Tetramorium solidum species group

Based on Bolton 1980 and Mbanyana et al. 2018

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Diagnosis

Mbanyana et al. (2018) - The following diagnosis, which is based on Bolton (1980), distinguishes the group from all other groups in the Afrotropical region: relatively large Tetramorium (HW 0.875–1.279 mm, WL 0.915–1.498 mm) with very well-developed, massively constructed heads (HW > 0.80 mm and in most cases exceeding 1.00 mm) equipped with strong mandibles; 12-segmented antennae; anterior clypeal margin with deep and wide median indentation, sometimes occupying half of the anterior clypeal margin, except for Tetramorium barbigerum, in which it is more shallowly impressed; large eyes (OI 24–26); frontal carinae absent or very short, if present ending before anterior eye level; frontal scrobes absent; ventral face of head (usually) with J-shaped ammocheate hairs; base of first gastral tergite always sculptured, even if weakly so.

Bolton (1980) - Antennae with 12 segments. Sting appendage dentiform to elongate triangular. Head massively constructed, HW > 0.80, often exceeding 1.00, equipped with powerful mandibles which are usually strongly sculptured. Anterior clypeal margin at least with a median notch or impression, often with an extensive semicircular emargination which may be very deep. Ventral surface of head usually with very long, conspicuous ammochaete hairs which in most species form a psammophore (absent only in dichroum). Frontal carinae very short or absent, when present ending in front of the level of the anterior margins of the eyes; often terminating just behind the frontal lobes. Antennal scrobes absent. Ventral margin of eye more or less flat, the anterior, dorsal and posterior margins curved so that the eye in profile resembles a reclinate letter D. Metapleural lobes short, low and rounded. Petiole node in dorsal view always broader than long. Pilosity on dorsal alitrunk, pedicel segments and first gastral tergite either absent (solidum-complex) or bizarre (setuliferum-complex), dense only in peringueyi-complex. Dorsal (outer) surfaces of hind tibiae with appressed pubescence in all but peringueyi and dichroum where short erect hairs are present. Base of first gastral tergite sculptured, even if only faintly shagreened; never completely smooth and shining.

Species complexes

Bolton (1980) - The group falls neatly into three complexes of closely related species based on the form of the pilosity which they show.

The first complex includes only peringueyi and dichroum, characterized by an abundant coat of short, stout erect hairs on all dorsal surfaces of the head and body, and also on the legs. Distribution of these hairs separates the two as in peringueyi erect short hairs are present on the antenna} scapes but absent from them in dichroum. T. dichroum is unique in the group as, although elongate hairs are present on the ventral surface of the head, there are no specialized ammochaete hairs.

In the second complex, which includes clunum, galoasanum and setuliferum, erect hairs are absent from the dorsal surfaces of the alitrunk, pedicel segments and first gastral tergite, but instead bizarre appressed pilosity is present. This consists of short, flattened, blunt hairs which are closely appressed to the surface of the sclerite on which they arise and which are usually silvery in colour and glittering in direct light.

The final and largest complex consists of barbigerum, glabratum, grandinode, jordani, pogonion, rufescens, signatum and solidum, in which the body is hairless or nearly so. Only solidum retains a few hairs on the dorsal alitrunk but none of the species have any hairs on the dorsal pedicel segments or the first gastral tergite. This complex also shows the strongest development of the psammophore and has the sculpture of the head and body finer than in other complexes of the group. Of the species included some have very definite diagnostic characters, such as absence of propodeal spines (jordani), presence of one or two pairs of erect fine hairs on the alitrunk (solidum), uniquely shaped pedicel segments (grandinode) or red colour (glabratum, rufescens) as opposed to the uniform black or blackish brown found in most species of the complex. Separation of the remainder rests on characters such as head shape, size of eyes, form of sculpture and overall size.

Mbanyana et al (2018) commented on these complexes but did not assign any of their newly described species into any of these complexes.

Notes

Mbanyana et al (2018) - Members of the Tetramorium solidum group are ground-nesting, seed-harvesting ants, mainly restricted to dry semi-desert areas of southern Africa. These ants appear to be arid-adapted and their distribution closely reflects current rainfall patterns in Africa.

Most of the species in this group occur in the southwestern parts of the Afrotropical region, where there is less rainfall, avoiding the moist grassland, savannah, woodland, and forest vegetation types in the east. The western region of southern Africa experiences an annual rainfall of about 100 mm to nearly 500 mm per annum (Venter et al. 1986). In addition, at least eight species were recorded from Kalahari Xeric Savanna and seven from the extreme dry regions of Namibia. Soil might also be influencing the distribution of these species. Tetramorium jordani is restricted to loose sandy areas and T. solidum extends more into the clay soils. Two species in the group have a wider distribution and are found further north in Africa. Bolton (1980) revised the group and listed 14 species. Other than this revision, and the recording of particular species in various ant surveys (e.g., Robertson 2000; Parr et al. 2002; Netshiliphala et al. 2005), very little has been published on the Tetramorium solidum group, despite the group’s apparent ecological importance as seed harvesters in semi-arid habitats in southern Africa. The highest diversity of species is concentrated in the Nama Karoo with 14 species recorded from this region. Nine species were recorded from the Succulent Karoo region, which is considered to be a major biodiversity hotspot (Myers et al. 2000).

Tetramorium rothschildi is the only species in the T. solidum group with branched hairs and the only species that has an equatorial distribution. We are not convinced that T. rothschildi is a genuine member of the group and hope that a currently ongoing molecular phylogenetic analysis will resolve the true relationship of T. rothschildi. Defining species in the T. solidum group based on morphology alone is very challenging due to the intraspecific variation in most species. High levels of intraspecific variation in T. signatum and T. rufescens may indicate the presence of several cryptic species. The issue of cryptic species is prevalent in many taxa (e.g., Bickford et al. 2007; Molino et al. 2011; Crespo & Lumbsch 2010) including ants (Seifert 2009; Schlick-Steiner et al. 2006).