Carebara is a worldwide genus of myrmicine ants, currently counting 230 valid species, concentrated mainly in tropical and subtropical regions. Most are subterranean, typically collected from leaf litter, under stones, or in decaying wood — habitats that make them easy to overlook and hard to study. The genus was established by Westwood in 1840 with C. lignata as its type species, and Carebara in the strict sense is mainly characterized by blind, small, typically monomorphic worker castes. Over the decades, several once-separate genera — Oligomyrmex, Paedalgus, Afroxyidris, Neoblepharidatta, Parvimyrma, and Pheidologeton — have been synonymized with Carebara based on morphological similarities, a consolidation later confirmed by phylogenetic studies using nuclear regions and ultraconserved elements. Notably, the former Oligomyrmex and Pheidologeton are distinguished by worker dimorphism and worker polymorphism, respectively, meaning their colonies contain workers of markedly different sizes and forms.

That dimorphism is precisely what makes the C. oni species group so interesting. Members of this group are distinguished from other dimorphic Asian Carebara by a combination of characters in both worker subcastes: nine-segmented antennae, six-toothed mandibles with the basal three teeth more blunt than the apical three, a subsquare head shape in full-face view with a cephalic index between 88 and 98, and — most strikingly — a massive mesosoma in major workers, with mesosomal length between 0.68 and 0.96 millimeters, a distinct promesonotal suture, a developed scutellum, and a band-shaped metanotum. Minor workers add their own signature: a concave median clypeus bearing a pair of acutely protruded lateral processes, four pairs of long erect hairs on the promesonotum, posteriorly situated propodeal spiracles, and a short petiolar peduncle with a low petiolar node. These are not subtle differences; they are the kind of structural features that taxonomists rely on when molecular data are not yet available.

The researchers did not simply accept earlier character lists, however. Liu and colleagues, in their 2024 revision of Chinese species, had suggested that C. altinoda, C. hunanensis, C. oni, and C. qianliyan form a distinct group based on a massive mesosoma, a short head, large size, and the presence of ocelli. But the new study found that some of these characters are unreliable. Total length, for instance, is highly influenced by measurement method and specimen condition, making it a poor diagnostic trait; mesosomal length serves as a more stable indicator instead. More importantly, the presence of ocelli — simple eyes on the heads of major workers — varies within species. Terayama had already noted intraspecific variation in C. oni, and the new samples confirmed that C. eguchii also includes individuals with and without ocelli. Using such variable traits as diagnostic characters, the authors argue, is inappropriate.

The examination of type material also overturned two presumed group members. Carebara altinoda, on review of its original description, turns out to have eleven-segmented antennae in both majors and minors, an unarmed propodeum, and no erect hairs on the promesonotum, petiole, or postpetiole in minors — features characteristic of the C. pygmaea species group. Carebara qianliyan, whose type specimen was examined directly, revealed a similar story: although its original description mistakenly stated nine-segmented antennae, the holotype actually exhibits eleven segments, along with an unarmed propodeum and the absence of erect hairs on the mesonotum, petiole, and postpetiole in minor workers. Both species, the authors conclude, likely belong to the C. pygmaea species group instead. The status of C. hunanensis remains uncertain until its type material can be examined, though the authors presume it is a member of the C. oni group.

Distinguishing the C. oni group from its closest relatives required careful comparison. Against the C. pygmaea species group, the C. oni group is set apart by nine-segmented rather than eleven-segmented antennae, six-toothed rather than five-toothed mandibles, a roundly convex rather than dome-like promesonotum in major workers, the presence rather than absence of propodeal spines in minor workers, and a lower rather than higher petiolar node in minors. Against the C. escherichi species group, the distinction is even simpler: the C. oni group possesses major workers, while the C. escherichi group does not. Minor workers of the C. oni group are further distinguished by a distinct metanotal groove, a low petiolar node, and the presence of propodeal spines — all features that are indistinct, higher, or absent in the C. escherichi group.

One of the most technically significant aspects of the revision is its treatment of intermediates — worker individuals whose morphology falls between the major and minor subcastes. Such intercastes have long been reported in Carebara, and all species in the C. oni group show gradual morphological differences within the major worker subcaste itself. The main variations observed include head length, mesosomal length, the number of ocelli (one to three), eye size, the forward convexity of the promesonotal suture in dorsal view, and the development of the propodeal spine. In C. eguchii, three intermediates were recognized: the first lacking ocelli entirely, the second bearing one median and one lateral ocellus, and the third — the holotype — with one median and two lateral ocelli. In C. bifida, ocellar number also varies from one to three, and the sculpture of the upper mesopleuron ranges from smooth and shiny to reticulate. In C. oni, five intermediates were documented, with eyes ranging from small and reduced to large and multi-faceted, and body color shifting from yellow in the first intermediate to brown in later ones. The authors designate the individual exhibiting stable morphological characters among the intermediates as the type specimen, a practice that anchors species descriptions in the most representative form available.

The two new species were collected from distinct Vietnamese habitats. Carebara eguchii was found on the western slope of Mt. Phansipan in Lai Chau Province, at elevations between 2,100 and 2,200 meters, with the holotype major worker measuring 3.12 millimeters in total length and bearing a reticulate promesonotum, a largely reticulate lateral pronotum, distinct reticulation on the dorsal propodeum, and a blunt subpetiolar process. Carebara bifida, named for the strongly forked median clypeus of its minor worker, was collected from dead wood in Cuc Phuong National Park in Ninh Binh Province, with additional non-type material from Quang Ninh, Bac Giang, and Lao Cai provinces. Its major worker is smaller, at 2.97 millimeters in the holotype, with a smooth and shiny promesonotum and pronotum, an acute subpetiolar process, and a weakly curved posterodorsal margin of the petiolar node. The species was previously documented in faunal surveys as Oligomyrmex sp. eg-1 and eg-2, unidentified dimorphic Carebara that had been waiting for formal description.

The identification keys provided in the revision are worker-based and rely on fine-grained sculptural and structural differences. For major workers, C. eguchii is separated by its reticulate dorsal promesonotum and blunt subpetiolar process, while C. bifida and C. oni are distinguished by the lower portion of the mesopleuron — half smooth and half reticulate in C. bifida, largely smooth in C. oni — and by the curvature of the petiolar node’s posterodorsal margin. For minor workers, C. oni stands apart with smooth and shiny vertex, lateral propodeum, and dorsal petiolar node, while C. eguchii and C. bifida are separated by pronotal sculpture, the presence or absence of the subpetiolar process, and the position of the propodeal spiracle relative to the propodeal declivity. These keys, modified from Terayama’s 1996 work and Liu and colleagues’ 2024 revision, give researchers a practical tool for identifying species in a group whose members nest in decayed wood across northern Vietnam, Taiwan, and Okinawa.

The authors are careful to note that their definition of the species group rests on morphological similarities among included taxa, and that molecular phylogenetic analysis will be necessary to test its monophyly. Until fresh material for molecular work is available, they argue, it is important to develop a sound morphology-based taxonomy — a stance that reflects a broader challenge in tropical Asian ant systematics, where many unidentified dimorphic Carebara species have been documented in faunal surveys across Vietnam, Brunei, Cambodia, China, Borneo, Indonesia, and beyond. With 36 species and subspecies recorded from China, 26 from Indonesia, and 14 from Vietnam, the region is clearly rich in Carebara diversity, yet few researchers have discussed species groups of dimorphic members based on worker morphology. This revision, the first attempt at such a classification system for dimorphic Carebara, may well open the door to discovering many more hidden species in the soils and decaying woods of tropical Asia.

Subject of Research: Taxonomy

Article Title: Taxonomic revision of the Carebara oni species group, with descriptions of two new species from Vietnam ( Hymenoptera , Formicidae , Myrmicinae )

Article References: Matsuura, K., Hosoishi, S., & Pham, H. T. (2025). Taxonomic revision of the Carebara oni species group, with descriptions of two new species from Vietnam (Hymenoptera, Formicidae, Myrmicinae). Deutsche Entomologische Zeitschrift, 72(2), 377-393. https://doi.org/10.3897/dez.72.171173

Image Credits: AI Generated

DOI: 10.3897/dez.72.171173

Keywords: ants, Carebara, taxonomy, Vietnam, new species, entomology, biodiversity, Taxonomic, revision, species, group, descriptions