Carebara species groups

The page below outlines one recent perspective on Carebara species groups. There is clearly a need for much more taxonomic work within this species rich genus that also exhibits a wide range of morphological diversity.

The following is based on Fischer et al. (2014) The establishment of the Carebara polita species group highlights the need to revise all Carebara species to better delimit species group boundaries in the genus and to rearrange the groups suggested by Fernández (2004, 2010) and later adapted to include the Indian Carebara fauna treated by Bharti and Kumar (2013). The establishment of new species groups will likely be necessary with ongoing taxonomic research and will help unravel the evolutionary history of this genus. Especially the Afrotopical region holds a large number of unrevised and new species, many of which are located in the BMNH collection in London.

The former Pheidologeton species could in future revisions be split into two species groups, one that shares characters with C. aberrans and C. affinis, where the minor and large major workers are connected through a continuous series of intermediate subcastes, and another group that shares characters with C. pygmaea and C. alperti, that are dimorphic and morphologically similar to some species of the former genus Oligomyrmex. In this publication we are establishing and focusing on the polita species group, which is mostly characterized by their minor workers’ morphology, i.e. head and mesosoma shape, medially smooth and shiny frons, present propodeal spines, and, most importantly, an elongate postpetiole in minor workers. The diagnosis for minor and major workers (where known) of the polita group is summarized below.

polita species group
Key to Carebara polita species group

Diagnostic characters of the polita species group

1. Antennae with usually eleven, but one species with nine segments; scapes, when laid back, never reaching or surpassing posterior head margin.

2. Eyes of minor workers small, in most specimens consisting of only one ommatidium, in major workers absent to multi-facetted, in all subcastes situated anterior to cephalic midlength and relatively close to anterior head margin.

3. Mandibles with four to six teeth, number in minor workers often one less than in major workers.

4. Posterior margin of head concave to weakly concave medially, or nearly straight.

5. Frons in minor workers medially smooth and shiny.

6. Minor workers with short to comparatively long propodeal spines present, in major workers sometimes reduced to a blunted angle (C. urichi).

7. Minor and/or major workers often with petiolar ventral process present as anteriorly directed small tooth, which is sometimes reduced or inconspicuous.

8. Petiole node in minor workers usually subangulate to rounded in profile, dorsally smooth and shiny, in major workers very well-developed and high, the dorsum rounded to angulate-subangulate and anterodorsally compressed.

9. Minor worker postpetiole relatively elongate, distincly longer than high (LPpI 131–173) and lower than petiole, major worker postpetiole compact, dorsally rounded, and in profile about as high as long (LPpI 83–107).

Comments on diagnosis of polita group worker caste

Range in head shape and sculpture, the number of distinctly expressed mesosomal sclerites in major workers, and the propodeum varying from having spines or not in this group is relatively large and comparable to the range found in former Pheidologeton species. Interestingly the major worker caste has shown itself completely elusive in two of the polita group species. Despite a multitude of collections from several localities C. brevipilosa and C. villiersi are known only from the minor workers so far. But C. polita has been collected without any major workers for almost a century, and C. nicotianae for more than 50 years. The minor workers of C. polita were described by F. Santschi in 1914 and majors have not been collected until 2001 by the late R. R. Snelling, but remained unidentified at first. If not for another collection of major workers together with queens and minor workers in 2008 by the first author, the fact that these two different worker subcastes belong to the same species, and one important link between Carebara and the former Pheidologeton, might have remained unnoticed.

Biogeographic notes on the polita species group

Six species occur in the Afrotropical region, two of them are widely distributed throughout sub-Saharan Africa: C. perpusilla in Kenya, Rwanda, South Africa, Tanzania, Uganda, Zambia, and Zimbabwe and C. silvestrii in Cameroon, Central African Republic, Equatorial Guinea, Gabon, Ghana, Ivory Coast, Kenya, Uganda, and Zimbabwe. Two species are widely distributed from Western to Eastern Africa: C. polita in Cameroon, Central African Republic, Gabon, Kenya, Tanzania, and Uganda and Carebara madibai sp. n. is known from the Central African Republic, Dem. Rep. Congo, Gabon, and Uganda. Carebara villiersi occurs throughout Western Africa and was found in Cameroon, Central African Republic, Gabon, Ghana, Guinea, and Ivory Coast. Carebara nicotianae seems restricted to Zambia and Zimbabwe in Southern Africa. Two of the species included in this group are present and widespread in the Neotropical Region: C. brevipilosa is reported from Brazil, Colombia, Costa Rica, and Panama, while C. urichi was found only in Belize, Brazil, Colombia, Costa Rica, Mexico, Panama, Peru, Suriname, and Trinidad.

C. phragmotica clade
Key to Carebara phragmotica species group

Fisher et al. (2015):

Shared characters of C. phragmotica clade species (all worker subcastes)

The characters listed below may not be autaphomorphic, since the majority of Afrotropical taxa remain poorly characterized and because of possible convergent evolution.

Phragmotic major workers present, with oval cephalic shield and anterolateral lobes covering the lateral base of the mandible. Antennae with 10 segments and 2-segmented club, the apical segment between combined length of antennal segments 3 to 9 and length of remainder of funiculus (antennal segments 2-9). Antennal scape relatively short, in minor workers failing to reach the posterior head margin by about lenght of 9th antennal segment, in majors of Carebara phragmotica ending at about midlength of head (SI 46-49), in phragmotic workers distinctly shorter and reduced (SI 21-34). Mandibles triangular and masticatory margin with five teeth, mandibles of phragmotic workers reduced and very small, about half as long as those of major workers in C. phragmotica (MDI 24-28). Anterior margin of clypeus in phragmotic workers very wide and straight to medially concave. Eyes minute and consisting of one ocellus, in phragmotic workers reduced and almost invisible, single median ocellus often present in major workers of C. phragmotica, but invisible or absent in phragmotic workers. Metanotal groove in profile impressed and propodeum higher than long. Propodeal teeth developed, relatively small and apically rounded to short-triangular and acute. Petiole quite massive in profile, with moderately long peduncle, a small, anteriorly pointing anteroventral tooth, and often with conspicuously convex ventral bulge, in dorsal view almost as wide as (minor workers) to wider than propodeal dorsum (majors and phragmotic workers). Postpetiole roundly subrectangular in dorsal view, between 1.2 and 1.5 times wider than petiole. In minor workers (of C. phragmotica and Carebara lilith) sculpture absent from head, promesonotum, dorsum of postpetiole and gaster.

Delimitation from other Carebara groups and species in the Afrotropical region

Here a general account of the Afrotropical Carebara fauna is given as well as information on how to differentiate species belonging to the phragmotica clade from the remainder of Carebara species that were found and described for the region, not including Madagascar. They can be divided into several groups of morphologically related species, some of which correspond to the preliminary groups defined by Fernández (2004): lignata complex for Carebara sensu stricto (before synonymization of Oligomyrmex), escherischi complex for former Paedalgus species, and with the former Oligomyrmex species roughly corresponding to the concinna complex, although it was defined for New World species, which have eleven antennal segments, contrasting to mostly 9- and 10-segmented Old World species. Two of these New World concinna complex species, Carebara brevipilosa and Carebara urichi, are now included in the polita group, but exact phylogentic relationships within and between the different faunas are still unresolved. Because we want to avoid creating polyphyletic species groups, we leave the definition of systematic species groups to larger-scale studies in the future.

Afrotropical Carebara species belonging to the former genus Pheidologeton are: Carebara aberrans (queen), Carebara diversa standfussi, Carebara hammoniae, Carebara hostilis, Carebara kunensis, Carebara mayri, Carebara solitaria (queen), and Carebara volsellata (male). They are mainly characterized by possessing eleven antennal segments, a markedly polymorphic worker caste with several intermediate worker subcastes, comparatively large, multi-facetted eyes, minor workers with antennal scapes usually surpassing the posterior head margin, and large major work ers, usually with one to several large occeli present. Morphologically, this group is closest to some species of the polita group, e.g. Carebara nicotianae and C. polita. The polita group can be distinguished from other Carebara by antennae with eleven segments (but only nine in Carebara madibai, eyes reduced, in minor workers usually consisting of a single ocellus, in majors sometimes larger and multi-facetted, but smaller than in former Pheidologeton species, major workers usually with high, weakly squamiform petiole node, and minor workers with postpetiole significantly longer than high in profile (Fischer et al. 2014). In Africa the polita group includes: Carebara madibai Fischer & Azorsa, Carebara perpusilla, C. polita, Carebara nicotianae, Carebara silvestrii, and Carebara villiersi. Species described in or assigned to the former genus Paedalgus (Forel) (escherischi complex in Fernández 2004) are: Carebara distincta, Carebara octata, Carebara pisinna, Carebara rara, Carebara robertsoni, Carebara sarita, Carebara sudanensis (queen), and Carebara termitolestes. They all share morphological characters that distinguish them from other Carebara species, i.e. nine antennal segments, mandibles with four teeth, metanotal groove not impressed, propodeum oblique in profile and declining towards posterior end without distinct angle, and propodeal teeth absent. Species of Carebara sensu stricto (definition before Fernández 2004, = lignata complex) are characterized by small workers and usually much larger queens, the workers usually with nine antennal segments, mandibles with three to four teeth, eyes and propodeal teeth absent, the propodeal dorsum often rounding into the posterior declivity without any angle. Species with matching morphologies are Carebara arnoldi, Carebara guineana, Carebara junodi, Carebara osborni, Carebara vidua, C. vidua var. fur, Carebara wheeleri (replacement name for Carebara silvestrii). Carebara ampla and its subspecies, Carebara bartrumi, Carebara langi, Carebara sicheli, and Carebara sudanica are all known from queens and/or males only, but their queens are usually very large and are morphologically close to C. vidua. It is unclear, however, how many species described by alates are synonymous with worker-based species. This has to be investigated in future studies, but colony collections with associated workers and alates are rare and difficult to achieve in a systematic way. Workers of Carebara fayrouzae, which occurs in Saudi Arabia, also have nine antennal segments and minor worker and queen morphologies closely match those of the above listed species in Carebara s. str. If they they should turn out to be closely related, then C. fayrouzae would be the first species in this group of which major workers have been found and described. In that case, it would not be unlikely that other species of Carebara s. str. and former Paedalgus are not monomorphic, but di- or even polymorphic as well. As Fernández (2004) pointed out, the currently available material is insufficient to answer this question. Workers of Carebara crigensis – described originally in its own genus Afroxyidris – which are morphologically similar to Carebara s. str., are characterized by antennae with ten segments, mandibles with two apical teeth plus one small basal tooth, eyes absent, and propodeum unarmed and rounded posteriorly.

Species from the phragmotica clade are part of a larger group of morphologically related species, which includes many taxa belonging to the former genus Oligomyrmex. Before its synonymisation under Carebara by Fernández (2004), Oligomyrmex was defined by possessing nine to eleven antennal segments (rarely eight), a markedly dimorphic worker caste, a well-developed metasternal process, anterior subpetiolar process present and radial cell of wing closed (Ettershank 1966). Including the two newly described species, workers of 30 valid species and subspecies of Afrotropical Carebara match this character combination. Twelve of them have nine antennal segments: Carebara alluaudi, C. alluaudi var. cataractae, Carebara angolensis, Carebara angolensis r. congolensis, Carebara convexa, Carebara donisthorpei, Carebara frontalis, Carebara jeanneli, Carebara latro, Carebara pumilia (replacement name for Carebara nana), Carebara santschii, and Carebara thoracica. It has to be noted though, that Weber (1952) later found specimens of C. thoracica with both, nine and ten antennal segments, as well as specimens that had nine segments on one antenna and ten on the other one. This character polymorphism seems to be not uncommon and can be observed in a few other species as well, calling to attention the relatively high plasticidy in some of the characters that are usually used for taxonomic delimitation.

One species, Carebara diabola, which was originally described in the genus Aneleus, has eleven antennal segments. Workers of the remaining 17 species have ten antennal segments, including Carebara elmenteitae, C. lilith and C. phragmotica. All of the latter three species are probably polymorphic, with the highly derived phragmotic majors as a distinct third subcaste, but the other two worker subcastes share many morphological characters with the other 14 species of this group. Some of them are easily distinguishable from the phragmotica clade, but several are strinkingly similar in their outer morphologies and only a complete taxonomic treatment will be able to draw more definitive species boundaries. In the following paragraph, is a short account of possibly related taxa, listing some supposedly stable characters that may be useful for their identification and delimitation.

Major workers of Carebara acuta are characterized by reticulate-punctate sculpture on head dorsum, with striae anteriorly, and propodeal teeth long and acute, minor workers without visible sculpture except for striae on anterior head (Weber 1952). The majors of Carebara africana (minors not described) without longitudinal rugulae on head, mandibles with six teeth, and propodeal teeth absent or reduced. Carebara arabica is morphologically very similar to phragmotica clade specimens, but its major workers are characterized by oblique posterior head corners with a distinct angle towards the median emargination, moderately large horns on the distal part of the posterior margin, and longitudinal rugulae moderately abundant, evenly spaced; the minor workers with reduced sculpture on meso- and metapleurae and absent or reduced propodeal teeth. Major and minor workers of Carebara arnoldiella are also lacking distinct propodeal teeth and head sculpture in major workers is strongly reduced and consists of only a few weakly developed rugulae. Head shape and sculpture of Carebara debilis major workers is very similar to C. phragmotica majors, but the type specimen possesses a very large median occelus, minute, rounded propodeal teeth, petiole longer than high and petiole node in profile widely convex, postpetiole in dorsal view very broadly elliptical, and some majors with only nine antennal segments; minor workers with very reduced propodeal teeth and only nine antennal segments. Carebara erythraea major worker’s head with very shallow posterior emargination, relatively few, short, longitudinal rugulae, frons and posterior sides almost smooth, propodeal teeth in major and minor workers not defined, but posterolateral lamella present. Major workers of Carebara incerta, Carebara khamiensis, and Carebara lucida are not described; their minors are characterized by absent or reduced propodeal teeth, short, in profile subtriangulate petiole with very short peduncle, mandibles in C. incerta and C. khamiensis with only four teeth, but five in C. lucida. The holotype of Carebara petulca, of which only the major worker is described, is characterized by densely rugulose head sculpture, a large median ocellus, small horns on the posterior head margin, relatively large eyes with six ommatidia, a distinct scutellum, and a high, posteriorly bluntly angled propodeum with two distinct teeth. Major workers of Carebara semilaevis, described in its junior synonym Carebara hewitti, are characterized by head densely rugulose, except for smooth anteromedian spot on frons, posterior head margin with very shallow emargination, horns absent, propodeal teeth in major and minor absent or reduced to rounded angles; minor workers petiole very short-pedunculate and about as high as long in profile. Carebara traegaordhi, Carebara ugandana, and Carebara vorax are described only from minor workers, the former two are defined by head and promesonotal dorsum mostly smooth and shiny, propodeal teeth absent or reduced to blunt angles, the petiole in C. traegaordhi being more compact than in C. ugandana, with shorter peduncle, and petiole node in profile more broadly convex and petiole ventrally convex. The head and body of C. vorax minors are covered with punctate-reticulate sculpture except for smooth spot anteromedially on frons and the anterior of pronotum, propodeum with well-developed lamella and teeth at its posterior corners, petiole relatively long-pedunculate.