Messor

Hita Garcia, Wiesel and Fischer (2013) - The more than 160 species (Bolton, 2012) are distributed across the Holarctic, Afrotropical, and Oriental regions, with its highest diversity found in the Palaearctic region. Messor is a genus of primarily granivorous ants that play an important role in seed dispersal. These ants are commonly encountered in savannahs, grasslands, or even more arid habitats like semi-deserts and deserts (Bolton, 1982).

Identification
Bolton (1981) - The closest relatives of Messor are the genera Aphaenogaster and Pheidole Westwood. Members of the latter genus are easily separated from Messor as the palp formula is reduced to 2,2, its species are dimorphic, and the antennal funiculus ends in a strongly defined 3-segmented club. Aphaenogaster, which is absent from sub-Saharan Africa, is more difficult to differentiate as its species, apart from being uniformly monomorphic, are very close to Messor and share most of its diagnostic characters, including the filiform to feebly clavate funiculi and high palp formula (PF) count. Of 55 species of Aphaenogaster dissected 31 had PF 5,3, and 24 had PF 4,3. For some reason, although species with the higher PF apparently outnumber those with the lower count, the zoogeographical distribution of the latter is much wider than that of the former. Aphaenogaster species with PF 5,3 are found in the Nearctic, Palaearctic and Oriental regions; species with PF 4,3 are also found in these three regions and in the Neotropical, Malagasy, Indo-Australian and Australasian regions as well.

Messor

Mostly polymorphic species (a very few feebly polymorphic and monomorphic species known).

Mostly with ammochaete hairs present (reduced in a few species).

Head massive and broad, in medium to large workers CI > 90 (range 95-125 in 64 species measured).

Metasternal process large to very large, always very conspicuous (45 species dissected).

Outer margins of mandibles strongly curved towards midline, the mandibles massive and heavy.

Aphaenogaster

Entirely monomorphic.

Mostly without ammochaete hairs (present in a very few species).

Head usually slender, CI 90 at maximum, generally much less (range 49-90 in 75 species measured).

Metasternal process small to absent, approaching size seen in Messor only in A. subterranea (55 species dissected).

Outer margins of mandibles not strongly curved towards midline, the mandibles triangular in shape and not massive.

Distribution
Bolton (1982) - The main base of the genus is in the Palaearctic region where about 70-80 species occupy a broad strip of territory reaching across the whole width of North Africa and the southern European countries, across the Near and Middle East and thence eastwards through the U.S.S.R. to China and Japan. Compared to this the faunas of other zoogeographical regions are relatively minor. The Afrotropical region has 12 species and Madagascar has 1; the Oriental region has 3-4 species and the Nearctic has 8, all distributed on the western side of the continent and formerly occupying a genus of their own, Veromessor, now synonymized. Species of Messor are absent from the Neotropical region, the Indo-Australian region and Australasia, nor do they occur on any of the Pacific island systems.

Fossils
Fossils are known from:.

Biology
Messor is a moderately sized genus of granivorous ants occurring in grassland and savannah, and in arid to desert situations. Knowledge of the detailed biology of the species is sparse, but good basic work has been done on some African species by Levieux & Diomande (1978), and Levieux (1979).

Species Uncertain
Unknown species of Messor are hosts for the eucharitid wasps and  (primary parasites) (Universal Chalcidoidea Database), and adults of the cricket  (Stalling & Cassar, 2020).

Castes
Polymorphic

Salata and Borowiec (2019) - A taxonomically challenging genus, Messor workers are polymorphic and very often the shape of the head, its sculpture cover and intensity, and the shape of propodeal convexity depend on the size of the specimen. Minor workers usually have sculpture weaker or, in some cases, even absent and the propodeum more or less rounded without special angulations or denticles, while major workers of the same species can bear very dense and robust sculpture and often distinct propodeal angulations or denticles (Steiner et al., 2018).

Worker Polymorphism
Salata and Borowiec (2019) - Workers are polymorphic and very often the coverage and intensity of sculpturing, the shape of the head, and the shape of propodeal convexity are size-dependent. Minor workers usually have weaker sculpture or, in some cases, it is not present. The propodeum of the minor is more or less rounded without special angulations or denticles, while major workers of the same species can bear very dense and robust sculpture and often distinct propodeal angulations or denticles (Steiner et al., 2018).

Nomenclature

 *  MESSOR [Myrmicinae: Pheidolini]
 * Messor Forel, 1890a: lxviii [as subgenus of Aphaenogaster]. Type-species: Formica barbara, by subsequent designation of Bingham, 1903: 277.
 * Messor subgenus of Aphaenogaster: Dalla Torre, 1893: 98; Forel, 1899c: 59.
 * Messor subgenus of Stenamma: Emery, 1895c: 298; Forel, 1903a: 693.
 * Messor raised to genus: Bingham, 1903: 277.
 * Messor senior synonym of Cratomyrmex: Emery, 1924d: 357; Bolton, 1982: 338.
 * Messor senior synonym of Veromessor (and its junior synonym Lobognathus): Bolton, 1982: 338.
 * CRATOMYRMEX [junior synonym of Messor]
 * Cratomyrmex Emery, 1892d: 572. Type-species: Cratomyrmex regalis, by monotypy.
 * Cratomyrmex subgenus of Messor: Santschi, 1920d: 378.
 * Cratomyrmex junior synonym of Messor: Emery, 1924d: 357.
 * Cratomyrmex revived from synonymy: Bernard, 1971: 6.
 * Cratomyrmex junior synonym of Messor: Bolton, 1982: 338.
 * LOBOGNATHUS [junior synonym of Messor]
 * Lobognathus Enzmann, J. 1947b: 152 [as subgenus of Veromessor]. Erroneous entry for Veromessor lobognathus and hence junior synonym of Veromessor: Brown, 1949a: 49
 * Lobognathus junior synonym of Messor: Bolton, 1982: 338.
 * VEROMESSOR [junior synonym of Messor]
 * Veromessor Forel, 1917: 235 [as subgenus of Novomessor]. Type-species: Aphaenogaster andrei, by subsequent designation of Emery, 1921f: 67.
 * Veromessor raised to genus: Wheeler, W.M. 1922a: 661.
 * Veromessor senior synonym of Lobognathus: Brown, 1949a: 49.
 * Veromessor junior synonym of Messor: Bolton, 1982: 338.
 *  SPHAEROMESSOR [unavailable name]
 * Sphaeromessor Bernard, 1985: 48. Unavailable name. Proposed without designation of type-species and therefore unavailable. Species included by Bernard (1985) are all referable to Messor: Bolton, 1995b: 46.

Bolton (1982):

Recent studies of Messor include those of Arnoldi (1977) on the fauna of the U.S.S.R., and Collingwood (1978) on the species of the Iberian Peninsula. The only previous synthesis of sub-Saharan African species is that of Arnold (1920), for the then-recognized South African forms, but no key was given in that revue. Creighton (1950) has keyed the North American species formerly in Veromessor.

Worker
Granivorous myrmicine ants, mostly strongly polymorphic but a few monomorphic or only weakly polymorphic. Head massively constructed in larger workers. Mandibles large and powerful, multidentate in smaller workers (up to 15 teeth) but this number usually decreasing with increased body size until in largest workers only a few massive teeth or an edentate crushing edge remains. Sometimes also in small workers the teeth are worn down to an edentate margin. Palp formula predominantly 4,3 but in largest workers usually 5,3 (30 species dissected). Median portion of clypeus broad and shield-like, broadly inserted between the widely separated frontal lobes; both median and lateral portions of clypeus unmodified except for a central impression of the anterior margin in some species. Frontal lobes short but conspicuous, at least partially concealing the antennal insertions. Frontal carinae absent. Antennal scrobes absent. Antennae 12-segmented, either filiform and without an apical club (in which case the flagellar segments gradually increase in size apically), or with a feebly defined incipient club where the apical 3-4 segments are slightly enlarged. Eyes present, moderate to large in size, situated at or just behind the midlength of the sides in full-face view. Ventral surface of head with elongate ammochaete hairs which usually form a psammophore. This may be reduced and non-functional in some species but the hairs are still conspicuous and generally longer than those found elsewhere on the body; in a few species the psammophore is better developed in smaller than in larger workers. With the alitrunk in profile the promesonotum swollen and convex, frequently dome-like and sloping down steeply behind to the metanotal groove which is weakly to distinctly impressed. Propodeum rounded to strongly bispinose posteriorly and on a much lower level than the convex promesonotum. Promesonotal suture fused and inflexible but its track represented by a distinct arched impression across the dorsum. Mesonotum bounded by impressions on all sides, its boundary easily discernible except in the smallest workers of a few species. Metapleural lobes absent or at most represented by a pair of low broadly rounded ridges. Propodeal spiracle large and conspicuous, circular to subcircular and situated approximately at the midlength of the propodeum or sometimes slightly behind the midlength, but never shifted conspicuously back towards the declivity. Basal posterior portion of mesopleuron just above the middle coxa with a few hairs projecting downwards and backwards. (Whether these are guard-hairs indicating the exit site of a gland is not known, but the hairs remain even in species where other body pilosity is very reduced or absent.) Spurs on posterior tibiae varying from very feebly pectinate through partially barbate and minutely barbulate to simple. Alitrunk ventrally with a strong metasternal process which is usually large to very large (reduced but still conspicuous only in Messor rufotestaceus and Messor vaucheri out of 45 species dissected). Petiole with a long anterior peduncle, the spiracle situated at about the midlength of the peduncle, well in front of the node. Petiole node in profile narrow and often bluntly triangular to conical in shape, but frequently a sloping differentiated dorsal surface is present where the anterodorsal angle is generally the highest point.