Rarest of African Hover Flies Revealed: New Species and First DNA Barcodes for Mysterious Genus Amphoterus

Deep in the indigenous forests of South Africa and Kenya, a group of hover flies so elusive that scientists have recorded them only a handful of times is finally coming into focus. In the first taxonomic revision of the genus Amphoterus in more than 65 years, researchers have redescribed its known males, described the female of one species for the very first time, unveiled a brand-new species from Kenya, and generated the first DNA barcodes ever obtained for this enigmatic lineage of Afrotropical hover flies. The work, published in the journal African Invertebrates, transforms a genus known from just three published records into a properly documented, photographable and genetically traceable branch of the fly tree of life.

Amphoterus was erected in 1915 by the Italian entomologist Mario Bezzi on the basis of a single male specimen collected in what was then British East Africa, now Kenya. The flies are medium-sized, measuring roughly 9 to 11 millimetres, and stand out among hover flies for their strikingly elongate antennae, broadly separated eyes in males, and wing venation that closely resembles that of the related genus Eumerus. Their dark, strongly punctured bodies and secretive habits have kept them almost invisible to science. For over a century, the genus rested on three brief descriptions, each founded on a single specimen, with only a handful of diagrams ever published.

The new revision, led by researchers working with material from seven natural history collections across Africa and Europe, changes that picture dramatically. The team redescribed the males of Amphoterus braunsi, described by van Doesburg in 1956 from Barberton in South Africa’s Mpumalanga province, and of the type species Amphoterus cribratus from Kenya. Crucially, they also described the previously unknown female of A. braunsi, drawing on a series of specimens accumulated over decades, including two females collected by the South African entomologist Jason Londt, in whose honour the new species is named.

That new species, Amphoterus londti, is known from a single female captured in a Malaise trap set in indigenous forest at Kibwe Forest in Kenya’s Eastern Province, at an altitude of about 925 metres. Despite being represented by just one specimen, its diagnostic features are unmistakable. The alula, a small lobe at the base of the wing, is almost three times as long as it is broad, noticeably narrower than in its relatives. Its postpedicel, the outermost segment of the antenna, is exceptionally long, about 4.1 times as long as high, compared with 2.8 to 3.1 times in the other species. The pedicel is shorter than the postpedicel, and the face is covered in dense white pollinosity rather than the yellow or golden dusting seen in the other species. The male remains unknown.

Distinguishing the three species relies on a combination of colour and structural characters laid out in the first identification key ever published for the genus. Amphoterus braunsi is the warm-toned member of the trio, with reddish brown legs, contrasting golden fasciae along the thoracic suture, dark reddish brown antennae and a golden-pollinose face bearing golden setae. The other two species are darker, with black to very dark brown legs, uniform thoracic sutures without golden bands, black antennae and faces dusted in yellow or white. Between those two, A. cribratus has a shorter postpedicel, yellow face pollinosity and a broader alula, while A. londti combines the elongate antenna, white facial dusting and narrow alula that set it apart.

The revision also brings modern imaging to a genus that had never been properly photographed. Using focus-stacking microscopy, the team produced high-resolution images of all three species in dorsal and lateral view, close-ups of the heads in frontal view, and detailed photographs of the male genitalia of both known males. These genitalia photographs reveal clear differences: the hypandrium of A. braunsi is broad in lateral view with trapezoid cerci, while that of A. cribratus is narrow with rounded cerci. Such characters are essential for confirming species boundaries in hover flies, where external appearance can be deceptively similar.

Perhaps the most significant technical advance is molecular. The researchers extracted genomic DNA from single legs of specimens and amplified a fragment of the mitochondrial cytochrome c oxidase subunit I gene, the standard DNA barcode region, using the classic Folmer primers. Six barcodes were generated and deposited in GenBank. The five barcodes from A. braunsi specimens showed a mean intraspecific genetic distance of just 0.004, confirming they belong to a single species, while the distance between A. braunsi and A. londti averaged 0.05, a magnitude consistent with differences observed between other Afrotropical hover fly subtribes. These are the first DNA barcodes ever obtained for the genus, providing an objective tool for matching males and females and for identifying future specimens.

Not every specimen could be sampled. The team deliberately avoided attempting DNA extraction from the century-old holotype of A. cribratus, reasoning that a specimen collected more than 100 years ago was unlikely to yield usable DNA and that damaging such a historically important type specimen was not worth the risk. The mystery deepens further: the female A. cribratus described by Brunetti in 1926 from Kalemie in the eastern Democratic Republic of the Congo, along with two specimens mentioned in a 1995 catalogue, could not be located in the collections searched and are now considered lost. The researchers note that Brunetti’s description differs from the new Kenyan species in the pattern of pollinosity on the thorax and fourth abdominal segment, supporting their conclusion that the two are not the same species despite coming from the same broad region.

The ecology of these flies is beginning to emerge from the new material. Nearly all A. braunsi specimens with habitat data come from indigenous forests in South Africa’s KwaZulu-Natal and Eastern Cape provinces, and live specimens observed at Rainbow Gorge in the Drakensberg were resting on leaves and flowers in sunny patches within the forest. Although never abundant, the flies show distinct temporal patterns: three specimens were caught over eight weeks of Malaise trapping at Shawswood, three more were hand-collected in a single day during the same period, and four were taken in just two days at Rainbow Gorge. This suggests adults may be locally common at particular times or in particular habitats, even if they are rarely encountered overall.

The revision is a reminder of how much biodiversity remains hidden in plain sight, even in well-studied groups like hover flies, many of which are important pollinators. With detailed descriptions, photographs, an identification key and DNA barcodes now in place, Amphoterus has moved from a taxonomic footnote to a genus that other researchers can actually work with. The authors suggest that further collecting in East and Central Africa, where Kenya alone contains three biodiversity hotspots, may well bring more specimens to light and clarify the habitat preferences and true distributions of these shadowy forest flies. For now, three species are recognised, one is known only from a single female, and one has lost its female to history, leaving plenty still to discover about one of Africa’s rarest hover fly lineages.

Subject of Research: Taxonomic revision of the rare Afrotropical hover fly genus Amphoterus, including a new species and the first DNA barcodes

Article Title: A revision of the hover fly genus Amphoterus Bezzi, 1915 ( Diptera , Syrphidae ) with the description of one new species

Article References: Midgley, J., Bellingan, T., & Jordaens, K. (2023). A revision of the hover fly genus Amphoterus Bezzi, 1915 (Diptera, Syrphidae) with the description of one new species. African Invertebrates, 64(2), 149-163. https://doi.org/10.3897/afrinvertebr.64.100481

Image Credits: AI Generated

DOI: 10.3897/afrinvertebr.64.100481

Keywords: Amphoterus, hover flies, Syrphidae, Diptera, taxonomy, new species, DNA barcoding, Afrotropical, Africa, forest insects, COI barcode, species description