Monomorium salomonis species group

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based on Bolton, 1987



Worker Monomorphic, usually with some size variation in any series but without allometric variation. Palp formula 2,2 (albopilosum, areniphilum, afrum, bicolor, damarense, drapenum, indicum, junodi, minor, niloticum, marshi, pharaonis, rufulum, salomonis, subopacum, sutu, viator, westi, reduced to PF 1,2 in some minute species (osiridis, rabirium, by in situ count) and in the socially parasitic noualhieri. Mandibles sculptured except in a few, usually very small, species. Masticatory margins of mandibles with 4 teeth which decrease in size from apex to base , the basal tooth not reduced to a minute denticle except in rufulum. Trulleum small to obliterated, when present either open or closed. Median portion of clypeus raised, projecting forward anteriorly, bicarinate to rounded along lateral margins of raised portion. Median portion of clypeus posteriorly broader than either of the frontal lobes where it passes between them. Anterior clypeal margin without a widely separated pair of teeth although anterior ends of clypeal carinae may be denticulate or sharply pointed in some species. Cephalic dorsum usually sculptured (not so in only a very few species), the sculpture ranging from dense blanketing reticulate-punctuation to faint superficial reticular patterning. Eyes distinct and moderate to large in size (0.19-0.40 x HW), generally with 6 or more ommatidia in the longest row; eyes situated at or very close to the midlengths of the sides of the head. Eyes circular to roughly oval, never reniform nor extended anteroventrally into a lobe. Head always longer than broad (CI < 90) and scapes usually relatively long (SI > 90, with very few exceptions). Antennae with 12 segments, terminating in a club of 3 segments. Metanotal groove moderately impressed to absent. Metanotal cross-ribs inconspicuous to absent; when present often short and masked by other sculpture. Propodeal spiracle circular to subcircular. Propodeum rounded to angular between dorsum and declivity, rarely the angle weakly dentate. Propodeal dorsum usually sculptured but never transversely striate; only very rarely the dorsum smooth. Petiolar spiracle at the node or immediately in front of the anterior face of the node. Body pilosity very variable in distribution and density, but with a marked tendency to reduce the pilosity, especially on the head and alitrunk. Alitrunk, petiole and postpetiole usually conspicuously sculptured. First gastral tergite frequently shagreenate or otherwise finely sculptured. (Workers examined: all included in this revision plus the following extralimital species. abeillei, algiricum, buxtoni, dichroum, hesperium, indicum, longi, medinae, niloticum, pallidus, salomonis, schurri, salomonis subnitidum, venustum, wroughtoni, plus 12 indeterminate species.)

Female Characters generally as worker but female much larger; only slightly larger than the conspecific male. Eyes usually larger than in worker, at or only slightly behind midlength of sides. Ocelli present except in extreme ergatoids where the eyes are also reduced to worker-size. Antennae with 12 segments, the apical club of 3 segments or rarely of 4 (effractor, santschii). In afrum the club is feebly 4-segmented as the eighth funicular segment is moderately enlarged. HW slightly to very distinctly greater than maximum width of mesoscutum. In very few females the two of about equal width but in ergatoids the head conspicuously wider than the mesoscutum. AIitrunk usually winged in virgins and with a full complement of flight sclerites, but several apterous or ergatoid forms are known which fail to develop wings and show a serial reduction of flight sclerites. (Apterous to ergatoid females are known in bicolor, albopilosum, rufulum, venustum, opacior, medinae, minor, hesperium, dichroum, advena, algiricum, damarense, biroi, grassei, libanicum, syriacum, of which the last seven are extreme ergatoids.) A few species are known which may produce both apterous and alate females. Mesoscutum very prominent and bulging anteriorly in some known or suspected social parasites (afrum, effractor, santschii), strongly overhanging the pronotum. Parapsidal grooves varying from conspicuous to absent. Axillae usually triangular in alate forms, separated mid-dorsally by a small gap. Axillae fused to mesoscutum in many apterous and ergatoid forms. Petiole and postpetiole varying from subconical and nodiform respectively to both strongly anteroposteriorly compressed. Forewings with cross-vein m-cu absent. Head and alltrunk conspicuously sculptured; first gastral tergite usually also sculptured, even if only faintly so. (Females examined: advena, afrum, albopilosum, algiricum, areniphilum, bicolor, damarense, delagoense, dichroum, dictator, effractor, herero, hesperium, indicum, junodi, medinae, minor, ocellatum, opacior, pharaonis, rufulum, salomonis, santschii, subdentatum, subnitidum, subopacum, venustum.)

Male About same size as or slightly smaller than conspecific female , much larger than worker. Palp formula 2,2 in all examined (perhaps reducing to PF 1,2 in minute species). Mandibles strongly developed and 4-dentate except in pharaonis-complex where they are relatively small and have 2-3 teeth. Scape cylindrical or subcylindrical, varying in length but usually quite short, about equal in length to funiculus segment 2, or slightly longer. First funicular segment not globular, remaining funicular segments not whip-like, but tapering apically in excelsior. Head capsule wider behind eyes than in front, maximum width of head about equal to maximum width of mesoscutum. Eyes large and approximately at midlength of sides; always a distinct space between the eye and the mandible, the eye not touching the clypeus. Ocelli large, not born on a turret and not breaking the outline of the occipital margin. Mesoscutum overhanging pronotum anteriorly, strongly produced and bulging forward in socially parasitic species (effractor, santschii). Notauli absent but mesoscutum with a narrow V-shaped anteromedian area which is only weakly sculptured or is smooth. Parapsidal grooves present to absent. Axillae small, triangular in dorsal view and separated dorsally by a groove; axillae usually fused to scutellum, sometimes also fused to mesoscutum. Propodeal spiracle far forward, in front of midlength of sclerite. Wings present in all known males, with cross-vein m-cu absent. Head and alitrunk sculptured, usually densely so. Predominant sculpture is reticulate-punctation throughout. First gastral tergite usually finely sculptured. Genitalia large to massive, partially retractile and bizzarely modified in some species. (Males examined: afrum, albopilosum, bicolor, delagoense, effractor, excelsior, indicum, junodi, medinae, minor, pharaonis, rufulum, santschii, subopacum, ocellatum, viator.)

Species Complexes

Afrotropical species-complexes of the salomonis-group (based on workers)

Note that distribution and density of standing pilosity is important in the species-level taxonomy of this group. Old or abraded specimens should be treated with circumspection. The complexes are based on the worker caste as relatively few females and males are known.

areniphilum complex

The areniphilum-complex. The single Afrotropical species currently occupying this complex, areniphilum, is recognized by its cephalic sculpture, which is finely reticulate to reticulate-shagreenate but usually overlaid by very fine dense scratch-like longitudinal striation, by its large eyes, and by its characteristically shaped alitrunk outline. The antennal scapes in areniphilum are of moderate length (SI 98-104) and the first gastral tergite has at most a single pair of standing hairs in front of the apical transverse row.

M. areniphilum is circum-Saharan in distribution and several related forms occur in North Africa. The taxonomy of the North African and Middle Eastern forms related to areniphilum is very confused and much in need of study.

Lush (2008) added a second species, subcomae, to this complex.

australe complex

A complex of 12 small to minute species (HW 0.42-0.57) in the salomonis-group in which the head is opaque, shagreenate-granular to punctulate-shagreenate, and frequently with the mid-dorsal strip of the head showing extremely fine longitudinal scratch-like striolae. The sculpture of the whole head usually has a smeared or silky appearance under low magnification as it is so fine. This form of cephalic sculpture is strongest developed in carbo, darkarense, minor, damarense, parvinode, opacior and sutu, tending to be somewhat fainter in disertum, and reduced in australe, anceps and termitarium. The first gastral tergite retains hairs in front of the apical transverse row (except in some samples of damarense), which may be evenly dispersed on the sclerite or restricted to 2-3 pairs on the basal half. The dorsal alitrunk lacks standing hairs in all species. Eyes are of moderate size (0.24-0.31 x HW except in sutu where they are large (0.35-0.38 x HW), and frequently are slightly in front of the midlength of the sides.

Of the 12 included species 7 occur only in the countries of southern Africa. Two, carbo and parvinode, are only known from Ethiopia and Sudan, one (sutu) is Kenyan, and dakarense is known from a single sample from Senegal.

Also included in this complex is senegalense, a nomen dubium. No type-material of this enigmatic form appears to have survived, but from Roger's (1862) brief description the species seems to fall here. It is just possible that senegalense may be a senior synonym of dakarense. Male and female are known for minor, females also for damarense and opacior; all other sexuals remain unknown.

bicolor complex

The complex contains species characterized by their distinctive colouring and sharply defined dense sculpture. The head and alitrunk are orange-yellow to red and the gaster dark brown to black, the latter often with steely or bluish reflections. The two colour zones contrast strongly. All surfaces of the head and alitrunk are blanketed by fine and very dense reticulate-punctate sculpture in which the individual punctures are small but very sharply defined. The sculpture is not reduced or effaced anywhere and the cephalic punctures do not have a smeared or run-together appearance. Standing hairs are always present on the first gastral tergite in front of the apical transverse row. Such hairs are usually absent from the dorsal alitrunk but are numerous in hirsutum and one pronotal pair may occur in some samples of rufulum.

Members of the complex are closest related to those of the opacum-complex, with which they share the same pilosity and form of sculpture. In the opacum-complex, however, the body is uniformly brown or black, or dark brown with a darker gaster; the contrasting colours of the bicolor-complex are not developed. Ignoring the difference in colour these two complexes could easily be combined, so closely are they related.

Of the species in the bicolor-complex four are of relatively limited distribution. M. hirsutum is known only from Ethiopia, westi from Kenya, personatum and dictator from Angola. M. bicolor is very widely distributed in the northern, western, central and eastern portions of the Afrotropical region, and also occurs in the southern Palaearctic, where it is replaced in part by a sibling-species, nitidiventre, around the eastern end of the Mediterranean. The final species, rufulum, is widely distributed in southern Africa and apparently replaces bicolor in Angola, Namibia, Botswana and Zimbabwe (where it was recorded as nitidiventre by Arnold (1916)). All appear to be species of savannah, arid zones, or forest clearings where there is well-drained soil and direct insolation.

M. bicolor was, prior to this study, one of the overworked 'form-species' previously much used in the salomonis-group. Any red and black Monomorium of the salomonis-group was described and appended to bicolor, forming a welter of infraspecific forms which quickly swamped the nomenclature and rendered accurate identification impossible. Of these names hirsutum, dictator, personatum, and dakarense are now regarded as valid species in the salomonis-group (the first three in the bicolor-complex); ebangaense is a valid species but belongs to the setigerum-group. The remainder are synonyms of the various bicolor-complex species.

Males are known for bicolor and rufulum, females for these two species and dictator; sexual forms otherwise unknown.

mediocre complex

A convenience complex to hold five southern African and one Kenyan species which lack pilosity on the alitrunk and on the first gastral tergite in front of the apical transverse row, and which may even lack the apical row itself. All show very reduced sculpture everywhere. In all species cephalic sculpture is represented by faint to vestigial superficial reticular patterning, similar to but fainter than that seen in the much larger species of the tchelichofi-complex.

The four small species esharre, osiridis, rabirium, and zulu have eyes which are in front of the midlength of the sides of the head, a condition very rare in the salomonis-group as defined here but characteristic of the setuliferum-group. In esharre the eyes remain close to the midlength, but in the other three the forward shift is obvious. It seems most probable that these species have acquired this character independently of the setuliferum-group as otherwise they do not conform to the diagnosis of that group. However, as the setuliferum-group itself is essentially a catch-all group, holding complexes of species which do not easily fit elsewhere, judgement on the correct placement of esharre and its allies must be deferred, pending further investigation.

In situ count of the palp formula shows that osiridis and rabirium have a reduced PF 1,2, rather than the PF 2,2 usual in the salomonis-group. Whether this applies to all species currently placed in the mediocre-complex remains to be seen as shortage of material and lack of suitably exposed mouthparts in the few specimens available precludes further investigation.

Because of their reduced sculpture and forward shifted eyes osiridis, zulu and rabirium may be suspected of being specialized monomorium-group members which have acquired sculpture, rather than salomonis-group members which have shifted eyes and reduced sculpture. I regard the three as specialized members of the salomonis-group, not only because they retain vestiges of the characteristic salomonis-group sculpture but also because they have grossly reduced pilosity, a trait not seen in the monomorium-group, and because they grade back into the mass of the salomonis-group, through the more normal members of this complex, namely esharre, mediocre and nirvanum]]. Sexual forms of all mediocre-complex members remain unknown.

opacum complex

A small complex of 6 species characterized by their uniformly dull brown colour and sharply defined dense reticulate-punctate sculpture which blankets the head and alitrunk. This complex consists of a central core of four species (junodi, micropacum, opacum, subdentatum) and two peripheral species included for convenience (afrum, albopilosum). Of these peripheral species afrum appears to be related to opacum but is noticeably more specialized, having lost its gastral pilosity and independently acquired its diagnostic modification of sharp posteroventral occipital corners. M. albopilosum, though sharing the characters of colour and sculpture with the rest of the complex, is abundantly hairy. The core-species of the complex are closely related to the members of the bicolor-complex' on one hand (see above), and to the subopacum-complex on the other. The latter complex, however, has very reduced cephalic sculpture.

M. afrum is widely distributed through the drier zones of the entire continent, but does not seem to occur in deserts. The remaining species are apparently restricted to the southern and eastern parts of the Afrotropical region. Males and females are known for afrum, albopilosum, and junodi; sexual forms of the rest remain unknown.

pharaonis complex

A small complex containing the very common cosmopolitan tramp-species pharaonis and its two close Indian relatives longi and wroughtoni. In these the cephalic dorsum and the alitrunk everywhere is blanketed by fine and very dense reticulate-punctulate sculpture. The eyes are slightly in front of the midlength of the sides of the head and are relatively small (0.18-0.21 x HW).

Antennal scapes are of moderate length, SI 105 or more. The metanotal groove is distinctly impressed. A pair of standing hairs is present on the pronotal humeri and usually a single pair also occurs on the mesonotum. The first gastral tergite has standing hairs more or less evenly distributed over the sclerite in front of the apical transverse row. Colour varies from uniform yellow to dark brown.

In the Afrotropical region only the bicolor-complex and the opacum-complex show uniform reticulate-punctate sculpture on the cephalic dorsum similar to that seen in pharaonis. M. pharaonis separates from all species of these two complexes by being smaller (HW 0.40-0.48 as opposed to a combined HW range of o· 52-0·80 for all species of the bicolor- and opacum-complexes), by being uniformly yellow in colour, and by having a single pair of stout standing hairs at the pronotal humeri and another single pair on the mesonotum. This combination of characters is not seen in any Afrotropical member of either the bicolor-complex or the opacum-complex.

As pharaonis, commonly called Pharaoh's Ant, is now distributed worldwide in the tropics, and is very widely spread in the subtropical and temperate zones in association with human habitations, the region of origin of the species has led to considerable differences of opinion. Arnold (1916) postulated South America as the original home of pharaonis, but as no close relatives exist there , and as there are very few endemic species of Monomorium in the New World (Kempf, 1972), and those which do occur all belong to the monomorium -group (DuBois, 1986), Arnold's postulate is improbable and is rejected.

The Afrotropical region, which has many species of the salomonis-group, was thought by Bernard (1952) to be the place of origin of pharaonis. However, his opinion of which species constitute close relatives of pharaonis is not accepted here (he included as close relatives Monomorium ilgii, Monomorium osiridis, Monomorium hannonis, Monomorium setuliferum and Monomorium termitarium). As no genuine close relatives of pharaonis occur in Africa it is reasonably certain that it is also an introduction in the region.

The most reasonable suggestion, and the one followed here, is that of Emery (1922), who considered India to be the most probable place of origin; a view repeated by Wilson & Taylor (1967). Having examined most extant Monomorium species during the course of this study, I conclude that the closest presently detectable living relatives of pharaonis are longi and wroughtoni, both of which are restricted to India, and the joint characters of these three form the diagnosis given above. Accepting that the three are close relatives and noting that the Indian subcontinent is the only place where all three are found together, it seems a reasonable hypothesis that all originated there. It does not of course account for the fact that pharaonis has gone on to become perhaps the most successful tramp-species in the world whilst the other two are still restricted to India. Sexual forms of pharaonis are known.

subopacum complex

The complex, which contains 8 Afrotropical species, shows cephalic sculpture which is much more reduced than in either of the complexes so far discussed (bicolor and opacum complexes). Instead of the head being uniformly sharply reticulate-punctate the sculpture here is reduced to reticulate-granular, shagreenate-punctate , or to a fine superficial reticular patterning which appears inlaid in the surface and does not roughen the surface. In general the alitrunk is more strongly sculptured than the head or both areas show distinct sculpture. The alitrunk does not show the characteristic outline of the areniphilum-complex.

Within the complex blanketing fine cephalic sculpture is shown by ocellatum and subopacum, whilst herero shows the same sculpture in reduced form, appearing roughly reticulate. The retaining species (delagoense, drapenum, kitectum, ophthalmicum, willowmorense) have cephalic sculpture reduced to fine superficial reticulation or reticular patterning only, as is also seen in the tchelichofi-complex; in this latter complex, however, the alitrunk also has very reduced sculpture. All species have one or more pairs of standing hairs present on the first gastral tergite in front of the apical transverse row.

This complex is intermediate in sculptural reduction between the opacum-complex, where sculpture is strong, and the tchelichofi-complex in which sculpture is very reduced indeed. All members of the subopacum-complex are restricted to southern Africa except for ophthalmicum from Ethiopia and subopacum itself. The latter is very widely distributed but appears originally to have been a circum-Mediterranean species which has subsequently been spread by commercial activity.

Males and females are known for subopacum and delagoense (described briefly by Forel, 1910b); the females of ocellatum and herero are known but both sexual forms of all the rest await discovery.

tchelichofi complex

The five species of this complex are characterized by the reduction of cephalic sculpture to a fine superficial reticular patterning which does not roughen the surface but rather appears inlaid into the surface. The alitrunk sculpture is also much reduced and in general the promesonotum is not more strongly sculptured than the head. Hairs are present on the first gastral tergite in front of the apical transverse row.

Known from four South African and one Ethiopian species the members of this complex form an apparently close-knit unit within the salomonis-group because of their very reduced sculpture, though whether this constitutes an apomorphy cannot presently be assessed. Superficially all the species seem quite smooth and glossy, but closer examination reveals the presence of extremely fine superficial reticular patterning, which appears inlaid in the cuticle. Loss of this fine patterning would leave the surface entirely featureless.

The tchelichofi-complex seems to form the final stage in a sequence of sculptural reduction which begins in the universal dense reticulate-punctation seen in the opacum-complex. This sculpture is reduced in the subopacum-complex either evenly over the entire body or partially, on the head, leaving the alitrunk dorsum more strongly sculptured than the cephalic dorsum. In tchelichofi and its allies the sculpture is reduced all over the head and promesonotum, so that the latter is no more strongly sculptured than the former. The only sexual form known in this complex is the male of excelsior.

viator complex

A complex of four Namib Desert species which are isolated within the salomonis-group by a combination of characters including reduced cephalic sculpture, which is superficially reticulate to reticulate-granular; never with sharply defined reticulate-punctate sculpture. The eyes are moderate to very large (0.26-0.40 x HW), and the scapes are long to very long (SI 111-130). Standing hairs are always numerous and fairly evenly distributed on the first gastral tergite in front of the apical transverse row.

Cephalic sculpture in this complex covers the same range of variation as is seen in the subopacum-complex, but the scapes there are shorter and the outline shape of the head tends to be different. In viator and allies the head in full-face view has the sides in front of the eyes approximately parallel to weakly divergent anteriorly, and the sides behind the eyes convergent posteriorly. In the subopacum-complex the sides tend to be evenly shallowly convex, broadest across the midlength and converging both in front of and behind the eyes. Two of the species noted by Marsh (1984) in his pitfall survey of Namib Desert ants are included here. Marsh's Monomorium sp. A = viator, and his sp. B = vatranum. The other two species included in the complex are mantazenum and marshi, also discovered in the Namib by Marsh.

The male of viator is known, all other sexuals are unknown for the members of this complex.


At present the salomonis-group includes those species immediately related to salomonis, plus the small complex of species surrounding pharaonis, and those forms once regarded , rightly or wrongly, as parasites belonging to the genera Wheeleriella, Paraphacota, Xenhyboma and Epixenus, which names are strictly synonyms of the salomonis-group within Monomorium. Excluded are the species now constituting the setuliferum-group and a number of other odd species previously wrongly included in this group.

One of the largest species-groups of Monomorium, the salomonis-group is fairly uniform and contains 48 currently recognized species in the Afrotropical region. Many more species are distributed throughout the width of Africa north of the Sahara, the Middle East and the eastern Palaerctic and Oriental regions. A few species occur north of the Mediterranean or on islands in that sea, and several species are accomplished tramps, being transported widely by commercial activity. The definition given above covers the entire group, not just the Afrotropical fauna.

Unlike most other groups within Monomorium, salomonis-group members are usually distinguished by their conspicuous fine sculpture and marked tendency to reduce the pilosity of the dorsal head and body. Forms with much-reduced sculpture are relatively rare in the group but these species, which may come I superficially to resemble members of the monomorium-group, usually also exhibit a marked lack of dorsal pilosity on the head and body. In smooth species of the monomorium-group dorsal pilosity is generally strongly developed. In general even species of the salomonis-group which have lost most or all sculpture, dorsally tend to retain it laterally on the alitrunk and everywhere on the propodeum.

Members of the salomonis-group are characteristically inhabitants of dry ground and well-drained soils, ranging from Mediterranean climate through savannah and semi-desert to hard desert conditions, and frequently nesting in exposed places where the soil receives direct insolation. It has been postulated (Bolton, 1986b) that this may in part be responsible for the frequent development of apterous and ergatoid females within the group. In the few species which have been observed salomonis-group members are scavengers and predators of small arthropods.

The Afrotropical species are divided into 8 complexes here, mostly for convenience and to point out obvious resemblances among various members of the mass of species. Extralimital species-complexes are not discussed and for the greater part remain uninvestigated, though all available material has been examined during this study. It is not claimed that the species-complexes mentioned below reflect any particular phylogenetic significance, too little is known of most species to warrant any such assumption; the salomonis -group as a whole does, however, appear to constitute a holophyletic group within Monomorium.

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