Revision of the southern African genus Stuckenbergomyia Smith , 1971 ( Diptera , Empidoidea ) and proposal of a new subfamily

The genus Stuckenbergomyia Smith is revised with the description of a new species from Namibia (S. namibiensis sp. nov.) and an undescribed species based on females from Western Cape Province of South Africa. The genus is fully illustrated and its phylogenetic relationships within the Hybotidae are discussed with the proposal of a new subfamily, Stuckenbergomyiinae subfam. nov.


Introduction
The genus Stuckenbergomyia Smith, 1971 is an odd genus of the family Hybotidae (Diptera, Empidoidea), characterised by a short spur vein arising from or near cell cua (anterior cubital). The genus is endemic to southern Africa, recorded from Mozambique, South Africa, Zimbabwe and now Namibia. Originally assigned to the Ocydromiinae, the genus is currently placed as incertae sedis in the Hybotidae (Sinclair 2003;Sinclair and Cumming 2006;Sinclair and Cumming 2017). Species of Stuckenbergomyia are believed to occur in dry montane grasslands (Smith 1969) and the form of the mouthparts suggests that they are flower visitors (Sinclair and Cumming 2017).
Aside from two species of Ocydromia Meigen, 1820 (Ocydromiinae) recorded from East Africa and South Africa, Stuckenbergomyia is the only other Afrotropical genus of Hybotidae not assigned to the subfamilies Tachydromiinae and Hybotinae (Smith 1969;Sinclair and Cumming 2017). The discovery of newly collected and better preserved specimens initiated this study and facilitated re-examination of the genus and subfamily assignment.

Material and methods
This study is based on material loaned from or deposited in the following institutions: Canadian National Collection of Insects, Ottawa, Canada (CNC); KwaZulu-Natal Museum, Pietermaritzburg, South Africa (NMSA); National Museum of Namibia, Windhoek, Namibia (NMNW).
Terminology used for adult structures follows those of Cumming and Wood (2017). Label data for primary types are presented exactly as they appear. Data are listed from the top downwards on the staging pin, with data from each label enclosed in quotation marks; lines are delimited by a forward slash mark. Additional information is included in square brackets. The repository of each type is given in parentheses. Secondary type data are abridged and listed alphabetically. Photographs were taken with a Leica camera model DFC5400 using Leica Application Suite X. Photo montages were created using Zerene Stacker 1.04. The distribution map was created with Simplemappr (Shorthouse 2010).
Diagnosis. The genus (as in Figs 1, 10, 20) is characterised by an elongate postpedicel, stout antennal stylus, three complete wing veins from cell dm (discal medial) and a short spur vein, that extends from or near cell cua. Redescription. Head: Holoptic with enlarged facets on upper two-thirds or dichoptic (facets not enlarged); eyes bare (ommatrichia absent). Face nearly completely membranous, U-shaped to base of antennal sockets. Ocellar triangle elevated in holoptic species; pair of ocellar setae very short, indistinguishable from other setae on triangle. Antenna inserted at middle of eye height; scape and pedicel subequal in length; scape bare, pedicel with circlet of subapical setulae; postpedicel elongate, tapered, 2-3× length of scape and pedicel combined; two-articled stylus, with short, apical peg-like receptor. Proboscis (as in Fig. 19) projected forwards, about as long as head. Palpus straight, strap-like; bearing numerous setulae, held obliquely to proboscis, with narrow palpifer (Fig. 10); lacinia absent. Labrum slender, straight without pair of apical epipharyngeal blades. Hypopharynx straight, slender, subequal in length to labrum. Labellum well developed, with pseudotracheae.
Distribution. Described species of Stuckenbergomyia are recorded from Mozambique, Namibia and South Africa. An undescribed female (in Natural History Museum, London), representing an additional species, is recorded from Zimbabwe ( Fig. 22) (Umtali [now Mutare], Vumba, 19°05'S, 32°45'E), but the specimen is said to be in poor condition (Smith 1969) and was not examined in the present study.
Remarks. Stuckenbergomyia can be identified by using the key to genera in Sinclair and Cumming (2017).
Specimens have only been collected by Malaise traps, yellow pans and light traps and there are no direct observations on habits. Adult mouthpart morphology (i.e. angle of proboscis; absence of epipharyngeal blades) and the observation of pollen grains in the dissected male and female abdomens of S. namibiensis, provides evidence that Stuckenbergomyia visits flowers and feeds on pollen.
Diagnosis. This species is characterised by the pair of dark vittae between acrostichals and dorsocentrals; broad wings, yellow legs and 5-7 pairs of long scutellar setae; male terminalia: nearly symmetrical, held at apex of abdomen, surstyli not clearly differentiated.
Description. Head: Holoptic with enlarged facets on upper two-thirds. Antenna with scape and pedicel paler than dark brown postpedicel; postpedicel length less than 4× basal width; stylus cylindrical, shorter than basal width of postpedicel; apical mechanoreceptor one-third length of stylus.
Remarks. This species was not included in the original list of Empidoidea from the Brandberg Massif (Sinclair 2000), because these specimens were collected subsequent to the initial surveys.
Diagnosis. This species is characterised by the faint brown vittae beneath acrostichals and dorsocentrals; broad wings and 4 pairs of long scutellar setae; male terminalia: symmetrical, held at apex of abdomen, surstyli broadly articulated lobes.
Redescription. Head: Holoptic with enlarged facets on upper two-thirds. Antenna dark brown; postpedicel length ca. 4× basal width; stylus cylindrical, about as long as basal width of postpedicel; apical mechanoreceptor one-third length of stylus.

Stuckenbergomyia tumbinensis
Diagnosis. This species is characterised by the biserial acrostichals and uniserial dorsocentral setae; narrow wings with undeveloped alula and 2 pairs of long scutellar setae; male terminalia: very large, symmetrical, held erect over right side of abdomen.
Redescription. Head: Dichoptic with facets not enlarged. Antenna with scape and pedicel paler than dark brown postpedicel; postpedicel length slightly longer than 4× basal width; stylus cylindrical, about as long as basal width of postpedicel; apical mechanoreceptor one-half length of stylus.

Stuckenbergomyia species A Figs 20-22
Diagnosis. This species is characterised by a narrow pair of vittae between acrostichals and dorsocentrals and broader brown vittae along supra-alars; broad wings and 7 pairs of whitish scutellar setae.
Remarks. Formal naming of this species should await association of male specimens.

Phylogenetic affinities
The subfamily assignment of Stuckenbergomyia has been problematic since the recognition of this enigmatic genus. Smith (1969) originally assigned this genus to the Ocydromiinae, but at that time, this subfamily included all genera not assigned to the other two subfamilies, the Hybotinae and Tachydromiinae. Stuckenbergomyia is currently un-assigned to subfamily in the family Hybotidae (Sinclair and Cumming 2006). The L-shaped male tergite 8 suggests affinities to Atelestidae, but Stuckenbergomyia is clearly correctly assigned to the Hybotidae due to the presence of a palpifer (sclerotised plate at base of palpus) (Fig. 19) and fore tibial gland (Fig. 12), in addition to the plate-like ventral apodeme (Fig. 5) (Sinclair and Cumming 2006). The combination of a truncate cell cua, shortened stylus, three veins emitted from cell dm, obliquely directed mouthparts and pollen feeding also suggests affinities with the Oedaleinae, but female segment 8 is not elongate and the epiproct is separate from the female cercus in Stuckenbergomyia (Fig. 6). In addition, despite feeding on pollen, epipharyngeal blades are present in the Oedaleinae, but absent in Stuckenbergomyia.
In all studied genera of Hybotidae, the spermatheca is stated to be either absent (i.e. not readily observed) or is membranous and unpigmented (Sinclair and Cumming 2006). The discovery of a sclerotised, pigmented spermathecal receptacle and duct in Stuckenbergomyia (Figs 2, 7) is the first time this plesiomorphic form has been observed in the family and suggests that the genus could be the sister group to the remaining Hybotidae. In the absence of additional phylogenetic informative characters, a new monotypic subfamily is proposed, Stuckenbergomyiinae subfam. nov. (type genus: Stuckenbergomyia Smith, 1971). The subfamily is named to provide stability in Hybotidae nomenclature and to highlight the importance of including this clade in future molecular and morphological phylogenetic studies.