Nesomyrmex species groups of the Afrotropical and Malagasy regions

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There is a Key to the Nesomyrmex species groups of the Malagasy region and a Key to the Nesomyrmex species groups of the Afrotropical region

N. angulatus species-group


Key to Afrotropical Nesomyrmex angulatus group workers

Afrotropical - Antennae with 12 segments; anterior clypeal lobe conspicuously convex, rounded and without a small median triangular projection; frontal carinae absent; propodeum always with moderately long to long spines, in profile distinctly longer than their basal width; petiole and postpetiole without large lateral spines, petiole sometimes with small denticles; all dorsal surfaces of body with short, blunt pilosity (with the exception of N. evelynae, which lacks standing hairs on most of first gastral tergite).

This group contains seven species that are widely distributed in the Afrotropical region, with one species extending into the Palaearctic and Malagasy regions. All species appear to be arboreal or subarboreal. The alpha taxonomy of the group appears straightforward based on the literature (Bolton 1982; Mbanyana & Robertson 2008). However, we find some species delimitations quite problematic and it might be necessary to revise the N. angulatus species group in the future after the accumulation of more material from additional Afrotropical localities.

Key to Malagasy Nesomyrmex angulatus group workers

Malagasy - Pronotal spines present or absent. Anterodorsal spines on petiolar node absent. Propodeal spines short to long and acute. Vertex ground sculpture areolate. Main sculpture on vertex not defined. Metanotal depression present or absent. Median clypeal notch present or absent. Median clypeal notch shape/depth: 0–23 μm. Antennomere count: 12. Absolute cephalic size (CS): 591 μm [418, 946]. Cephalic length vs. maximum width of head capsule (CL/CWb): 1.218 [1.057, 1.490]. Postocular distance vs. cephalic length (PoOc/CL): 0.40 [0.359, 0.444]. Scape length vs. absolute cephalic size (SL/CS): 0.676 [0.519, 0.866]. Eye length vs. absolute cephalic size (EL/CS): 0.260 [0.193, 0.317]. Petiole width vs. absolute cephalic size (PEW/CS): 0.431 [0.330, 0.522]. Postpetiole width vs. absolute cephalic size (PPW/CS): 0.496 [0.361, 0.585]. Petiolar node height vs. absolute cephalic size (PEW/CS): 0.250 [0.185, 0.311].

N. brevicornis species-group


Key to Nesomyrmex brevicornis group species

Malagasy

This group is split from the Nesomyrmex madecassus-group (sensu Csősz and Fisher 2015) based on several features, such as the remarkably deep mesopropodeal depression, the short scape (SL/CS: 0.49–0.72, in contrast to SL/CS of madecassus-group species: 0.72–0.83), the short and high petiole (PEL/ CS: 0.34–0.49, in contrast to that of madecassus-group species: 0.44–0.58), the relatively long petiolar node (NOL/CS: 0.20–0.34, in contrast to that of madecassus-group species: 0.28–0.39, and the shorter mesosoma (ML/CS: 1.08–1.36, in contrast to that of madecassus-group species: 1.26–1.50) that distinguish workers of brevicornis-group from those of madecassus-group species.

All species of the brevicornis-group are arboreal; most nest in twigs or hollow branches, but some have very specialized associations with live stems or particular species of trees. Nesomyrmex cingulatus, for example, is known only from live stems from spiny thicket near Tsimanampetsotsa. Nesomyrmex minutus sp. n. is unique in nesting only in the stems of Macphersonia gracilis (Sapindaceae) and represents one of the few known ant plants in Madagascar.

N. cataulacoides species-group

Afrotropical - Antennae with 11 segments; anterior clypeal lobe conspicuously convex and rounded, without a small median triangular projection; frontal carinae absent; propodeal spines very well developed, long and spiniform; petiole and postpetiole each with a pair of large and conspicuous lateral spines; all dorsal surfaces of body without standing pilosity.

The N. cataulacoides species group holds only one morphologically bizarre and extraordinary species, which is impossible to confuse with any other Nesomyrmex species from the Afrotropical or any other region. The 11-segmented antennae, lack of standing pilosity on all dorsal surfaces, and the extreme spinosity render N. cataulacoides immediately recognizable. The possession of spines/teeth on the anterior pronotal corners, the anterior and posterior propodeum, and on both waist segments is unique within the genus. The affinities of N. cataulacoides to other Nesomyrmex are difficult to ascertain, mostly due to the extreme morphological specializations. So far it is not possible to associate it closely with any other Nesomyrmex species or species group. Currently, the species is known only from Cameroon and Kenya. Based on the few collections available, this species is strictly arboreal. The observed disjunctive distribution is most likely a sampling artefact due to the scarcity of collecting in Central African canopies, and we expect that N. cataulacoides will be collected in intermediate countries in the future.

N. hafahafa species-group

Key to Nesomyrmex hafahafa group species

Malagasy - The three pairs of dorsal spines (pronotal spines, propodeal spines and antero-dorsal spines on petiolar node) makes the appearance of this group extremely unique; no similar species group has been found either in the Malagasy region or on the African continent.

Pronotal spines present. Anterodorsal spines on petiolar node present. Propodeal spines long and acute. Vertex ground sculpture areolate. Vertex main sculpture rugulose. metanotal depression absent. Median clypeal notch present. Median clypeal notch shape/depth: 15–31 μm. Antennomere count: 12. Absolute cephalic size (CS): 1059 μm [930, 1200]. Cephalic length vs. maximum width of head capsule (CL/CWb): 1.074 [1.0, 1.143]. Postocular distance vs. cephalic length (PoOc/CL): 0.378 [0.342, 0.403]. Scape length vs. absolute cephalic size (SL/CS): 0.890 [0.835, 0.984]. Eye length vs. absolute cephalic size (EL/CS): 0.232 [0.210, 0.264]. Petiole width vs. absolute cephalic size (PEW/CS): 0.267 [0.203, 0.353]. Postpetiole width vs. absolute cephalic size (PPW/CS): 0.523 [0.430, 0.586]. Petiolar node height vs. absolute cephalic size (PEW/CS): 0.142 [0.107, 0.186].

N. humerosus species-group

Afrotropical - Antennae with 12 segments; anterior clypeal lobe short, flat-margined, and never convex, with small median triangular projection; frontal carinae present, but weakly developed; propodeal spines very well developed, long and spiniform; petiole and postpetiole without lateral spines; all dorsal surfaces of body with short, blunt pilosity.

This group contains only one species, which is morphologically quite unique in the Afrotropical region. Of special importance are the short, flat-margined anterior clypeal lobe with a small median triangular projection and the barrel-shaped petiolar node with its small, triangular node. These characters are in slightly modified ways also seen in several Neotropical and Malagasy species, while they are absent in the other Afrotropical species groups. However, the fact that N. humerosus shares these characters with species from other regions does not necessarily mean that N. humerosus is more closely related to them. It could also be an independent African lineage and similarities with species from other regions might be based on convergence. Despite the fact that N. humerosus does not resemble most species from the N. simoni group, it still shares characters with some species, such as the large eyes and the shape of the dorsal mesosomal outline, and it could be that N. humerosus is just an aberrant N. simoni group member. At present, it is not possible to deduce the biogeographical and phylogenetic affinities of this peculiar species. Currently, N. humerosus is known to occur in Kenya, Tanzania and Yemen. It is a rather rarely collected species and our scarce knowledge is based on just four collection events. Based on a sample collected in Kenya by the first author, it seems to live on vegetation, but it was also sampled from the ground in Tanzania and Yemen. It is possible that the species also occurs in other East African countries, such as Somalia and Mozambique, which are greatly under-sampled.

N. madecassus species-group

Pronotal spines absent. Anterodorsal spines on petiolar node absent. Propodeal spines short, lamelliform to absent. Vertex ground sculpture smooth. Vertex main sculpture not defined. Metanotal depression present. Median clypeal notch present or absent. Median clypeal notch shape/depth 0–15 μm. Antennomere count: 12. Absolute cephalic size (CS): 571 μm [405, 785]. Cephalic length vs. maximum width of head capsule (CL/CWb): 1.231 [1.092, 1.567]. Postocular distance vs. cephalic length (PoOc/ CL): 0.479 [0.407, 0.544]. Scape length vs. absolute cephalic size (SL/CS): 0.718 [0.492, 0.831]. Eye length vs. absolute cephalic size (EL/CS): 0.249 [0.1934, 0.279]. Petiole width vs. absolute cephalic size (PEW/CS): 0.217 [0.181, 0.256]. Postpetiole width vs. absolute cephalic size (PPW/CS): 0.331 [0.243, 0.398]. Petiolar node height vs. absolute cephalic size (PEW/CS): 0.122 [0.072, 0.158]. somyrmex madecassus and ca. seven other taxa from the Malagasy zoogeographical region will be revised in the forthcoming revisionary work.

Key to Malagasy Nesomyrmex madecassus species-group workers

N. sikorai species-group

Key to Malagasy Nesomyrmex sikorai group workers

Malagasy - Pronotal spines present or absent. Anterodorsal spines on petiolar node absent. Propodeal spines short to long and acute. Vertex ground sculpture not defined. Vertex main sculpture areolate. Metanotal depression present. Median clypeal notch present or absent. Median clypeal notch shape/depth 0–15 μm. Antennomere count: 12. Absolute cephalic size (CS): 750 μm [634, 890]. Cephalic length vs. maximum width of head capsule (CL/CWb): 1.218 [1.075, 1.382]. Postocular distance vs. cephalic length (PoOc/CL): 0.461 [0.411, 0.511]. Scape length vs. absolute cephalic size (SL/CS): 0.816 [0.761, 0.872]. Eye length vs. absolute cephalic size (EL/CS): 0.232 [0.201, 0.284]. Petiole width vs. absolute cephalic size (PEW/CS): 0.243 [0.206, 0.326]. Postpetiole width vs. absolute cephalic size (PPW/CS): 0.359 [0.306, 0.426]. Petiolar node height vs. absolute cephalic size (PEW/CS): 0.175 [0.149, 0.205]. Nesomyrmex sikorai (Emery, 1896), Nesomyrmex retusispinosus (Forel, 1892) plus ca. ten more Malagasy species will be revised in a forthcoming revisionary work.

N. simoni species-group

Afrotropical - Antennae with 12 segments; anterior clypeal lobe conspicuously convex and rounded, without a small median triangular projection; frontal carinae absent; propodeum usually unarmed, rarely with short teeth, in profile no longer than their basal width; petiole and postpetiole without lateral spines; usually all dorsal surfaces of body with (mostly) long, fine or (rarely) short, blunt standing pilosity, sometimes pilosity reduced on a few body parts, but never completely absent from all dorsal surfaces.

The 17 species of this group are all endemic to South Africa. In contrast to the members of most other Afrotropical groups, all N. simoni group species nest and live on the ground. Mbanyana & Robertson (2008) revised this group extensively, provided a sound and functional species identification key, and presented detailed descriptions of all species. Consequently, in this study we do not go into further details concerning the N. simoni group and refer to Mbanyana & Robertson (2008).

References