New Wasp Species from Congo Defies Classification, Hints at Hidden Lineage

A single wasp specimen collected in 1957 from the forests of what was then the Belgian Congo has turned out to be something no entomologist expected: an entirely new species that refuses to fit into either of the two major species groups of the African social wasp genus Ropalidia. The insect, now formally described as Ropalidia chromis in the journal African Invertebrates, was recovered from the collections of the California Academy of Sciences in San Francisco, where it had rested for decades before modern taxonomic scrutiny revealed its remarkable mixture of traits.

Ropalidia is a widespread genus of social wasps distributed across the Ethiopian, Oriental and Australian regions, with 49 species known from the African mainland. The genus is easily recognized by a hallmark anatomical feature: the second tergum and sternum of the metasoma are merged into a single unit, an arrangement that sets Ropalidia apart from nearly all of its relatives, with only three New Guinea endemics showing overlapping sclerites instead. On the African mainland, most species fall into two large assemblages, the capensis-group and the non-capensis-group, distinguished by morphology and supported by genetic analysis, while only a handful of less common species occupy separate lineages.

The new species presents a genuine taxonomic dilemma. In a comparative analysis spanning more than a dozen anatomical features, Ropalidia chromis matches the capensis-group in some characters and the non-capensis-group in others, while several traits resemble neither assemblage at all. Its wing length of 8.9 millimetres exceeds the maximum recorded for the capensis-group, which ranges from 6.2 to 8.5 millimetres, yet sits at the lower boundary of the non-capensis-group, which spans 8.5 to 12.1 millimetres. The female antenna is clavate, meaning the eighth flagellomere is about twice as wide as long, a condition shared with the capensis-group but absent in the non-capensis-group, where the ratio reaches only 1.5.

Other characters pull in the opposite direction. The elongated second submarginal cell of the wing, with its extended median angle, aligns with the non-capensis-group, as does the clypeus colour pattern, which in Ropalidia chromis features a centrally attached spot rather than the transversal apical yellow line typical of capensis-group members. The scape, the basal segment of the antenna, is conspicuously elongated at roughly twice the length of the first flagellomere, whereas capensis-group species commonly show a ratio of 1.5 and non-capensis-group species have a scape of roughly equal length to the first flagellomere. The first metasomal segment, T1, is strongly angulate in lateral view, enclosing an angle of about 110 degrees, and deeply and coarsely punctate laterally, a combination not seen in either group, where T1 punctation is weak and the shape is globular or rounded.

Beyond these mixed affinities, the species carries features that appear autapomorphic, meaning they are unique derived traits not shared with either group and sufficient on their own for species determination. The most striking is the markedly depressed area above the antennal sockets, an anatomical arrangement not recorded in any other African Ropalidia, where the corresponding region is flattened. The clypeus, the facial plate covering the mouthparts, displays a biphasic punctation pattern: a fine basal cuticular sculpture overlaid with evenly spaced, well-defined smaller punctures. This dual sculpturing is characteristic of numerous species in the genus Polistes but has never been observed in Ropalidia, making it a remarkable case of homoplasy, the independent evolution of similar features in separate lineages. The general appearance of the wasp, with its thin gena at most half the width of the compound eye and its depressed supraantennal area, even substantially resembles members of the subfamily Eumeninae, a group of mostly solitary wasps.

The holotype, a single female, was collected 45 miles east of Kama in what is now Maniema Province of the Democratic Republic of the Congo, at an elevation of 750 metres, by collectors E.S. Ross and R.E. Leech in August 1957. The specimen was studied and photographed using a Leica S9i stereoscopic microscope with an integrated camera, and images were stacked with Helicon software to produce sharp composite photographs of the head, mesosoma and metasoma. The male of the species remains unknown, and the description is based entirely on this one female, a reminder of how much undiscovered diversity may still lie hidden in museum drawers and uncollected habitats across central Africa.

The species name chromis is a Latinized form of the word chrome, chosen in reference to the silvery-yellowish setae and pubescence that cover the head, mesosoma and metasoma. This metallic sheen is one of the most conspicuous features of the insect, with short silvery-yellow pubescence about half the length of the anterior ocellar diameter covering the entire mesosoma, somewhat longer whitish setae on the propodeal excavation, and golden pubescence with yellowish protruding setae extending over the translucent yellow lamella of the second tergum. The basal colour of the body is black, but the clypeus is tricolourous, bearing a black border, a brown median spot nearly reaching the apex, and yellowish-white lateral areas, while the metanotum carries two large yellow spots occupying two thirds of its total surface.

Interestingly, both the colour and the morphology of Ropalidia chromis follow a broad east-west pattern previously documented across African Ropalidia. A recent revision of the genus identified an intriguing trend: species and populations in eastern Africa tend toward increasing yellow or reddish colouration and weaker punctation, while those in western and central Africa become darker, often black, with much coarser punctation. This gradient holds both across species and within intra-specific clusters, notably in Ropalidia guttatipennis and Ropalidia aethiopica, which both become darker or even black in Cameroon, Gabon and the Democratic Republic of the Congo. Ropalidia chromis, with its black basal colour and coarse punctation, sits squarely within the central African end of this spectrum.

Several morphological features of the new species are found in other wasps that occur exclusively in this part of Africa, suggesting converging evolutionary processes that favour such traits. The angulate and strongly punctate T1 is also present in Ropalidia salebrosa, a newly described species in the same recent revision. The coarser T2 punctation resembles that of Ropalidia crassipunctata, which also shares the thin gena. The very strongly punctate mesopleuron, with punctures becoming reticulate dorsally, is likewise present in Ropalidia brazzai. These parallels indicate that the environmental conditions of western and central Africa may repeatedly drive the evolution of darker bodies, coarser sculpturing and angular metasomal segments across independent lineages.

The discovery underscores both the value of historical museum collections and the unresolved diversity of African social wasps. That a specimen collected more than six decades ago could reveal a species with a unique combination of traits, including a depressed supraantennal area and biphasic clypeal punctation never before seen in the genus, hints at a separate phylogenetic lineage whose evolutionary history remains to be traced. As taxonomists continue to apply modern imaging and comparative methods to material held in institutions worldwide, further surprises from the wasp fauna of central Africa seem likely, and Ropalidia chromis stands as a vivid reminder that the continent’s insect diversity is still being written.

Subject of Research: Description of a new social wasp species, Ropalidia chromis, from the Democratic Republic of the Congo

Article Title: A new species of the genus Ropalidia Guérin-Méneville from central Africa ( Insecta , Hymenoptera , Vespidae )

Article References: Polašek, O. (2024). A new species of the genus Ropalidia Guérin-Méneville from central Africa (Insecta, Hymenoptera, Vespidae). African Invertebrates, 65(1), 9-14. https://doi.org/10.3897/afrinvertebr.65.103539

Image Credits: AI Generated

DOI: 10.3897/afrinvertebr.65.103539

Keywords: Ropalidia, new species, Democratic Republic of the Congo, Vespidae, social wasps, taxonomy, central Africa, Polistes, morphology, species groups, convergent evolution, holotype