Taxonomic revision of the Afrotropical genus Megatrigon Johnson, 1898 (Diptera: Syrphidae)

The genus-group taxon Megatrigon Johnson, 1898, stat. nov., is revised and treated as a valid genus within the Merodontini (= Eumerini). Extensive diagnoses are given for the genus and for its three constituent species groups: argenteus group [11 spp.], nivalis group [monotypic], sexfasciatus group [3 spp.]. Five new generic combinations are proposed within Megatrigon : M. argenteus (Walker, 1852) comb. nov., M. flavimarginatus (Hull, 1964) comb. nov., M. jacobi (Herve-Bazin, 1913) comb. nov., M. nivalis (Hull, 1964) comb. nov. and M. ochreatus (Hull, 1964) comb. nov. All species of the argenteus group are revised and nine new species are described: Megatrigon apiformis sp. nov., M. argentifrons sp. nov., M. argentimaculatus sp. nov., M. cooksoni sp. nov., M. immaculatus sp. nov., M. magnicornis sp. nov., M. natalensis sp. nov., M. sexmaculatus sp. nov., M. tabanoides sp. nov. Within the sexfasciatus group, M. jacobi (Herve-Bazin, 1913) comb. nov. is treated as a senior synonym of Eumerus connexus Hull, 1964 syn. nov., but no further work is done at the species level due to insufficient material.


Introduction
The hover fly genus Eumerus Meigen, 1822 contains some 300 species (Pape & Thompson 2010), that are generally medium-sized flies, with a distinctive habitus showing strong metalegs and a rounded or broadly oval abdomen.Larvae of all species, when known, are associated with various bulbs and tubers, although they may also develop in fruits and damaged or decaying parts of succulent plants (Ricarte et al. 2008) (the record of E. superbus Shannon, 1927 from a cycad cone ("reared from a Macrozamia cone, shipped from australia"; Shannon 1927: 84) needs further study).Larvae are usually saprophagous although some appear to be at least partly phytophagous (Rotheray & Gilbert 2011), and a few species are agricultural and horticultural pests (e.g., Ricarte et al. 2008).adults are known as flower visitors and may be expected to be significant pollinators (Ssymank et al. 2008;Speight 2014). in a cladistic analysis of the tribe Merodontini edwards, 1915(= eumerini Smirnov, 1924), doczkal & Pape (2009) provided strong evidence that Eumerus is paraphyletic with regard to a wellsupported 'Merodon clade' containing the genera Merodon Meigen, 1803, Platynochaetus Wiedemann, 1830, Lyneborgimyia doczkal & Pape, 2009 and Azpeytia Walker, 1865.Unexpectedly, two species, Eumerus argenteus Walker, 1852 and E. sexfasciatus (Johnson, 1898), emerged in their analysis as sister to the Merodon clade and this clade was given the informal name argenteus group.one of the included species, sexfasciatus, is the type species of the genus-group name Megatrigon Johnson, 1898, which has, however, not been recognized as valid by subsequent authors, and all species described from this group have been listed under Eumerus in recent catalogues and databases (Smith & vockeroth 1980;Pape & Thompson 2010).at present, no fully resolved phylogenetic tree for the world fauna of Eumerus (s.l.) is available, but certain clades are already well supported, and we need names (formal or informal) for proper communication about them.The argenteus group sensu doczkal & Pape is very well characterised by several distinct autapomorphies (doczkal & Pape 2009), which makes this group one of the most strongly corroborated clades of the tribe Merodontini.We here initiate a splitting of Eumerus by re-instating and redefining the nominal genus Megatrigon Johnson, 1898.diagnoses of the genus, of the three included species groups -argenteus group [11 spp.], nivalis group [monotypic], sexfasciatus group [3 spp.] -and of all species of the first two groups are given.The sexfasciatus group is left untreated at the species level due to insufficient material.

Material and methods
only pinned, dry specimens were available, and when dissections were necessary the specimens were softened in a humidity chamber to allow for separation of the terminal segments with fine forceps and soaking the terminalia in 10% koH overnight at room temperature.after washing in water, acetic acid and again in water, male terminalia were transferred to glycerol for examination under a dissecting microscope at appropriate magnification.The terminalia were subsequently stored in a glass or plastic microvial pinned with the source specimen.
in order to facilitate the future recognition of undescribed species, all species treated are described employing the full set of taxonomically useful characters we have observed.

SaMC
= South african Museum (as part of the iziko Museums), Cape Town, South africa UMo (oUMnH) = oxford University Museum of natural History, oxford University, oxford, United kingdom coll.Bkm = private collection of Werner Barkemeyer coll.M. Hauser = private collection of Martin Hauser (eventually to be deposited in CSCa) a large part of the available specimens of Megatrigon is in more or less bad condition, i.e., damaged or dirty, and many have faded colours due to age and/or improper storage.Thus, the colours described here could be darker in fresh specimens.The often bad condition and the very low number of specimens (sometimes singletons only) of almost all species usually did not allow for cleaning or optimal preparation before taking photographs.For the preparation of the diagnoses of Megatrigon and the included species groups, all available specimens have been used (i.e., including the specimens of the sexfasciatus group, which are not treated at the species level in the present account).identification of females is still unresolved, except for the distinct female of M. nivalis (Hull, 1964) comb. nov., but we have in a few cases listed females as paratypes or as additional material, although species descriptions are based on males only (with the noted exception of M. nivalis).

Terminology
Morphological terminology follows mainly Mcalpine (1981) for the non-genitalic morphology, Stuckenberg (1999) for the antenna, and Sinclair (2000) for the structures of the male terminalia, with terms for characters not treated there adopted from Speight (1987), Hippa & Ståhls (2005)

Remarks
The holotype is without the posteroventral patch of setae on katepisternum.This is quite unusual among Megatrigon and all other Merodontini (currently only known from Lyneborgimyia magnifica doczkal & Pape, 2009) and probably this is an individual aberration (the number of posteroventral katepisternal setae is very low -usually less than 10 -in most Megatrigon, lower than usually seen in the Merodontini).in all other characters this specimen fits the diagnosis of Megatrigon given below.The male of Megatrigon sexfasciatus is unknown (probably not present in the available material).
Wing (Fig. 5).Cross-vein sc-r well developed, C ending far before apex of wing, R 4+5 moderately sinuous (as in Eumerus tricolor), M 1 without stump veins, dm-cu almost perpendicular to Cua 1 , usually with stump vein, Cua 1 slightly more than 2 x as long as dm-cu, bm-cu about as long as basal part of Cua 1 or slightly longer, Cua about 3 x as long as Cua 2 , a 1 +Cua 2 smoothly curved, Cup short and ending at about half length of Cua, wing broad, about 2.35 × as long as wide, wing margin with undulated membrane, costagium with long setae at posterior margin (except in Megatrigon nivalis), marginal fringe of upper calypter 'glued ' (cf. fig. 32i-k in Hippa & Ståhls 2005).
Legs.Procoxa short and stout, mesocoxa posteriorly usually bare (except for M. jacobi comb.nov.); metafemur shiny, thickened and curved dorsally, about 2.5 × as long as deep, light yellow setae on metafemur exceed relatively large apical flange, usually with shiny, bare, longitudinal area ventrally; ventral face of metafemur slightly convex or straight, with a low anteroventral preapical flange bearing weak bristles on strong finger-like outgrowths, and with a few weak bristles posteroventrally preapically (Fig. 6C); metatibia with a sharp bare (a few isolated setae may occur) anteroventral ridge in proximal half, protibia with irregularly spaced anterolateral apical setae; protarsus short and stout, tarsomeres 2-4 as wide as long; claws enlarged.abdomen.Tergites strongly curved ventrally at margin; anterior corners of tergite 2 inflated, border between tergites 1 and 2 not crenulated; lateral margins of tergites microtrichiose; ♂ sternite 4 without projections, with incised posterior margin and a fine desclerotised median line, lateral margins simple, sternite 6 bare, sternite 7 with a basal bump.anterior surstyle lobe absent, posterior surstyle lobe narrow (Fig. 13a, e:y), subepandrial sclerite microtrichiose (Fig. 13C-d, G:z).Setae on various part of the body serrated (Fig. 9B), except in M. nivalis comb.nov.The included species can be arranged into three well defined species groups, each defined by several autapomorphic character states.

Diagnosis
katepisternum posteriorly setose on full width (♂) or with a small gap above the ventral pile patch (♀).anepisternal setae thick, glistening, without serration.Base of wing extensively bare of microtrichia, including parts of cells c, r 1 , br, bm, cup, and narrow stripes along the longitudinal veins.The bare flat posterior margin of tergite 1 extensively microtrichiose.♂: Sternite 4 with flat caudal lobes, entirely microtrichiose.Genital pouch about half as wide as abdominal segment 4 at anterior margin.Male genitalia (Figs 10M, 11M-n, 12L): posterior surstyle lobe directed caudally, fused with median surstyle lobe for most of its length, with a sharp ridge at the border with median lobe, apex of posterior lobe with a small hook.Median surstyle lobe setose on about the posterior 0.67 of outer and ventral surface, on median surface covered by microtrichia on most part and with a sharp longitudinal ridge near dorsal border on about posterior half.Cerci with blunt apex, ventral surface membraneous and without setae.Subepandrial sclerite strongly curved (in lateral view), and its anterior end sclerotized at the sides only, sclerotized but translucent in the middle, the posterior part microtrichiose on nearly full length, without trichiose pillow-like structure.Hypandrium with a moderately wide base in ventral view and narrow in lateral view, with a transverse bulge immediately anterior to the attachment of the phallus, without 'shoulders' between wide base and narrower distal half, apex slightly produced beyond ctenidium and blunt.ejaculatory apodeme small.♀: tergite 5 (Fig. 8a, B:x) and sternite 5 with anteriorly directed setae, ovipositor with thick blunt setae and a dorsal subapical tooth (Fig. 8C:y).

Diagnosis
See diagnosis of nivalis group.

Type material
SoUTH aFRiCa: ♂, holotype, northern Cape, Belmont, 23 Feb. 1934, J. ogilvie (BMnH).[examined.]The holotype is in bad condition, with both metalegs and right mesoleg missing (all from femur on), head glued to the thorax, left mesoleg soaked in an unknown substance.
Wing.Costagium with yellow setae, posterior setae short (about as long as diameter of costa distal to costagium).Cells c, r 1 , bm, cup and anal lobe with large areas bare of microtrichia, as well as narrow bare stripes along parts of the veins and at the posterior margin of the wing including alula, veins yellowish brown at base, darker brown towards apex.abdomen (Fig. 8).Conical [similar to Merodon avidus (Rossi, 1790)], lateral margins of tergite 2 convergent posteriorly, tergite 3 is 2 × as wide as long, tergite 2 with a median fascia, tergites 3+4 each microtrichiose anteriorly and with a median fascia, tergite 4 with microtrichiose posterior margin.Tergite  (Hull, 1964) comb. nov. (♂).
2 posteromedially (posterior to fascia) with a large triangular black setose area, most part of tergite 3 (except for fascia and lateral margins) black setose, tergite 4 with small black setose area between anterior and median fascia, otherwise tergites with whitish pilosity.except for the lateral margin setae are very short, those on posterior margin of tergite 2 not distinctly exceeding margin of tergite, tergites pitted (alveoli).Genital pouch wider than half width of tergite 4 anteriorly.Sternite 1 large.Sternites 2-4 with semi-decumbent, comparatively short, white setae.

Distribution
afrotropical -namibia, South africa (Fig. 16).♂: sternite 4 with flat caudal lobes, entirely microtrichiose.Genital pouch less than half as wide as abdominal segment 4 at anterior margin.Posterior surstyle lobe directed medially, its apex forming a strong twisted hook (Fig. 13a:y).Median surstyle lobe setose along ventral margin on full length, on median surface extensively covered by microtrichia and posteriorly with a small inner accessory lobe pointing medially (Fig. 13d:x).Cerci (Fig. 10:z) with blunt apex, ventral surface membraneous and without setae.Subepandrial sclerite strongly curved (in lateral view, Fig. 10:w), and its anterior end sclerotized at the sides only, sclerotized but translucent in the middle, posterior part with a strongly trichiose pillow-like structure, which is densely furnished by unusually long and thick microtrichia (Fig. 13C, d:z), otherwise bare.Hypandrium (Fig. 12:a-J) with a moderately wide base (in ventral view), without 'shoulders' between wide base and narrower distal half, apex distinctly produced beyond ctenidium and pointed.ejaculatory apodeme small.The male terminalia of all included species are very similar (Figs 10-12), the significance of the observed small differences between species needs to be proved by more material.♀: tergite 5 and sternite 5 with posteriorly directed setae, ovipositor with moderately thick setae, without tooth.
The argenteus group can be subdivided into two subgroups, at least the first being monophyletic.a full description is given for M. argenteus comb.nov., which is the only fairly common species of this group (all others being rare or known from singletons only).The descriptions of the remaining species focus on the differences to M. argenteus.

Head (Figs 2a, 3a
). ellipsoidal in frontal view (about 1.3 × as wide as deep); in lateral view about 1.5 × as deep as long; dichoptic eyes, distance between eyes relatively small, 0.13-0.14× width of head; cuticle of head capsule brown-metallic bronze, shiny; face covered with long white setae; shiny, Fig. 8. Megatrigon nivalis (Hull, 1964)  except narrow, short microtrichiose stripes along eye margins (missing in lower part), and median facial stripe wide below antennae (in some specimens connected with microtrichia along eye margin) and then tapering towards mouth edge; the width of face 0.37-0.38 × width of head; frons with semi-erect yellow setae, cuticle well visible, shiny; sometimes with reduced stripes of microtrichia near the eye margins; vertex covered with long, dark yellow setae (sometimes dark brown); shiny in the central part, but microtrichiose anteriorly (around front ocellus), laterally, along the eye margin, postocellar microtrichiose spots relatively small, separated by the distance between posterior ocelli; the width of vertical triangle 0.22-0.25 × width of head; microtrichia on post-ocular orbit dense, dorsally narrowly exceed postocular ridge and connected with postocellar microtrichiose spots, postocular orbit dorsally only little wider than at its narrowest part, 0.08 × width of head; ocellar triangle almost isosceles; with median longitudinal groove; antenna (Fig. 3a) brown, sometimes pedicel paler (orange brown) than postpedicel or end of pedicel and beginning of pospedicel paler (orange brown); setae on scape and pedicel predominantly white, except brown setae on dorsal part; postpedicel ellipsoidal, about 1.5 × as long as deep; 1.7-1.8× as long as pedicel.
THorax.Scutum black, laterally with golden lustre, anteriorly on median part dark blue, covered with moderately long, mostly relatively dense, (semi) erect yellow hairs, sparse on the blue anteromedian area; two lateral white microtrichiose stripes reaching the posterior end of scutum, two submedian microtrichiose stripes extending close after the transverse suture (occassionally almost absent) and more or less visible microtrichia on transverse suture, some specimens with a little trace of median microtrichiose stripe.Supra-alar setae usually black, inserted on enlarged tuberculate alveoli.Proepimeron covered with dense microtrichia and short, sparse, yellow setae.anepisternum, anterior anepimeron, posterodorsal and ventral part of katepisternum, metasternum covered with long, yellow hairs.Microtrichia reduced or missed on median part of anepimeron, and a large spot on katepimeron (barrette).Scutellum black, with yellow rim; setosity of long yellow setae.Mediotergite mostly shiny, only median 0.25-0.33 of width sparsely covered with microtrichia.Wing.Membrane slightly brown infuscated, with brown veins and dark, dense microtrichia; at least proximal half of cell br largely without microtrichia; setae on costagium black; halter with yellow pedicel and capitulum.
Legs.Brown, except light brown-orange on following parts: trochanters (more or less), proximal and distal part of pro-and mesofemur, large part of pro-and mesotibia (except weak submedian dark ring), proximal half and apex of metatibia, pro-(sometimes tarsomere 1 darkened dorsally) and mesotarsus and ventral surface of metatarsus (sometimes also tarsomeres 2-5).Hairs on legs predominately light yellow, long, except some short black setae on anterior and dorsal part of pro-and mesofemur, darkened setae on antero-ventral part of mesotibia and tarsi dorsally (especially metatarsus).White microtrichia distinct on pro-and mesofemur (very dense posteriorly, more inconspicuous on remaining parts), proand mesotibia dorsally, proximal 0.33 of metatibia dorsally (i.e., reduced compared with other species).
abdomen (Figs 1a,7a,9).Cuticle black, except more or less orange translucent areas on tergites 2+3 covered by dense silvery reflecting microtrichia ('silver spots'); broad (tergite 2 is 1.9-2.4,tergite 3 is 2.2-3.5 × as wide as long); silver spots on tergites 2+3 large, almost reaching the corners, tapering towards middle of tergite; distance between silver spots on tergite 2 relatively large, 0.2-0.25 × width of tergite; in tergite 3 small, 0.11 × width of tergite; tergite 4 usually with small white non-reflecting microtrichiose spots in median part; tergites light yellow setose, except large area of silver spots without setae, and short black setae on tergite 2 posteriorly, black cuticle of tergite 3, anteromedially and below microtrichiose spots on tergite 4 mixed with yellow setae.Sternites brown, densly microtrichiose, covered with long light yellow hairs of more or less even length.maLe geniTaLia (Figs 10a,11a,12a,.Cerci relatively large, shaped as one quarter of a circle.Median surstyle covered with relatively narrow, longitudinal stripe of microtrichia interiorly; deeply hollowed posteriorly towards posterior surstyle; with inner, acute accessory lobe; ventral margin more or less straight.Posterior surstyle with hooked process recurved in the direction of median surstyle and at the base with external, long setose, bump.Base of epandrium as high as long.Hypandrium sickleshaped, with rather wide distal part, tapering towards apex; base wide, without deep groove. PuParium (Fig. 14; n=6).integument rough, dull brown.dorsally, two pupal spiracles (Fig. 14a:x) project from the middle of the operculum, widely separated by a distance of 4 × their length.These processes are conical structures, ≈ 0,04 mm long, pointed apically and with numerous disc-shaped tubercles, distributed irregularly on almost the entire surface (Fig. 14B:y), except the apex (Fig. 14C).each tubercle has 4-8 spiracular openings radially arranged (Fig. 14d).Surface between tubercles and on apex of process rough, reticulate, divided in hexagonal areas.Posterior respiratory process (Fig. 14e-F) with total length ≈ 0.23 mm, length between transverse ridge and centre of spiracular plate (a): ≈ 0.30 mm, width at level of the ridge (b) ≈ 0.45 mm, a/b= 0.67; short and broad (like in Merodon), wider than long, strongly ridged in proximal half (spiracular plate (Fig. 14e) not studied due to poor condition of specimens, but seems to have four pairs of spiracular openings like in Merodon, not three pairs like in Eumerus).Cephaloskeleton as in Fig. 4G.

Diagnosis
Frons covered with very dense, long, silvery-white setae and microtrichia; on vertex microtrichia present anteriorly, along lateral sides of ocellar triangle and as small, but distinct postocellar spots (Fig. 2B).

Etymology
The species epithet, which should be treated as a noun in apposition, is derived from the Latin words argentum (= silver) and frons (= forehead), alluding to the dense, silvery microtrichiae that completely cover the frons.

Head (Figs 2b, 3b
). Lateral margins of face more parallel while in M. argenteus comb.nov.slightly divergent; distance between eyes larger than in M. argenteus comb.nov., 0.19 × width of head; ocellar triangle eqilateral, without median longitudinal groove; on face microtrichiose stripes along eye margin wide at the level of antennae, but tapering and reduced at lower part, connected with microtrichia below antenna by tiny oblique microtrichiose line; median facial stripe well developed; microtrichia on postocular orbit do not exceed post-ocular ridge.
THorax.Mesoscutum with two lateral microtrichiose stripes and two submedian microtrichiose stripes ending well behind level of transverse suture; supra-alar setae yellow; microtrichia on mediotergite medially occupied more than 0.5 of width.
Wing.Membrane not infuscated, with light brown veins, microtrichia brown; cell br above vena spuria narrowly bare; costagium with yellow setae.

Diagnosis
Microtrichia pattern of head (Fig. 2C): Frons shiny anteriorly, microtrichiose posteriorly; between frons and vertex bare transversal line; vertex covered with microtrichia, except ocellar triangle, small central area behind ocellar triangle, short longitudinal line medio-posteriorly and head edge; microtrichia of post-ocular orbit exceeded post-ocular ridge and joined with postocellar microtrichia.

Etymology
The species epithet, which is formed as an adjective, is derived from the Latin words argentum (= silver) and macula (= spot), alluding to the large silver spots on abdominal tergites 2+3.

Head (Figs 2C, 3C
). Microtrichiose stripes on face along eye margin wide and long, extended till tentorial pit and joined with microtrichia below antenna by oblique microtrichiose stripe; distance between eyes a little larger than in M. argenteus comb.nov., 0.17 × width of head; ocellar triangle isosceles, without median longitudinal groove; colour of postpedicel (Fig. 3C) dictinctly darker than pedicel.
THorax.Mesoscutum with two, wide, lateral microtrichiose stripes and two weak submedian microtrichiose stripes starting as triangular markings and ending as small spots at the level of transverse suture; supra-alar setae yellow; median about 0.50 of width of mediotergite covered by microtrichia.
Wing.Membrane not infuscated, with yellow veins and microtrichia; cell br above vena spuria completely covered with microtrichia; costagium with yellow setae.
Legs. dark submedian ring on pro-and mesotibia weak and visible only ventrally, microtrichia on metatibia distinct in proximal half, dorsally; all tarsi yellow-orange, without shading; all setae of legs light yellow; tarsomere 1 of pro-and metaleg not swollen like in M. argenteus comb.nov.; tarsomere 1 of protarsus anterodorsally without a line of minute black spinules between the yellow trichia.

abdomen (Figs 1, 7C
). Tergites light yellow setose; tergites 2+3 with very large silver spots, covering most of the tergite's width except for a narrow brown goblet-shaped median part; distance between silver spots on tergite 2 smaller, 0.19 × width of tergite; spots on tergite 3 not tapering towards the middle of tergite, but being more rounded; silver spots on tergites 2+3 covered with relatively long, dense, adpressed, yellow setae except antero-lateral corner; tergite 4 with two small indistinct microtrichiose spots near to anterior margin.Male genitalia in Figs 10C, 11C, 12C.

Etymology
The species epithet, which is formed as an adjective, is derived from the Latin words magnus (= large) and cornus (= horn), alluding to the long antennae.

Description
LengTH.Body 10 mm, wing 6.5-7.0 mm.2d, 3d).Upper part of face across the whole width, at the level and below antennae, completely covered with microtrichia, thereby microtrichiose stripes along eye margin and median facial stripe connected at that level; frons and vertex shiny (only few microtrichia in the middle of frons, anterior and lateral to anterior ocella), without postocellar spots or with very few microtrichia; microtrichia on post-ocular orbit do not exceed post-ocular ridge; length of post-ocular orbit large, 0.11-0.13× width of head; distance between eyes large, 0.21-0.23 width of head; ocellar triangle equilateral and without median groove.
Wing.Membrane not infuscated, veins and microtrichia light brown, cell br above vena spuria almost all covered with microtrichia; costagium with yellow setae.
Legs (Fig. 6C).setae on legs all yellow; pro-and mesotibia with indistinct dark submedian ring; all tarsi yellow.1d, 7d).Tergites 2+3 and posterior margin of tergite 4 brown-yellow with black alveoli, setae on tergites short, yellow and in some parts black: posterior 0.25 of tergite 2, submedian and posterior part of tergite 3; tergite 2 medially covered with white microtrichia, tergite 3 with tiny microtrichiose line extended from anterior margin to the centre of tergite, tergite 4 without microtrichiose spots.Male genitalia in Figs 10d, 11d, 12d.Distribution afrotropical -South africa (eastern Cape) (Fig. 15).2F); on face microtrichiose stripes along eye margin and median facial stripe very reduced, present only as a tiny line but on the whole length; setae on vertex more dense and intensively yellow, and on the frons also yellow, not silvery-white like in M. argentifrons sp.nov.(Fig. 2e); microtrichia on mesoscutum reduced to traces of lateral and submedian stripes (Fig. 4B).

Etymology
The species epithet, which is formed as a geographical genitive, is derived from the name of the province in South africa, kwaZulu-natal, from where the type series of this species originate, i.e., the type locality.
THorax (Fig. 4B).Microtrichia on mesoscutum reduced so that only traces remain of the submedian and lateral microtrichiose stripes.
Wing. almost entirely covered by microtrichia, only few microtrichia missing in cell br basally, and above vena spuria.

Diagnosis
Golden-haired species with three pairs of silver spots on tergites 2-4 (Figs 1F, 7F).Silver spots on tergite 3 completely and densely setose, setae directed towards the lateral margin, i.e., turned ca.90° in relation to longitudinal axis.vertex (Fig. 2G) completely covered with dense, yellow-golden microtrichia except narrow bare stripe along the posterior margin.

Etymology
The species epithet, which is formed as an adjective, is derived from the Latin words sex (= six) and macula (= spot), alluding to the three pairs of silver spots on abdominal tergites 2-4.
THorax.Submedian microtrichia present on mesoscutum only anteriorly as triangular markings and as small spots at the level of transverse suture, besides two distinct, long lateral microtrichiose stripes; supraalar setae yellow; microtrichia on mediotergite covered about 0.4 of width.
Legs.Setae on legs all light; of tarsi, only tarsomere 1 and to a lesser extent tarsomere 2 of metaleg dorsally darkened.1F, 7F).Tergites covered with relatively long, dense yellow-golden setae, without black ones; lateral silver spots on tergites 2-4 triangular, relatively large: on tergites 2+3 almost reaching the corners, tapering towards the middle of tergite where they approach each other; spots on tergite 4 anterior, small, widely separated from each other and lateral margin, but touching anterior margin.Male genitalia in Figs 10F, 11F.

Etymology
The species epithet, which is formed as an adjective, is derived from the Latin words apis (= [honey] bee) and forma (= shape), alluding to the honey bee-like appearance of this species.
THorax (Fig. 4C).Microtrichia on mediotergite covered about one third of width.

Diagnosis
Black species with white, connected microtrichiose fasciae on tergites (Figs 1H, 7H) and bicoloured wing resulting from dense microtrichia, white in proximal 0.6 and brown in distal 0.4 (Fig. 5B).Frons shiny; vertex and post-ocular orbit white microtrichiose except area between posterior ocelli, medioposterior part, spots at eye corner and posterior dorsal margin of head (Fig. 2i).

Etymology
The species epithet, which is formed as a male genitive, is derived from the personal name of the collector, Mr. daniel Montague Cookson.

Head (Figs 2i, 3H
). distance between eyes, 0.17 × width of head; width of vertical triangle 0.26-0.29 × width of head; postocular orbit wider, the width of postocular orbit dorsally 0.13 × as wide as head; ocellar triangle equilateral, without median longitudinal groove; microtrichia on face well developed and similar to M. ochreatus comb.nov.: white microtrichiose stripes along eye margin distinct, connected with wide median microtrichiose facial stripe below antennae by narrow oblique stripe.
THorax.Mesoscutum with lateral microtrichiose stripes wide and long, posteriorly connected with scarce prescutellar microtrichia; indistinct submedian stripes ending at conspicuous microtrichiose spots on transverse sutures; traces of median stripe ending near the level of transverse suture; supra-alar setae yellow; mediotergite almost entirely covered by microtrichia.
Legs.only apex of pro-and mesofemur pale, not the base; tarsi yellow, only tarsomere 1 of metaleg brown dorsally; metatibia dorsally with white microtrichia in proximal half that reach the scar.

Diagnosis
dark brown-black species with almost no markings and blue lustre, covered with short white hairs (Fig. 1i); face shiny, apart from white microtrichiose area below antennae, only traces of usual microtrichia pattern present; frons and vertex shiny without microtrichia, both inflated and without border between them, cuticle smooth without indistinct alveoli (Fig. 2J); tergites dark brown without any markings (except in a female tentatively assigned to this species with three pairs of weak, white, oblique microtrichiose fasciae on tergites 2-4) covered with white setae (Fig. 7i); tergite 4 with two small depressions on posterior half.

Etymology
The species epithet, which is formed as an adjective, is derived from the Latin negation prefix in-(= "not"; here in the assimilated form im-), and macula (= spot), alluding to the lack of spots or other distinct markings on the abdominal tergites.

Additional material
SoUTH aFRiCa: 1 ♂ 1 ♀, Western Cape, Uniondale district, Bo kouga, Mar.1954, museum staff leg.(SaMC).These specimens are not designated as paratypes, as they differ in small details from the type of M. immaculatus sp.nov.and for that reason may not be conspecific.More material is needed to explore the significance of the observed differences.
THorax.Mesoscutum with lateral microtrichiose stripes on notopleuron shifted medially (can be reduced), submedian stripes ended at the level of transverse suture; bare area on katepisternum and anterior anepimeron are larger than usual.
Legs. almost completely dark (pro-and mesotarsus slightly darkened dorsally) except pale ends of pro-and mesofemur and tibia and ventral surface of tarsi; legs entirely white setose; metafemur with blue-violet lustre anterodorsally.

Description
LengTH.Body 10.0 mm, wing 7.5 mm.2K, 3J).Face largely microtrichiose: microtrichiose stripes along eye margins long, wide, especially at the level below antennae where joined median facial stripe and narrow oblique stripe; face and gena wider than in M. argenteus comb.nov., gena 1.5 × as wide as postpedicel (in M. argenteus comb.nov.gena and postpedicel of equal width); distance between eyes larger than in M. argenteus comb.nov., 0.19 × width of head; postocular orbit wider than in M. argenteus comb.nov., the width of postocular orbit dorsally 0.13 × width of head; postpedicel longer than in M. argenteus comb.nov., 1.7 × as long as deep; 1.7 × as long as pedicel; ocellar triangle equilateral without median longitudinal groove.
Wing.Membrane irregulary yellow-brown infuscated, densely covered with microtrichia that is mainly yellow-brown, except white microtrichia along the posterior edge and wing base; costagium with yellow setae.
Legs.Setae yellow; all tarsi yellow, metatibia almost entirely dark, with white microtrichia in proximal half dorsally that reach the scar.

Diagnosis
katepisternum with a posterodorsal and a small posteroventral pile patch, with or without a few short setae below the dorsal patch.The bare, flat posterior margin of tergite 1 (almost) entirely without microtrichia.Base of wing entirely microtrichiose or at most with small bare areas in cells br, bm, or cup.anepisternal setae saw-like.Tarsomere 1 of protarsus anterodorsally without black spinules between the yellow setae.♂: Sternite 4 with inflated caudal lobes, partly without microtrichia, and with a small accessory lobe close to the base of the lobes.Genital pouch large, more than half as wide as abdominal segment 4 at anterior margin.Posterior surstyle lobe (Fig. 13e:y, F:y) straight, without hook, directed medially.Median surstyle lobe (Fig. 13e, F:w) setose along posterior half of ventral margin, on median surface without accessory lobe, without microtrichia, its posterior end extended, forming a wide rounded lobe directed posteromedially.Cerci (Fig. 10L:z) with pointed apex, with sclerotized and setose ventral surface.Subepandrial sclerite strongly curved, its anterior part almost parallel to the posterior part, a complete sclerotized bridge at its anterior end, the posterior part microtrichiose on full length, without a bulging 'pad' (Fig. 13G:z).Hypandrium (Figs 11L,12k) with a very wide base and lateral 'shoulders' between the wide proximal and the narrower distal part, apex slightly produced beyond ctenidium and blunt.ejaculatory apodeme large.♀: tergite 5 and sternite 5 with posteriorly directed setae, ovipositor with moderately thick setae, without tooth.
= Eumerus connexus Hull, 1964, syn. nov. (holotype in BMnH, examined).Megatrigon sexfasciatus Johnson, 1898 (holotype in BMnH, examined).due to the lack of sufficient material, the species of the sexfasciatus group are not (re)described in the present account.Megatrigon jacobi comb. nov. (Figs 1L,2M,3L,7L,13e) is a distinctive species readily recognised by the reduced mouthparts and related features of the lower parts of the head (similar to Merodon bombiformis Hull, 1944).The identity of M. sexfasciatus remains obscure.The material that has been available for the present study consists of males of several species that in external characters are very similar to the female holotype of M. sexfasciatus.at present we cannot assign any of the available males to the nominal taxon M. sexfasciatus.There are several 'morphs' similar to the type of M. flavimarginatus comb.nov. in the available material.However, as most of these are represented by singletons, we are unable to assess the significance of the observed, often slight, morphological differences and for that reason we refrain from describing them here.More material in good condition is needed to uncover the species diversity in the sexfasciatus group, but we expect that a good number of distinct species will be discovered.

Key to species of Megatrigon
The currently known species are probably a fraction only of the species actually in existence, and users of the key should expect to encounter species that are not included.The key should be used mainly as a step in the identification process, and proper identification requires comparisons with the diagnoses and descriptions, including a thorough study of the terminalia.as the identification of females is still unresolved and/or the assignment to the corresponding males is uncertain, the key can be used for males only (except for the very distinct female of M. nivalis comb.nov.).

Discussion
Eumerus (s.l.) is arguably the morphologically most diverse genus-group taxon of the family Syrphidae.as currently recognized, it contains about 300 described species (Pape & Thompson 2010), and many more await discovery and description.There is growing evidence that Eumerus (s.l.) is paraphyletic (e.g., doczkal & Pape 2009), which means that Eumerus will have to either be widened to encompass the Merodon genus group with the morphologically rather distinct genera Merodon, Platynochaetus, Lyneborgimyia and Azpeytia, or to be split into a number of genera.Merodon is in itself a speciesrich genus, containing some 160 described species (Pape & Thompson 2010), and dozens of species are awaiting description (a. vujić, pers. comm.).Merodon is well defined and has been in use for considerable time, and transferring its species to Eumerus may seem as nomenclaturally destabilising by going against current usage, and a transfer would moreover create many secondary homonyms.
From a practical point of view, splitting Eumerus (s.l.) into smaller, named units may seem inescapable as a means of efficient communication, and whether these are given informal or scientific names, and in case of the latter whether a rank of genus or subgenus is applied, is largely a matter of personal preferences.as mentioned above, the species here united in the genus-group taxon Megatrigon share several autapomorphies (e.g., lower calypter of wing with long pile dorsally, metafemur with striae, male genitalia with long microtrichia on subepandrial sclerite, well-defined and narrow posterior surstyle lobe, and S shaped hamus of hypandrium), which are so distinct that this arguably is one of the most strongly corroborated clades of the tribe Merodontini, and the rank of genus is here considered as fully justified.
Megatrigon is an endemic afrotropical taxon with its centre of diversity in the dry southern parts of the continent .Most of the recognized species are present in the Cape Region, which is one of the most species-rich centres of bulbous plants. of almost 1200 species of bulbous plants in this region, three-quarters are endemic (Manning et al. 2002).it seems quite probable that species of Megatrigon, like the sapro-phytophagous genera and phylogenetically close relatives Merodon and Eumerus, are associated with geophytes during the larval stage, and the predominantly southern african distribution could be explained by the marked diversity of the potential host plants in this region.all species of Megatrigon currently known are rare in collections, and a considerable number is still known from one or two specimens only, thereby indicating that a considerable number of species are yet to be discovered.