dimorphism (Figs 5C,D,7,8).
Thorax: Scutum and scutellum black with bronze lustre, covered with dense, erect graywhitish or yellow pile. Scutum with barely visible 2 longitudinal microtrichose vittae. Wing hyaline with dark-brown veins, and densely covered with microtrichia, except basal edges of cells BM and CuP. Femora and tibiae brown-black, except for paler knees and base of tibiae; tarsi dark brown dorsally (except for usually paler tarsal segments on pro-and mesolegs), light brown ventrally. Pile on legs yellow. Metafemur ( Figure 5A) slightly curved.
Female (Figs 5C, 7A, 8D). Similar to the male except for typical sexual dimorphism and shiny frons with narrow line of microtrichia along eye margin, mostly covered with black pile. Postalar callus and postpronotum can be yellow-red or brown.
Etymology. The epithet lydicus is Latin, meaning "from Lydia", and refers to the region of origin of the holotype, viz. western Turkey, which once was included in the Kingdom of Lydia and neo refers to the new name for this species known from unpublished manuscript. It is to be treated as an adjective.
Thorax: Scutum and scutellum black with bronze lustre, covered with dense, erect yellow or whitish pile. Wing hyaline with dark-brown veins, and dense microtrichia. Femora brown-black, knees and most of tibiae (or at least both ends) yellow-red; tarsi usually yellow dorsally and light brown ventrally (in some specimens all tarsi can be brown). Pile on legs yellow. Metafemur thick, slightly curved ( Figure 5B) and covered with long pile. Abdomen: Lateral sides red-orange to red-brown, medially black; terga 2 and 3 can have reddish triangular vittae or maculae; terga 2-4 each with more or less distinct white transverse fascia of microtrichia interrupted in the middle (lacking in some specimens) ( Figure 6B); pile on terga erect, whitish yellow and very long on lateral sides; terga 2-4 medially with adpressed pile, variable in colour (from all black except for white pile on microtrichose fasciae to predominantly pale).   Male genitalia (Figure 12): Anterior surstyle lobe bent inwards ( Figure 12B), with ventral margin slightly convex ( Figure 12A); median parts of surstylus with one inner thorn ( Figure 12B); posterior surstyle lobe wide and triangular, pointed apically (Figure 12A). Hypandrium wide, with smooth thecal ridge ( Figure 12D). Female (Figs 5D, 7B, 8C). Similar to the male except for typical sexual dimorphism; face shiny, almost lacking microtrichia; frons shiny, with distinct line of microtrichia along eye margin. Scutum usually with two lateral and three medial longitudinal microtrichose vittae.
Distribution. Species distributed in North Africa ( Figure 10).
Below, we redescribe an Afrotropical species of the Merodon desuturinus species group that is morphologically closely related to M. desuturinus. Curran, 1939 Figs 13A, C, D, 14 Type material. Holotype: male, in AMNH. Original label: Zimbabwe, Sanyati Valley S. Rhodesia, 9-10.1925, leg. R. H. R. Stevenson, det. Curran. Diagnosis. Face covered with microtrichia; black terga without lateral orange maculae, and terga 3 and 4 each with very narrow microtrichose fascia, approx. 1/10 of tergal length. Morphologically related to the species M. desuturinus from which it can be distinguished by the following features: eye contiguity is approx. 8 facets long ( Figure 13A) (the eyes are separated in M. desuturinus, Figure 13B); tarsi entirely pale (dark brown dorsally in M. desuturinus); male genitalia: posterior surstyle lobe with narrow apex pointed upwards ( Figure 14A) (triangular in M. desuturinus, Figure 11A); hypandrium with ventral margin of theca angled and folded ( Figure 14C) (rounded and unfolded in M. desuturinus, Figure  11C). M. cuthbertsoni is known only from Zimbabwe (Sanyati Valley in southern Zimbabwe), whereas M. desuturinus is endemic to just a few high Balkan mountains (Europe).  Figure 13C) brown, basoflagellomere 1.1 times as long as wide; arista brown and thickened basally and dark brown apically, 1.3 times longer than basoflagellomere, covered with short, dense mi-crotrichia. Face and frons black, covered with long whitish yellow pile and sparse silver microtrichia. Oral margin shiny black, slightly protruded. Vertical triangle isosceles ( Figure 13A), three times longer than eye contiguity, shiny black except in front of anterior ocellus that has white microtrichia, covered with long whitish yellow pile. Ocellar triangle slightly isosceles. Eye contiguity approx. 8 facets long. Eye pile as long as scape, pale. Occiput with whitish yellow pile, along the eye margin with dense white microtrichia and posteriorly with metallic bluish greenish lustre.

Merodon cuthbertsoni
Thorax: Scutum and scutellum black with bronze lustre, covered with dense, erect yellow pile. Pleuron covered with grey-green microtrichia and the following parts with long yellow pile: posterior part of anterior anepisternum, posterior anepisternum (except anteroventral part), anepimeron, metasternum, and anterior, posterodorsal and posteroventral parts of katepisternum; katatergum with short, dense, erect, light-brown pile. Wing hyaline, with dense, brown microtrichia. Calypter pale yellow. Haltere with light brown pedicel and yellow capitulum. Femora dark brown-black, except for usually paler apex; tibiae dark brown with pale basal and apical parts; all tarsi yellow. Metatrochanter without processes. Metafemur ( Figure 13D) thickened and slightly curved.
Abdomen: Black with bronze reflections, as long as mesonotum. Terga 2-4 each black with more or less distinct white transverse fascia of microtrichia, interrupted in the middle; pile on terga erect and yellow, except for central parts of terga 2-4 that are covered with adpressed black pile. Sterna blackish brown, covered with long pale yellow pile.
Female. Unknown. Distribution. Species endemic to Zimbabwe.

Discussion
The Merodon desuturinus clade was first mentioned by Mengual et al. (2006), and Milankov et al. (2008b) showed that this group represents an evolutionarily independent lineage among Merodon taxa. However, each of this study included only one species of the group (M. cabanerensis and M. desuturinus, respectively) in their analyses. Radenković et al. (2018) recently confirmed the monophyly of the M. desuturinus group in their analysis that examined an additional eight species of the group in relation to 27 other Merodon taxa, as well as its close relationship to the albifrons group. Based on adult morphological, molecular, and distributional data, Radenković et al. (2018) found that the M. desuturinus species group consists of two clearly separate lineages and represents an important link between the Palaearctic and Afrotropical faunas. They proposed that diversification in the M. desuturinus group most likely occurred during fundamental shifts in African climate. During the Pliocene-Pleistocene epoch, favourable conditions for Merodon species (increased aridity and open grasslands) in Africa most probably allowed faunal transitions from the Eastern Mediterranean (including SW Asia), with one lineage migrating to South Africa and another to the western Palaearctic.
The main morphological diagnostic character that separates these two lineages is the presence of a dense and strong yellow-to-red brush of pile on the metatrochanter in Afrotropical species, which is lacking in Palaearctic taxa. The Afrotropical lineage comprises the M. melanocerus subgroup of five taxa (Radenković et al. 2018), the M. planifacies subgroup, and the species M. cuthbertsoni that is morphologically related to the Palaearctic taxon M. desuturinus. The M. planifacies subgroup is characterised by a distinct apomorphic character, i.e., a reduced oral margin covered by microtrichia. M. cuthbertsoni is endemic to Zimbabwe, but its systematic position remains unclear. Currently, there is no genetic data on M. cuthbertsoni since only old museum material exists.
Our revision of the Palaearctic species from the M. desuturinus group has resulted in the delimitation of four species. This lineage consists of closely related yet clearly morphologically distinct species. The most distinctive species is M. murorum, based on the shape of the male genitalia and its reddish abdomen.
Two of these Palaearctic taxa are endemo-relicts; M. cabanerensis is known only from a restricted area in central Spain and Morocco and M. desuturinus is found only on four high mountains of the Balkan Peninsula, of which two are in Montenegro (Durmitor and Orijen) and two are in Serbia (Kopaonik and Stara planina). Merodon neolydicus Vujić, nom. n. is present in several countries of the Eastern Mediterranean (Greece, Turkey, Syria, Lebanon, Israel) and Iran, while M. murorum is distributed in North-West Africa (Algeria, Morocco, Tunisia) (Fig. 10).
Based on the distributions of the Palaearctic lineage of the M. desuturinus species group on high mountains of North Africa, in the Eastern Mediterranean and on the Iberian and Balkan peninsulas, they can be considered as oromediterranean relicts (Fig. 10).