Revision of the Merodon bombiformis group (Diptera: Syrphidae) – rare and endemic African hoverflies

In the present work, the Afrotropical species of the bombiformis species group, part of the aureus lineage, are revised. Six species are recognized, based on a combination of morphological and genetic features. Three of these species are new to science: Merodon lotus Vujić & Radenković sp. nov., M. vittatus Vujić & Likov sp. nov., and M. zebra Vujić & Radenković sp. nov. Redescriptions are provided for the other three species: M. bombiformis Hull, 1944, M. multifasciatus Curran, 1939, and M. nasicus Bezzi, 1915. The female of M. bombiformis is described. The name Merodon edentulus Macquart, 1855 is considered here as a nomen dubium. One new synonymy is proposed: M. apimima Hull, 1944 syn. nov. (junior synonym of M. multifasciatus). The distribution of the bombiformis species VUJIĆ A. et al., Revision of the Merodon bombiformis species group 89 group is discussed. The larval host plant of M. multifasciatus is identified as Gladiolus. A key to the identification of both males and females of the bombiformis group is provided.


Introduction
The phytophagous hoverfly genus Merodon Meigen, 1803 (Diptera: Syrphidae: Eristalinae: Merodontini) is distributed across the Palaearctic and Afrotropical regions and comprises more than 160 species (Ståhls et al. 2009;Vujić et al. 2012). Merodon equestris (Fabricius, 1794) as an introduced anthropogenic species from Europe is also widely distributed across the Nearctic (Speight 2020). The highest diversity of genus Merodon is recorded around the Mediterranean basin, a phenomenon which has been associated with the great variety of geophytes in this region, as those are the host plants of larvae of Merodon (Andrić et al. 2014;Ricarte et al. 2017). Although the immature stages of only eight species of Merodon have been described and the larval host plants are postulated only for eight additional species, based on still incomplete evidence, the connection of larvae of Merodon with underground storage organs of the family Hyacinthaceae Batsch ex Borkh. is well recognized (Ricarte et al. 2008(Ricarte et al. , 2017Andrić et al. 2014;Preradović et al. 2018). It might be expected that the congruence between geophyte diversity and species-richness of Merodon, highlighted by Ricarte et al. (2017) for the European Mediterranean zone, would be paralleled in South Africa, since it is there that the geophyte diversity is at its maximum (Pfosser & Speta 2004). But, surprisingly, nowhere within the Afrotropical Region is there a high diversity of species of Merodon.
The rest of the Afrotropical species of the genus Merodon belong to the aureus lineage. Šašić et al. (2016) defined species groups within this lineage, such as the M. aureus, M. bombiformis, M. funestus, M. nanus and M. spinitarsis species groups. Revision of all available material from museums and detailed analyses of newly collected specimens from our own expeditions to RSA resulted in the delimitation of the six species considered here, all belonging to the M. bombiformis group.
The aims of the present study were: 1) To review material belonging to the bombiformis group held in the collections of several entomological institutions and private individuals, 2) To define and describe the taxa within the bombiformis species group, including mapped geographic distributions and new species, 3) To examine the genetic diversity exhibited among the members of this species group and infer the phylogenetic relationship, using one mitochondrial gene (cytochrome c oxidase subunit I (COI)) and two nuclear genes (28S rRNA and 18S rRNA), 4) To present an illustrated key and a discussion on the different taxonomic characters used, 5) To discuss the larval host plant data available for the bombiformis species group.

Morphological study
The present study is based on the examination of all available material of the Merodon bombiformis species group deposited in the museums and universities listed below. The following abbreviations of museums and entomological collections are used in the text to indicate where the material studied is deposited: Numbers given alongside the abbreviation FSUNS in the examined material refer to unique identifiers of the specimens, stored in the database of the Faculty of Sciences, University of Novi Sad. The distribution map ( Fig. 1) was generated with the mapping software DIVA-GIS (Hijmans et al. 2012).
Relationships of species to different vegetation types were based on the map of natural vegetation of Africa (Sayre et al. 2013).
For the study of male genitalia, dry specimens were relaxed in a humidity chamber. The genitalia were extracted with a hook-tipped entomological pin before being cleared by boiling in warm 10% potassium hydroxide (KOH) for 3-5 min. This was followed by brief immersion in acetic acid to neutralize the KOH, and then by immersion in ethanol to remove the acid. The prepared genitalia were stored in microvials containing glycerol pinned to the source specimen.
Digital photographs were taken with a Leica DFC 320 digital camera attached to a Leica MZ16 stereo microscope, while for drawings a Leica MZ16 binocular microscope was used with an FSA 25 PE drawing tube. The photographs were stacked using CombineZ software (Hadley 2006).
Body length was measured in lateral view in millimetres (mm) using an eyepiece micrometer from the lunula to the end of the abdomen. Wing length was measured from the tegula to the apex of the wing.

Molecular analysis
The genomic DNA of six hoverfly specimens, belonging to three species from the Merodon bombiformis species group (M. bombiformis, M. multifasciatus and M. lotus Vujić & Radenković sp. nov.) was extracted from mid and hind legs using the sodium dodecyl sulfate (SDS) extraction protocol described by Chen et al. (2010). Fresh material required for high-quality DNA extraction was only available for these species. Species of the M. bombiformis group for which we were unable to obtain fresh material were not included. Genomic DNA vouchers are deposited at the Faculty of Sciences, Department of Biology and Ecology, University of Novi Sad (FSUNS).
We amplified one mitochondrial (COI gene) and two nuclear genes (28S rRNA and 18S rRNA genes). The Folmer fragment (DNA barcode) on 5′COI gene was amplified using LCO1490 and HCO2198 primer pair (Folmer et al. 1994), the 28S rRNA gene using F2 and 3DR primer pair (Belshaw et al. 2001), while for amplification of 18S rRNA we used 2F and b2.9 primer pair designed by The Crandall Lab (for primer sequences see Vujić et al. 2019b

Sequence processing and phylogenetic analysis
The COI, 28S rRNA and 18S rRNA gene sequences were edited for base-call errors using BioEdit ver. 7.0.9.0. (Hall 1999). Additional sequences of several species representing the main Merodon lineages following Radenković et al. (2018) were downloaded from GenBank and joined to the sequence matrices of all tree genes. The COI and 18S rRNA gene sequences were aligned manually, while 28S rRNA gene sequences were aligned using MAFFT ver. 7 (Katoh & Standley 2013), available from the European Bioinformatics Institute (EMBL-EBI) web services (McWilliam et al. 2013). The aligned COI, 28S rRNA and 18S rRNA gene sequences were concatenated and combined into a single sequence matrix. The ML tree was constructed using RAxML ver. 8.2.8 (Stamatakis 2014) using the CIPRES Science Gateway web portal (Miller et al. 2010) under the general time-reversible (GTR) evolutionary model with a gamma distribution (GTRGAMMA) (Rodríguez et al. 1990) following instructions in the manual. The applied command line was as follows: raxmlHPC-HYBRID -T 4 -n result -s infile.txt -m GTRGAMMA -p 12345 -q part.txt -f a -N 1000 -x 12345 -o E._pulchel. Nodal supports were estimated using rapid bootstrapping with 1000 replicates. The tree is rooted with Eumerus pulchellus Loew, 1848 as outgroup. The uncorrected pairwise (p) distance matrix for 5′ COI gene sequences among specimens of the M. bombiformis species group was estimated using MEGA ver. 7 software (Kumar et al. 2016). GenBank accession numbers of all sequences are provided in Supp. file 1.   Hull, 1944

Redescription
Original description is insufficient, without any illustrations and based on a single male specimen from South Africa (Hull 1944).
THorax (Figs 6a, 10a). Mesonotum matte black, except lateral side of scutum including postpronotum and postalar callus, and posterior margin of scutellum reddish-yellow, covered with reddish pile; scutum with five pollinose longitudinal vittae (Fig. 10A). Pleuron covered with gray pollinosity and the following parts with whitish pile: anterior part of proepimeron, posterior part of anterior anepisternum, most of the posterior anepisternum except anterior end, antero-ventral and postero-dorsal part of katepisternum, anepimeron, and metasternum; in some specimens anterior anepisternum and katatergum reddish. Wing hyaline, with dense microtrichia and yellow to brown veins. Calypter pale yellow. Halter with yellow pedicel and capitulum. Legs usually dark brown to black (in some specimens partly reddish), except yellow basal third of tibiae. Metatrochanter without calcar. Metafemur moderately thickened, with less developed apicoventral triangular lamina, not dentate, only the apical dens is distinct, posteriorly oriented ( Fig. 6A). Pile on legs predominantly yellow. abdomen (Fig. 5A). Predominantly reddish-yellow (Fig. 5A). Reddish parts of terga 2-4 covered with yellow pile and dense pollinosity, while dark parts with short adpressed black pile; sterna from yellow to dark brown, covered with white or yellow pile. The species is distributed in the Republic of South Africa and Lesotho. On the basis of our data, the flight period was found to be from November to February. Merodon bombiformis was collected in different vegetation types, brush-grass savanna, temperate and mountain grasslands and montane foresttundra. On Drakensberg Mountain, at the locality between Maclear and Naudes Nek, it was found near the stream, within bushes of Leucosidea sericea Eckl. & Zeyh. (Rosaceae) and Gladiolus sp. on rocky soil (Fig. 24). Leucosidea sericea grows in Afromontane regions of southern Africa, and is the sole species of this genus (South African National Biodiversity Institute 2004). Stuckenberg (1956) described the puparia of M. bombiformis, reared from larvae found feeding on commercially grown Gladiolus sp. (Iridaceae) corms. Specimens from Pietermaritzburg (5 Nov. 1955, 19 Nov. 1955 were reared from bulbs of Gladiolus L. too.  Curran, 1939 (USNM 2052372;FSUNS ID 04512 Bezzi, 1915 (RBINS;FSUNS ID 21976). Scale bar = 2 mm. Vujić & Radenković sp. nov. urn:lsid:zoobank

Etymology
The specific epithet 'lotus' is a Latin adjective meaning 'elegant, fine', also 'washed/clean', implying clean, non pollinose terga, as the main diagnostic character of this species.
THorax (Figs 6e, 10e). Mesonotum black, covered with reddish-yellow pile; scutum without pollinose longitudinal vittae, but postpronotum and anterior part of scutum pollinose (Fig. 10E). Pleuron black, covered with sparse gray pollinosity and the following parts with whitish to yellow pile: anterior part of proepimeron, posterior part of anterior anepisternum, most of the posterior anepisternum except anterior end, antero-ventral and postero-dorsal part of katepisternum, anepimeron, metasternum. Wing brownish, hyaline, with dense microtrichia and dark brown veins. Calypter yellowish. Halter with yellowish pedicel and capitulum. All three femora dark brown to black; tibiae mostly reddish, medially black to dark brown; tarsi reddish, except blackish apical segment (Fig. 6E). Metatrochanter without calcar. Metafemur moderately thickened, with less developed apicoventral triangular lamina, more or less denatate, the apical dens more or less distinct (Fig. 6E). Pile on legs predominantly yellowish. abdomen (Fig. 5E). Black, covered with yellow to whitish pile on lateral sides. Tergum 1 covered with short whitish to yellow pile; terga 2 and 3 medially with shorter black pile, except two medial fasciae with whitish pile and some whitish pile at posterior margin; anterior half of tergum 4 covered with short black pile, posterior half with longer yellowish pilosity, medially with two oblique fasciae covered with whitish pile. Sterna dark brown, covered with yellowish pile.

Diagnosis
Large (

Redescription
Original descriptions are insufficient, without any illustrations and based on a single female from South Africa (Curran 1939) and male and female types (Hull 1944).   Length: large species, body 10-12 mm, wing 8-9 mm (n = 5).
THorax (Figs 6b, 10b). Mesonotum black, except postpronotum and posterior margin of scutellum brown-reddish, covered with reddish-yellow pile; scutum with five pollinose longitudinal vittae (as on Fig. 10B). Pleuron black to dark brown, covered with gray pollinosity and the following parts with whitish to yellow pile: anterior part of proepimeron, posterior part of anterior anepisternum, most of the posterior anepisternum except anterior end, antero-ventral and postero-dorsal part of katepisternum, anepimeron, and metasternum. Wing hyaline, with dense microtrichia and yellow to brown veins. Calypter pale yellow. Halter with yellow pedicel and capitulum. All three femora dark brown to black; tibiae mostly reddish, medially black to dark brown; tarsi reddish ( Fig. 6B). Metatrochanter without calcar. Metafemur moderately thickened, with less developed apicoventral triangular lamina, more or less dentate, the apical dens is distinct (Fig. 6B). Pile on legs predominantly whitish to yellow. abdomen (Fig. 5B). Black to dark brown. Tergum 1 black, usually pollinosed, covered with short whitish to yellow pile; tergum 2 with pollinose posterior margin and narrow medial fascia (in some specimens interrupted in the middle) covered with whitish pile except posterior half of tergum (between pollinose fasciae) covered with short black pilosity; terga 3 and 4 with pollinose posterior margin and broad medial pollinose fasciae (Fig. 5B); anterior half of tergum 3 covered with short black pile; tergum 4 covered with long golden-yellow pilosity (Fig. 5B). Sterna reddish-brown, covered with white or yellow pile. Female (Figs 10B, 14B, 15B, 16B) Similar to the male except for normal sexual dimorphism: metafemur usually with small apical dens on triangular lamina (Fig. 16B); frons with non pollinose medial vitta (Fig. 15B); vertex predominantly covered with black pile. Hull (1944) described Merodon apimima "related to multifasciatus Curran in type of femoral armament and abdominal fascia; distinct in the black front, yellow pilose vertex and, brown humeri, absence of red on second abdominal segment, beside other details of pattern" based on six males and one female. All these characters mentioned in original description can be regarded as intraspecific variability of M. multifasciatus. Based on our study of the type material of both species, we regard M. apimima as a junior synonym of M. multifasciatus. In the original description of species, Hull (1944) did not state the etymology of the species name, so we cannot conclude whether he treated "apimima" as a noun or an adjective. According to the International Code of Zoological Nomenclature (Article 31.2.2) (International Commission on Zoological Nomenclature 1999) if the author does not indicate whether the species name is a noun or an adjective, as in this case, it has to be treated as a noun. When the species name is a noun simple or compound, in apposition, it does not need to agree in gender with the generic name, therefore the original spelling has to be retained (Article 34.2.1). In some references (Smith & Vockeroth 1980;Dirickx 1988) M. apimimus is used because authors considered "apimima" to be an adjective. But in Systema Dipterorum (Evenhuis & Pape 2020), which is the most comprehensive database related to the Syrphidae, the original spelling of M. apimima is applied. (Fig. 1) This species occurs in Malawi, the Republic of South Africa and the Democratic Republic of Congo. On the basis of our data, the flight period was found to be throughout the whole year. Merodon multifasciatus was found in deciduous forest-woodland savanna and brush-grass savanna.

Larval biology
The material from Nylstroom (17 Feb. 1908) was bred from bulbs of Gladiolus (immature stages undescribed). Specimens collected in Emgwenya (Waterval Boven) (6 Apr. 2018) were found on flowers of different plant species inside a small spot (20 × 40 m) with the natural population of Gladiolus sericeovillosus Hook., along a small periodical spring. It is a strong indication that this plant is the primary host for larvae of M. multifasciatus.

Diagnosis
Small (6-7 mm), black species with narrow, white pollinose, tergal fasciae. Oral margin distinctly protruded (Fig. 9F); face and frons without pollinosity, except narrow vitta along eye margins; terga 2-4 with medial pollinose fasciae (Figs 5F, 14E); tergum 4 covered with whitish to yellowish pile; male genitalia with S-shaped posterior lobe of surstyle, gradually narrowing toward tip (Fig. 19D: pl), anterior margin of surstyle rounded in lateral view (Fig. 19D: (Bezzi 1915). In the same work, he presented diagnostic characters for, and notes on, the third Ethiopian species, M. nasicus, referring to 1914 as the year of its description (Bezzi 1915: 102), based on material from Eritrea in the collections of the Hungarian Museum. Bezzi (1915) also stated that there was a single female specimen of this species from Njoro, in British East Africa. Subsequently, in the Catalogue of the Diptera of the Afrotropical Region (Smith & Vockeroth 1980), and in the Diptera Database (Evenhuis & Pape 2020), this Njoro specimen was erroneously considered as the holotype. Shortly after his 1915 monograph, Bezzi published a description of Lampetia (Merodon) nasica (Bezzi 1921), based on a male and female from Eritrea in the collection of the Hungarian Museum. This is actually the unpublished description from 1914 (Bezzi 1915). However, based on International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature 1999), the oldest available name for a taxon is its valid name, therefore the publication with the first appearance of the name of the taxon (Bezzi 1915) should be considered as valid. Following from this conclusion, in addition to specimens from Eritrea, cited in both publications (Bezzi 1915(Bezzi , 1921, one female specimen from Kenya (Bezzi 1915), deposited in BMNH, also belongs to the syntype series. The Diptera collection in the Hungarian National Museum in Budapest, including the syntypes from Eritrea, was destroyed by a fire in 1956. However, in order to stabilize nomenclature should additional specimens of the type series be found, we designate the specimen from BMNH as lectotype.

Redescription
Original description without any illustrations was based on single male and female (Bezzi 1921).

Male
Head (Figs 4F, 9F, 11F, 12F). Antenna (Fig. 4F) reddish-brown; pedicel elongated, approximately as long as basoflagellomere (relation scape : pedicel : basoflagellomere = 1.0 : 3.0 : 3.0); basoflagellomere concave dorsally, with acute apex; arista reddish-brown, thickened basally, 1.3 times as long as basoflagellomere. Face black, without pollinosity, except along eye margins and covered with whitish to yellowish pile, except on bare medial vitta that occupies ½ width of face. Oral margin strongly protruded (Fig. 9F). Frons black, without gray pollinosity, except eye margins, covered with whitish to yellowish pile. Vertical triangle isosceles (Fig. 12F), usually dark brown, predominantly covered with a long, yellow pile. Ocellar triangle equilateral. Eye pile black in upper half and gray in lower, as long as scape. Eye contiguity about 15-18 ommatidia long. Occiput black, partly pollinose, covered with yellowish to whitish pile. THorax (Fig. 6F). Mesonotum dark brown, covered with light yellowish to yellowish pile; scutum without or with indistinct pollinose longitudinal vittae; postpronotum and anterior margin of scutum with sparse gray pollinosity. Pleuron dark brown, covered with gray pollinosity and the following parts with whitish to yellow pile: anterior part of proepimeron, posterior part of anterior anepisternum, most of the posterior anepisternum except anterior end, antero-ventral and postero-dorsal part of katepisternum, anepimeron, metasternum. Wing hyaline, mostly covered with microtrichia; veins yellow to brown. Calypter pale yellow. Halter with yellow pedicel and capitulum. All three femora dark brown to black, except reddish apex; tibiae mostly reddish, medially dark brown; basal three tarsomeres yellowish to reddish, the rest blackish (Fig. 6F). Metatrochanter without calcar. Metafemur moderately thickened, with less developed apicoventral triangular lamina, more or less dentate, the apical dens is distinct (Fig. 6F). Pile on legs predominantly whitish to yellow. abdomen (Fig. 5F). Dark brown with yellow to whitish-yellow pile on lateral margins. Tergum 1 usually with pollinosity at least laterally, covered with short pile; tergum 2 with narrow medial fascia interrupted in the middle, covered with whitish pile; tergum 2 medially mostly covered with short black pilosity; terga 3 and 4 with or without narrow pollinose posterior margin and distinct medial fascia interrupted in the middle; tergum 3 covered with short black pile medially; tergum 4 covered with longer whitish to yellowish pilosity, except few black pile in anterior half. Brown sterna pollinose, covered with white or yellow pile.
Female (Figs 14E,15E,16E,20) Similar to the male except for normal sexual dimorphism: metafemur usually with very small apical dens on triangular lamina (Figs 16E, 20B); frons with non pollinose medial vitta (Fig. 15E); vertex shiny, non pollinose, mostly covered with black pile.   Bezzi, 1915, lectotype (BMNH NHMUK010369943). Scale bar = 2 mm. (Fig. 1) Merodon nasicus is known from Kenya, Ethiopia, Eritrea and Burundi. On the basis of our data, the flight period was found to be in March and from September to October. This species occurs only on high mountains at elevation between 2000 and 2500 m a.s.l. in vegetation of montane forest-tundra (Sayre et al. 2013).

Etymology
The specific epithet 'vittatus' 'wearing or carrying a vitta', an adjective, refering to the broad golden gray fasciae on the terga 2-4.
THorax (Figs 6C, 8b, 10C). Mesonotum black, except lateral side of scutum including postpronotum, postalar callus, and posterior margin of scutellum red-brown; covered with grayish-yellow pile; scutum with five pollinose longitudinal vittae (Fig. 10C). Pleuron black to dark brown, covered with gray pollinosity and the following parts with whitish to yellow pile: anterior part of proepimeron, posterior part of anterior anepisternum, most of the posterior anepisternum except anterior end, antero-ventral and postero-dorsal part of katepisternum and anepimeron. Wing hyaline, with dense microtrichia and yellow to brown veins. Calypter pale yellow. Halter with pale yellow pedicel and capitulum. All three femora dark brown; tibiae mostly reddish, medially dark brown; tarsi reddish (Figs 6C, 8B). Metatrochanter without calcar. Metafemur moderately thickened, with less developed apicoventral triangular lamina, more or less dentate, the apical dens is distinct (Figs 6C, 8B). Pile on legs predominantly whitish to yellow. abdomen (Fig. 5C). Black to dark brown. Tergum 1 with gray pollinosity, covered with black basal sockets of pile and with short pale yellow pile; tergum 2 with pollinose posterior margin and narrow medial fascia, interrupted in the middle, covered with pale yellow pile; terga 3 and 4 with pollinose posterior margin and broad medial fasciae (Fig. 5C); tergum 3 covered with pale yellow pile except anterior half with few short black pile; tergum 4 covered with long golden-yellow pilosity (Fig. 5C). Sterna reddish-brown, covered with pale yellow pile.

Etymology
The specific epithet 'zebra' was given as a reference to the African equine, zebra with a distinctive black and white striped coat, pattern like in this Merodon species. It is to be treated as a noun in apposition. The name 'zebra' was proposed by W. Hurkmans in an unpublished manuscript, but unfortunately the designated holotype belongs to M. multifasciatus. We decided to keep this descriptive name for this African endemic.
THorax (Figs 6d, 10d). Mesonotum black, except postpronotum and posterior margin of scutellum brown-reddish, covered with reddish-yellow pile; scutum with five pollinose longitudinal vittae (as on Fig. 10D). Pleuron black to dark brown, covered with gray pollinosity and the following parts with whitish to yellow pile: anterior part of proepimeron, posterior part of anterior anepisternum, most of the posterior anepisternum except anterior end, antero-ventral and postero-dorsal part of katepisternum, anepimeron and metasternum. Wing hyaline, with dense microtrichia and yellow to brown veins. Calypter pale yellow. Halter with yellow pedicel and capitulum. All three femora dark brown to black; tibiae mostly reddish, medially black to dark brown; tarsi reddish, except darker apical two tarsomeres (Fig. 6D). Metatrochanter without calcar. Metafemur moderately thickened, with less developed apicoventral triangular lamina, more or less dentate, the apical dens is distinct (Fig. 6D). Pile on legs predominantly whitish to yellow. abdomen (Fig. 5D). Black to dark brown. Tergum 1 black, usually with pollinosity, covered with short whitish to yellow pile; tergum 2 with pollinose posterior margin and narrow medial fascia, in some specimens interrupted in the middle, with whitish pile except posterior half between pollinose fasciae covered with short black pilosity; terga 3 and 4 with pollinose posterior margin and broad medial fasciae; anterior half of tergum 3 covered with short black pile; tergum 4 covered with long golden yellow pilosity. Sterna reddish-brown, covered with white or yellow pile.

Names of Merodon in the Afrotropical fauna with unresolved status
Merodon edentulus Macquart, 1855 nom. dub., stat. rev. Macquart, 1855: 90. This name was introduced by Macquart (1855) based on a single specimen from South Africa. The type material is lost. Curran (1939) stated that this can scarcely be a Merodon because it is said to   have no thorn (lamina) on the metafemur. Type locality is Cape of Good Hope (RSA) from where no specimen of Merodon was collected. The description is insufficient, even for an identification to the genus level. In the same publication, Macquart (1855) described one other exotic Merodon from South America, Merodon angustiventris Macquart, 1855 which was re-identified as a member of the genus Quichuana Knab, 1913(Thompson 1988. Therefore, we regard the name Merodon edentulus to be a nomen dubium.

Variability of species in the Merodon bombiformis group
A variability in shape of the triangular lamina of the metafemur is present in most of the species of the Merodon bombiformis group, as seen in figures of M. bombiformis (Fig. 21A-D), M. lotus sp. nov. (Fig. 22A-C) M. multifasciatus (Fig. 21F-H), M. nasicus (Fig. 23A-B) and M. zebra sp. nov. (Fig. 22D-F). The triangular lamina is more or less dentate, and the apical dens is distinct (M.

Key to the species of the Merodon bombiformis species group
Female of M. vittatus sp. nov. is unknown.
1. Species with reddish-yellow/golden face, frons (Figs 9A, 15A), lateral sides of scutum (Figs 10A, 14A) and most of terga (Figs 5A, 14A); oral margin reduced (Fig. 9A); antennal segments very short (Fig. 4A) Bezzi, 1915 Collecting effort In the period 2011-2018, eight expeditions to RSA were organized with 235 field working days. During these days, 187 specimens of Merodon were collected, among them 11 specimens belonging to the M. bombiformis group, identified as three species. A single specimen of M. bombiformis was collected on field day 108 (in 2016) on the Drakensberg Mountains (Fig. 24). Subsequently, we focused on two other taxa of the M. bombiformis group, previously detected in RSA. Namely, Merodon multifasciatus was recorded on day 215 (in 2018) in the province Mpumalanga, near Emgwenya (Waterval Boven) (Fig. 25). A third species was known from a single specimen deposited in the Natural History Museum, London (previously unpublished), with Ceres as locality on the label. We revisited the Ceres area 18 times in the period 2015-2018 and found a population of M. lotus sp. nov. on day 230 (in 2018) of our fieldwork (Fig. 26).

Molecular analysis
The specimens of the three collected The ML phylogenetic analysis of the concatenated dataset (1858 bp long) consisting of one mitochondrial gene (COI) and two nuclear ribosomal genes (28S and 18S) produced the tree presented in Fig. 27 (100). Merodon bombiformis represents a separate branch within the aureus lineage, but this placement on the tree has low support (bootstrap value <50).

Discussion
The majority of species of Merodon are present in Southern Europe and Asia Minor, which coincides with the high diversity of geophytes (their larval host plants) in these regions. Out of a total of ca 160 species of Merodon, based on recent literature and the present work, only 16 are known from the Afrotropical region. Of the five monophyletic lineages of Merodon (Vujić et al. 2019a), only two, aureus and desuturinus, have representatives in the Afrotropical region, as well as in the Palaearctic. The Merodon desuturinus lineage consists of two clearly separate groups of species based on both adult morphological and molecular data: the Palaearctic group with four species ) and the Afrotropical group with 10 ( Radenković et al. 2018;Djan et al. 2020 Merodon bombiformis possesses some distinct apomorphic characters, such as a reduced mouthpart, reddish-yellow face and frons, convex frons, pedicel with strong setulae, basoflagellomere short, rounded, with a medial fossette. Reduced mouthparts (oral margin) covered by microtrichia are also present in the M. planifacies subgroup Djan et al. 2020). During 32 fieldwork days in the Drakensberg Mountains, spread over seven years of research conducted in RSA, members of our team collected 51, and observed twice as many, specimens of the M. planifacies subgroup (Djan et al. 2020).
There were no observed flower visits by any of the adult specimens during all of these fieldwork days. Adults do not visit flowers because they do not feed due to an adult life strategy adapted to fasting, and therefore they do not need well-developed mouthparts (Vujić pers. obs.). We can suppose that the same situation exists in M. bombiformis. We collected 11 specimens from the M. bombiformis group during our fieldwork. Two specimens of M. multifasciatus were recorded on flowers of Nidorella auriculata; all of the eight specimens of M. lotus sp. nov. were on flowers of Coleonema sp. By contrast, a single specimen of M. bombiformis was recorded sitting on the leaves of bushes.

Placement of the Merodon bombiformis group in the Merodon phylogeny
As stated, previous studies on the phylogeny of Merodon resolve five evolutionary lineages: albifrons, aureus, avidus-nigritarsis, desuturinus, and natans (Vujić et al. 2019a). The results of all of these studies were based on analysis of COI and 28S rRNA gene sequences (Mengual et al. 2006;Vujić et al. 2012;Radenković et al. 2018), while a new study on the phylogeny of Merodon based on a combined analysis of molecular and morphological data is currently under preparation (Veselić in prep). In this paper, we present a preliminary phylogeny of Merodon based on combined COI, 28S rRNA and 18S rRNA gene sequences with the main goal to resolve the M. bombiformis group relationship in the aureus lineage.
We confirmed the monophyly of the five Merodon lineages, although the aureus and albifrons lineages both have low bootstrap support (< 50), possibly as a result of a limited number of species per lineage included in the analysis. The M. bombiformis group is resolved within the aureus lineage as paraphyletic.
However, due to a limited number of specimens included in the analysis, our results must be interpreted with caution. Thus, more comprehensive analysis which will include additional specimens to resolve the phylogeny of the M. bombiformis group with more certainty.

Distribution and diversity
The Merodon bombiformis group comprises only six species with an extremely low number of collected specimens. Before we started our research, only 42 specimens of the M. bombiformis group had been detected in all available collections (26 males and 16 females). The initial aim of our research in the Republic of South Africa (RSA) was to collect fresh specimens of Merodon bombiformis for molecular analysis in order to understand the position of this taxon in the phylogenetic tree of the M. aureus lineage. During eight expeditions, together providing a total of 235 field days (2011-2018), 187 specimens of Merodon were collected, only 11 of them belonging to the M. bombiformis group. The other 176 specimens belong to the M. desuturinus lineage, and have partly been reported on in recent publications Djan et al. 2020). We found specimens of all three species of the M. bombiformis group known from RSA and these were used in the molecular analysis presented here.  (Goldblatt et al. 2006). These distributional patterns suggest that diversification of these taxa began when North America was already clearly separated from Eurasia. The absence of the genus Merodon on the American continent can thus be linked to historical events in the Hyacinthaceae and Iridaceae. However, the greatest diversity of Hyacinthaceae and Crocoideae is found in South Africa (Pfosser & Speta 2004;Goldblatt et al. 2006), which is not in congruence with the exceptionally high diversity of Merodon in the Eastern Mediterranean (more than 100 species) as opposed to South Africa (11 species) Djan et al. 2020). Migration of bulb species from southern Africa to northern Africa and Eurasia was possible during the late Early Miocene, giving rise to free exchange of flora and fauna between Africa and Eurasia (Rögl 1998;Pfosser & Speta 2004). Radenković et al. (2018)  ). All of these records originate from localities belonging to the Eastern Afromontane Biodiversity Hotspot, which includes several widely scattered, but biogeographically similar mountain ranges in Eastern Africa (Critical Ecosystem Partnership Fund 1996), with vegetation of montane forest-tundra (Sayre et al. 2013). This chain of localities in East Africa supporting representatives of the M. bombiformis group continues toward the south with three species which alternate and partly overlap in their ranges. Merodon zebra sp. nov. is recorded from Congo and Tanzania  All habitats in RSA where we collected specimens from the Merodon bombiformis group are either outside protected areas, or in protected areas with high pressure of human impact, or at sites both unprotected and heavily impacted by human activity. Merodon bombiformis was found on a small patch of natural habitat (0.7 ha) on the slopes of the Drakensberg Mountains far from protected areas, on the east and north of the mountain range. Merodon multifasciatus was recorded on the edge of the city (Emgwenya (Waterwal Boven)) with high human impact and a very small remaining population of the assumed host plant. This situation is similar to that of the habitat of M. lotus sp. nov. (0.5 ha), which belongs to Ceres Bergfynbos Reserve (BIOPAMA 2011), but subject to everyday human influence, being on the edge of city infrastructure. Despite our extremely intense collecting efforts in more than 50 localities in the RSA, most of them in protected areas, all specimens found were outside well-preserved National Parks and Reserves. Conservation efforts to protect these small populations of the extremely rare species of the M. bombiformis group thus need to be concentrated on the remaining small patches of natural habitats supporting their host plants. Specimens of the genus Gladiolus are clearly visible during the flowering season and very often collected by the local communities.

Host plants
The data gathered from both the literature (Stuckenberg 1956)