Description of the female of Syrittosyrphus opacea Hull, 1944 (Diptera, Syrphidae, Eristalinae) with additional notes on the genus

The taxonomy of Syrphidae is far from being complete in the Afrotropical Region and many species have been described from a single sex only. One of these is the enigmatic monotypic genus Syrittosyrphus Hull, 1944, of which, so far, only the male of Syrittosyrphus opacea Hull, 1944 was described from the Eastern Cape Province in South Africa. Here, we re-describe the male and describe the female. We summarise all known distribution records from South Africa (Eastern Cape, KwaZulu-Natal and Limpopo Provinces) and Zimbabwe (Vumba), of which several are new. We also provide notes on the species’ ecology.


Introduction
The Afrotropical Region is relatively poor in genera and species of hoverflies or flower flies (Diptera: Syrphidae) (Ssymank et al., in press). Nevertheless, it harbours a number of enigmatic, endemic hoverfly genera. A case in point is the monotypic genus Syrittosyrphus Hull. Syrittosyrphus is a genus within the subfamily Eristalinae and is assumed to be part of the tribe Milesiini although its phylogenetic relationship to other Milesiini remains unknown (see Thompson 1972Thompson , 1974Hippa 1990). Hippa (1990) suggested that the closest relatives include Milesia Latreille, Pterallastes Loew and Palumbia Rondani (Hippa 1990), while Hull (1944) suggested that the genus was related to the genus Korinchia Edwards. Phylogenetic analysis of anchored hybrid enrichment molecular data seems to confirm that the sister group of Syrittosyrphus is Korinchia + (Palumbia + Pterallastes) which do not have species in the Afrotropics (Moran et al., unpublished data).
Recently, a complete key to the genera of the Afrotropical Region was compiled (Ssymank et al. in press). Older keys to the Afrotropical genera, such as Curran (1927) and Vockeroth (1969) were incomplete and none included the genus Syrittosyrphus Hull. This genus is monotypic and endemic to southern Africa and its single species Syrittosyrphus opacea Hull, 1944 was described on a single male from Somerset East (Cape Province, South Africa) and deposited at the Natural History Museum UK (NHMUK, formerly the British Museum of Natural History -BMNH). The description of the species is brief and lacks illustrations. Later, Smith and Vockeroth (1980) and Dirickx (1998) list the holotype in their catalogues. Hull (1949) illustrated S. opacea, while Hippa (1990) illustrated the head, metathoracic spiracle, male hind leg and male and female genitalia. Hippa (1990), in his study on the genus Milesia Latreille, studied material from a variety of museums, including the two holdings which, at that time, had S. opacea in their collections, i.e. NHMUK and the KwaZulu-Natal Museum (KZNM, Pietermaritzburg, South Africa; historic acronym: NMSA). It is unclear whether Hippa illustrated the holotype from NHMUK (the only specimen in the collection) or a male from the KZNM. The female illustrated by Hippa (1990) must be one of the females in the collection at KZNM, but apart from the illustration of the female genitalia, morphological details of the female are lacking. Pictures of the male lateral and dorsal habitus and detail of the scutum, taken by X. Mengual (Zoologisches Forschungsmuseum Alexander Koenig, Bonn, Germany) are provided in http://syrphidae.myspecies.info/taxonomy/term/768. Whittington (1998) studied the D. Cookson collection from Vumba in the Eastern Highlands of Zimbabwe which comprises one male and one female of S. opacea and which are stored at the KZNM. Although the female of S. opacea was obviously known to both Hippa (1990) and Whittington (1998), so far, a description of the female is lacking and the generic diagnosis is based on morphological characters of the male only. Here, we describe, for the first time, the female of S. opacea. Since the female differs in some morphological characters from the male, a description of the female is important to properly diagnose the genus and identify the species. This also allowed us to include the genus in the key to the genera of Afrotropical Syrphidae (Ssymank et al., in press).

Material and methods
Study material was obtained from the following museums and personal collections: Morphological terminology followed Cumming and Wood (2017), except for the segments of the first, second and third pair of legs, for which we have used the prefix pro-, meso-and meta-, respectively. We refer to pollinosity as microtrichia in the form of dust-like pruinescence (Cumming and Wood 2017, p. 89). Body length and wing length were measured using a reticule in a Zeiss SV11 stereomicroscope. Digital images were obtained using the set-up as outlined in Brecko et al. (2014). Stacking was done using the software ZERENE STACKER (www.zerenesystems.com/cms/home). Genitalia were dissected using forceps and soaked for 36 hours in a cold 10% potassium hydroxide (KOH) solution, after which they were transferred to acetic acid for 24 hours. Afterwards, they were transferred to glycerine. Digital images were taken with a Leica MZ16 microscope using LEICA APPLICATION SUITE (LAS) automontage software (version 3.8). From the specimen voucher AB59880784 (KMMA), we obtained a DNA barcode (GenBank accession number MW698743). Procedures for DNA barcoding followed Jordaens et al. (2015).
Thorax (Figs 1,2A and 3A): Dark brown to black, scutum yellow pollinose along margins and suture and in anterior part, also along median line, leaving large brownish-black non-pollinose areas with blackish setae. Scutellum yellow, width to length ratio 2.5:1, with premarginal sulcus, long yellow densely setose (setae ca. 1/3-1/2 length of scutellum); subscutellar fringe well developed with dense long yellow setae of ca. 0.45 mm. Pleurae brownish, mostly densely grey pollinose; postpronotum with a small pronounced non-pollinose bulge leading into an elevated rim of the scutum, with yellow and a few black setae intermixed; anterior anepisternum, posterior part of proepimeron and anterior part or posterior anepisternum bare; proepisternum, posterior part of anepisternum and anterior anepimeron with long yellow setae; katepisternum with posteriodorsal tuft of long setae and ventrally setose; meron, metepimeron, katepimeron, katatergite, posterior anepimeron, except for some short microsetae, bare; metasternum densely long yellow setose. Ampulla yellow. Plumula yellowish. Thoracic spiracles with whitish to ochre vestiture.   Hippa (1990)]: reddishbrown, tibia with basal 1/3 to 1/2 yellow. Mesofemur posteriorly grey pollinose, anteriorly with a broad median pollinose band; short adpressed yellowish setae; tarsae ventrally with short stout black setulae. Mesotibia abruptly broadened in apical 1/4. Mesofemur posterolateral with a row of longer whitish setae (length ca. 1/2 diameter of mesofemur). Mesotibia slightly dilated apically with apicoventrally a short comb of short black stout setulae and a small pointed tooth on the opposite side. Metacoxa (Fig. 4C) with two black spurs of ca. 1 mm long. Metafemur with posterolateral row of longer whitish setae (length ca. 1/3 of diameter of metafemur); ventrobasally with a low ridge carrying a black comb of short stout black setulae; apicoventrally with a depression that carries a ridge with a row of black stout setae on the anterior side. Metatibia yellow, apical 2/3 brownish; dilated towards apex, apical half carinate posteriorly; anteriorly with an oblique depression at 1/4 from apex.
Wing (Figs 1, 4D): hyaline, along anterior margin yellowish with darkened patches below pterostigma extending to cross-vein r-m, wing tip slightly darkened. Anal cell very long. Cell r 1 widely open, in apical half widened and close to wing margin suddenly narrowed, ending at wing margin. Vein R 4+5 with a deep oblique sinuate loop into cell r 4+5 . Spurious vein well developed, as thick as other veins, extending into cell r 4+5 . Anal cell microtrichose with basal area bare. Basal half of wing up to cross-vein r-m with sparse microtrichia, with bare areas along veins; apical 1/3 of wing (including entire cell r 4+5 ) densely microtrichose. Calypters white with an orange-brown margin, long yellow-white pilose. Halteres yellow-brown.
Abdomen (Figs 1, 2A, 3A): long and conical; setae from tergites I to IV mostly yellowish short and adpressed, only on lateral margins longer yellow setae. Tergite I yellow to ochre, densely greyish pollinose with the exception of a narrow non-pollinose posterior margin. Tergite II with large orange maculae, with lateral, anterior and posterior margins brownish to black; slightly darkened along median line; mostly nonpollinose. Tergite III narrowly yellow along anterior margin, otherwise dark brown; with sparse, but distinct yellowish to grey pollinosity, pollinosity more dense along median line and towards posterior margin. Tergite IV brownish-black, only anterior part of median line greyish pollinose. On tergites II-IV, the dark coloured parts have short blackish setulae. Hypopygium with brownish-black long setae. Sternite I brownish, densely grey pollinose. Sternite II yellow with black median line; only along anterior margin whitish pollinose. Sternite III anterior and posterior margin yellow, with central broad brownish-black fascia, uniform, but weak pollinose. Sternites I-III with very long dense setae of ca. 2 mm length. Sternite IV brownish-black, distinctly greyish pollinose laterally, with long setae that form a dense comb along lateral margin, leaving a median non-pollinose, almost bare, area of approx. 1/4 to 1/3 of sternal width.

Discussion
We here have described, for the first time, the female of the Afrotropical hoverfly, Syrittosyrphus opacea, which allowed us to update the diagnosis of this monotypic genus. As a result, the genus now correctly keys out in a key to the genera of Syrphidae of the Afrotropical Region (Ssymank et al., in press). We obtained a single DNA barcode and the uncorrected p-distance with any other Afrotropical hoverfly species is ≥ 10%. Its closest relative in the Afrotropical Region seems to be an undescribed species from the Ruwenzori Mountains in Uganda which we currently cannot attribute to any known genus (Jordaens et al., unpublished data).
Syrittosyrphus opacea seems to be a SE African endemic and, so far, has only been recorded from South Africa (nine localities) and Zimbabwe (one locality) (Fig. 7). The species is very rare with, up to now, five males and seven females known (see list of Material Studied). Very little is known of its ecology, but all specimens were collected from November to mid-March, except for one which was collected in June. Judging from the indigenous forest map of South-Africa ( fig. 12.2, p. 586 or fig. 12.3 at Cathedral Peak Area and Royal Natal National Park are not indicated on the map in Mucina and Geldenhuys (2006), but these areas still have small forest patches (J. Midgley, pers. comm.; as in fig. 12.5, p. 590 in Mucina and Geldenhuys 2006). Additionally, the Vumba mountains (Zimbabwe) would classify as Northern Afrotemperate Forest. Flower visits have not been observed, but the short stout proboscis (2.6-3.1 mm) with broad labellum suggests that the species is visiting open flowers. Adults have been observed sitting on the ground at forest margins or in sun-lit patches on the forest floor. Some have been collected from Malaise traps. Immature stages are unknown.
With the detailed re-description of the male and the description of the female provided here, both sexes of Syrittosyrphus opacea can now be unambiguously identified. The available records of this rare SE African endemic species suggest a distribution linked to three forest types along the Eastern Escarpment (Northern Afrotemperate Forest and Northern and Southern Mistbelt Forests). Based on these habitats, together with vegetation maps, future research may close distribution gaps and clarify the biology.
Gonçalves Miranda and an anonymous referee for their valuable comments which have improved the manuscript considerably. We also thank Gil Felipe Gonçalves Miranda for a fruitful discussion on the female genital morphology. Financial support was received through the Directorate-general Development Cooperation and Humanitarian Aid (DGD) to the RMCA (project DIPoDIP: Diversity of Pollinating Diptera in South African Biodiversity Hotspots) and the joint Belspo-NRF South Africa project DIPTATEACH (Diptera Museum Collections as a Source for Taxonomic Research and Teaching Activities).