Monomorium monomorium species group

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based on Bolton (1987) and Heterick (2006)

Species

Diagnosis

Worker Monomorphic, frequently with size variation in any series but without allometric variation. Mandibles unsculptured, the masticatory margin usually with 4 teeth which decrease in size from apex to base. More rarely the mandible with 3 teeth plus a minute basal denticle; a very few species with only 3 teeth and none with 5 teeth. Trulleum small to obliterated, when present frequently closed. Palp formula predominantly 2,2 but reduced to 1,2 in minute species. Median portion of clypeus raised, usually projecting forward anteriorly and longitudinally bicarinate but the carinae feeble or fading anteriorly in a few species. 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 the anterior ends of the clypeal carinae may project as sharp angles or teeth. Cephalic dorsum unsculptured and glassy smooth except for scattered hair-pits. Eyes always present and distinct , size small to large (0.15-0.38 x HW) and generally with 4 or more ommatidia in the longest row. Eyes usually situated in front of the midlength of the sides in full-face view; close to or at the midlength in only a few species-complexes. Eyes roughly circular to elongate-oval in profile, never reniform or extended anterolaterally into a lobe. Head always longer than broad (CI 72-89), scapes very variable in length (SI 72-1 10). Antennae with 10 to 12 segments, terminating in a strong club of 3 segments. Metanotal groove moderately to strongly impressed, with distinct cross-ribs. Propodeal spiracle circular to subcircular. Propodeal dorsum rounding into declivity, not angulate or dentate. Promesonotal dorsum unsculptured. Propodeal dorsum usually unsculptured but rarely it may be reticulate-punctate; never transversely striate or rugulose. Petiolar spiracle at the node. Body pilosity variable in distribution but usually conspicuous, only extremely rarely absent from the dorsal alitrunk. Mesopleuron and metapleuron often smooth but may retain faint sculpture. Petiole, postpetiole and gaster usually unsculptured, very rarely otherwise. (Workers examined: all included in this revision plus about 50 extralimital forms of the group - see Bolton 1987 for species list)

Female Characters generally as worker but female much larger; female slightly smaller to slightly larger than con specific male. Eyes larger than in worker and positioned at or close to the midlength of the head. Ocelli present. Mandibles as worker but dentition much reduced or bizarre in some socially parasitic species. Antennae with 11 or 12 segments, with a 3-segmented club. HW greater than maximum width of mesoscutum or the two about equal. Alitrunk usually winged and with a full complement of flight sclerites, but several apterous forms are known (Monomorium carbonarium, Monomorium ebeninum, Monomorium floricola, Monomorium mictilis, Monomorium minimum). Alitrunk long and narrow in dorsal view, long and low in profile. Parapsidal grooves distinct to absent. Axillae triangular in dorsal view, separated by a small mid-dorsal gap or just meeting at the midline; axillae partially to entirely fused to mesoscutum in apterous females. Forewings with cross-vein m-cu absent and the venation frequently much reduced, with many veins faint to vestigial and not tubular. Head, alitrunk and gaster usually unsculptured but some with weak sculpture on the head behind the lateral portions of the clypeus and behind the frontal lobes. Lateral alitrunk sometimes sculptured in part. First gastral tergite unsculptured. Pilosity always present on dorsal surfaces of body, often abundant. (Females examined: - see Bolton 1987 for species list.)

Male Slightly smaller to slightly larger than the conspecific female, much larger than the worker. Mandibles meeting medially at full closure, with 3 teeth and frequently also with a minute basal denticle. Palp formula 2,2 or 1 ,2. Scape cylindrical or subcylindrical, variable in length but usually about equal to the second funicular segment or a little longer. First funicular segment not globular, the remainder of the funiculus not strongly tapering apically, not whip-like. Head capsule wider behind the eyes than in front, the maximum head width about equal to the maximum width of the mesoscutum. Eyes large and sited just in front of the midlength; always a space present between eye and mandibular base in full-face view, the eye not touching the clypeus. Ocelli large but not born on a turret nor breaking the outline of the occipital margin. Mesoscutum overhanging pronotum anteriorly. Notauli absent and mesoscutum usually lacking a narrow V-shaped anteromedian area which is less sculptured than the surrounding area. Parapsidal grooves present to absent. Axillae small, triangular in dorsal view and separated by a small gap medially. Propodeal spiracle in front of the midlength of the side. Venation as alate female. Head sculptured, remainder of body variable but usually smooth, sometimes the mesoscutum and scutellum sculptured. First gastral tergite unsculptured. Genitalia large and partially exserted. Body with pilosity dorsally. (Males examined: - see Bolton 1987 for species list.)

Species Complexes

Afrotropical and Malagasy species-complexes of the monomorium-group (based on workers)

altinode complex

This quite conspicuous complex includes 9 species which are very widely distributed in sub-Saharan Africa. The distinctive structure of the clypeus is shared with the katir-complex and the leopoldinum-complex, but both of these lack the apomorphic petiole and postpetiole configuration shown by the altinode-complex. The female of occidentale is known but otherwise all sexual forms are unknown in this complex.

The members of this complex appear to constitute a holophyletic lineage and are linked by possession of the following characters. Clypeal carinae are sharp and conspicuous, close together posteriorly and widely divergent anteriorly. The anterior clypeal margin has a prominent median section which is flanked by a pair of teeth, denticles or projecting acute angles at the apices of the clypeal carinae. Antennae 12-segmented and the scapes not reaching the occipital margin when laid straight back. With the head in full-face view the eyes are distinctly in front of the midlength of the sides; in profile the eyes are usually elongate-oval and have a maximum diameter 0.20-0.28 x HW. The head capsule in profile is somewhat dorsoventrally flattened, with the dorsum, venter or both flat to shallowly convex. Usually the head becomes deeper posteriorly. Petiole node high and narrow in profile (Figs 84-88), anteroposteriorly slightly compressed and the peduncle subtended by a small or inconspicuous anteroventral process. Postpetiole high and narrow, also anteroposteriorly compressed and with a high vertical anterior face.

bequaerti complex

based on Bolton (1987) and Heterick (2006)

Monomorium bequaerti is known only from Zaire, and pulchrum only from Zimbabwe. Males and females have not yet been found for either species.

A small complex characterized by possession of 11-segmented antennae and a relatively large postpetiole. In profile the postpetiole is equal to or somewhat more voluminous than the petiole, and has a long, gradually sloping posterior face.

boerorum complex

based on Bolton (1987) and Heterick (2006)

A large complex of species comprising all those forms which do not fit any of the above complexes, and hence merely lumped here for convenience.

Species with 11 antennal segments and usually showing reduced eyes in which a single median longitudinal row of 2-4 ommatidia is enclosed by an outer ring of ommatidia.

A number of species with 12-segmented antennae also show a similar eye structure to that noted above.

The following tramp species also has the aforementioned eye structure.

The remaining species, which have 12-segmented antennae, all show a normal eye with two or more longitudinal rows of ommatidia within the outer ring. These species also tend to have a narrow blade-like subpetiolar process.

Despite this there is a tendency for variation in the characters mentioned and a few exceptions to the characters. It is not possible to make any meaningful division of the complex at the present time.

The vast majority of species included here are restricted to the territories of southern Africa. Monomorium vaguum is found elsewhere on the continent but may conceal more than one valid species. Only three species are found away from southern Africa, namely inquietum (Zaire), spectrum (Gabon), and trake (Ghana).

exiguum complex

based on Heterick 2006


Based on comparative type material I have seen, Monomorium exiguum is part of a complex that includes, at least, the exclusively African Monomorium mictilis, Monomorium rosae and Monomorium taedium. An 11-segmented antenna, an elongate and flattened head capsule, weakly developed clypeal carinae, a dorsally rather flattened rather than evenly convex propodeum and the low, strongly conical form of the petiole are common to all of these taxa. Monomorium rosae is placed in a different complex than Bolton (1987) on the basis of the appearance of the worker postpetiole, but in actual fact, the shape of the postpetiole in larger, darker specimens of Monomorium exiguum approaches that of M. rosae, if it is not identical. The degree of obliqueness seen in the posterior face of the postpetiole appears to be proportional to the size of the worker, rather than a distinct feature at the species level, let alone the species-complex level, in all three species mentioned above.

The distinction between the above four species, if indeed it truly exists, is minimal. In appearance the workers form a continuum, with the bright yellow M. mictilis being the smallest species and the very dark M. rosae the largest. To give just one instance, the relevant measurements supplied by Bolton (1987) for M. exiguum (40 specimens) and M. rosae (12 specimens) certainly give this reviser pause for thought! With M. exiguum in regular font, M. rosae in bold, these read: TL (i.e., total length); 1.5–1.7/1.6–2.0 HL 0.36–0.42/0.42–0.50, HW 0.28–0.32/0.33–0.40 CI (=CeI) 74–80/76–82 SL 0.22–0.27/0.28–0.35 SI 74–84/85–94 PW 0.17–0.21/0.21–0.25 AL (i.e., mesosoma length) 0.36–0.44/0.42–0.56. Apart from the larger size, the only real difference that I can discern between a M. rosae syntype from the Democratic Republic of Congo and large, brown specimens of M. exiguum I have seen from Madagascar is the presence of faint sculpture on the lower mesopleuron in M. rosae. Fresh M. rosae material, which I have not seen, is said by Bolton to be ‘blackish-brown to black’.

Monomorium mictilis is separated from M. exiguum by Bolton (1987) on the basis of the presence or absence of erect infrahumeral setae. These are supposedly absent in M. mictilis and long and erect in M. exiguum. In fact the setae are present, but short and appressed in M. mictilis. This particular character does not seem to be useful as a means of separating similar species in the M. monomorium species group, at least in Madagascar. Where hundreds or even thousands of workers are available for examination, I have noticed variability in the number, length and alignment of the promesonotal setae, including differences in the length and alignment of the infrahumeral setae. This phenomenon may not be recognizable where only a few, isolated specimens are available for study. For now, I would allow for the separation of M. mictilis from M. exiguum, as all Malagasy specimens I have seen of the latter have erect or semi-erect infrahumeral setae (albeit of different lengths). The same applies to Monomorium taedium, for which I have seen three paratype workers. Apart from their somewhat larger size (HW = 0.34 mm) and lack of erect infrahumeral setae they look exactly like brown M. exiguum. Interestingly, the postpetiole of the paratype specimens of M. taedium is quite globose, as in smaller M. exiguum.

Another Monomorium with an 11-segmented antenna, M. nigricans, is of uncertain affinities, but may also belong to the M. exiguum complex.

flavimembra complex

based on Heterick (2006)

Workers of Monomorium micrommaton and M. chnodes bear a close resemblance to hirsute, yellow workers of M. termitobium. The queen of M. micrommaton is relatively large, with a proportionately massive mesosoma and a broad, cordate head, quite different characters from those of the queen of M. termitobium. Monomorium chnodes possesses a square propodeum (unlike that seen in any workers of Monomorium termitobium) with a large propodeal spiracle, and some queens and workers have a five-toothed mandible — a feature otherwise unknown in the M. monomorium species group. However, preliminary molecular data place Monomorium chnodes close to M. platynode, and both species possess a very short clypeus that, when seen in profile, descends towards the arc of the mandibles at almost 90 degrees. Monomorium platynode, in fact, may represent a radiation derived from M. chnodes, with a reduction in the size and dentition of the mandible.

In Monomorium chnodes, M. flavimembra, M. lepidum, M. platynode, and M. versicolor, the clypeal carinae are obsolete or only weakly defined and the anteromedian clypeal margin is depressed and moderately to strongly declivous when seen in profile— in the case of M. chnodes and M. platynode being almost vertical, as mentioned above. In four of these species, the fourth (i.e., basal) tooth is greatly reduced or absent. Only M. chnodes has a strongly defined basal tooth. In M. chnodes, M. flavimembra, and M. lepidum, the petiolar node is more-or-less conical and the postpetiole is rounded, but in M. platynode and M. versicolor the nodes are high and the petiolar node is strongly cuneate. Monomorium bifidoclypeatum is very similar to these species and almost certainly also belongs to this complex.

iyenasu complex

Monomorium iyenasu

A single rather strange species from Tanzania, is included here. Known only from the worker it is immediately diagnosed by its relatively large size for a member of the monomorium-group (HW > 0.70), 12-segmented antennae with short scapes (SI <80), small eyes (0.19 x HW) and very dense pilosity. It lacks obvious relatives among the Afrotropical fauna and may represent an introduction from outside the region.

katir complex

The four large-eyed species of this small complex appear to be derived from the same source as the altinode-complex, that source may well be the leopoldinum-complex. All three share the same characteristic clypeal structure and it may be postulated that the altinode-complex consists of relatives of the leopoldinum-complex which have evolved a specialized petiolar and postpetiolar structure, whilst the katir-complex consists of relatives of the leopoldinum-complex which have evolved enlarged eyes. The female of balathir is known, males remain unknown in this complex.

The clypeus, antennae and head shape correspond to that seen in the altinode-complex but the eyes , situated in front of the midlength, are relatively very large (0.30-0.38 x HW), with their posterior margins at or very close to the midlength of the sides. Also the petiole is subconical in profile and the postpetiole rounded, lacking the characteristic shape of the foregoing complex.

leopoldinum complex

The 6 species included here have much the same general appearance as the two complexes noted above, but they lack the specialized petiole and postpetiole of the altinode-complex and the enlarged eyes of the katir-complex. All species of the leopoldinum-complex are of eastern or southern Africa, their sexuals are not known.

Clypeus, antennae and head shape as described for the altinode-complex. Eyes in front of the midlength of the sides but of moderate size (0.18-0.27 x HW). Petiole subconical and postpetiole rounded, lacking the high narrow aspect of the altinode-complex.

malatu complex

Of the five species represented here four have have 12 antennal segments, while dolatu has only eleven. Most of the species are of west or central African origin but disoriente is known only from Tanzania. Sexual forms of all species remain to be discovered.

Characters of this small complex are the same as those of the strangulatum-complex, and species with both 11 and 12 antennal segments are also included here. The structure of the petiole is, however, different, the node of malatu-complex members being high and either narrowly subconical or cuneate in profile. The anterior peduncle is short and stout, and is subtended by a relatively large anteroventral process which is usually in the form of a broad, anteriorly truncated lamellate strip. In most species the standing hairs of the head, alitrunk and gaster tend to be blunt or truncated apically.

rhopalocerum complex

On the whole these species are remarkably similar to the members of the schultzei-complex but have shorter scapes, forward shifted eyes and somewhat more flattened heads. Females are known for exchao (=termitobium) and rhopalocerum, males for exchao (=termitobium) alone.

Clypeal structure as in the schultzei-complex and also matching that complex in alitrunk, petiole and postpetiole structure. Eyes in the rhopalocerum-complex are more elongate than in schultzei and allies and are very obviously situated in front of the midlength of the sides. The antennal scapes when laid straight back from their insertions fail to reach the occipital margin (except in binatu [=termitobium] where they just reach) , and the antennae are always 12-segmented. The head capsule in profile is shallowly biconvex, its deepest point at about the midlength.

Heterick (2006) Though Monomorium termitobium (along with the apparently closely related M. xuthosoma and M. sakalavum) are associated with Bolton’s rhopalocerum complex, as here conceived the morphological parameters of this complex far exceed those designated by Bolton.

schultzei complex

The species included here form a close-knit complex of related forms and probably represent a holophyletic lineage. The species are restricted to eastern and southern Africa (Heterick added the Malagasy species denticulum to this complex), and are apparently related to the rhopalocerum-complex, whose members share a similar overall distribution. Males of the schultzei-complex remain unknown but females of arboreum, firmum and schultzei are represented in collections.

Clypeal carinae sharp, close together, parallel or only feebly divergent anteriorly. Anterior margin of projecting median portion of clypeus without prominent acute angles, teeth or denticles. Antennae 12-segmented and relatively long (SI 95-110), the scapes when laid straight back reaching or slightly exceeding the occipital margin or rarely failing to reach the margin only by a mere fraction of their apical width. Eyes in profile appearing round or subcircular (rather than elongate-oval), in full-face view the eyes at or close to the midlength of the sides; in general the posterior margins of the eyes are at the midlength. Head capsule in profile distinctly biconvex, not dorsoventrally flattened. Head deepest just behind level of eye or close to the midlength. Promesonotum and propodeum in profile each forming a distinct convexity, separated by the metanotal groove . Petiole node small and subconical in profile. Subpetiolar process inconspicuous, either a small anteroventral lobe or a narrow strip. Postpetiole in profile low and rounded, smaller than the petiole and lacking a high vertical anterior face.

strangulatum complex

Four species with 12 antennal segments, and strangulatum having only 11. Of the five all but egens are obviously closely related, but in egens the anterior clypeal margin tends to be concave and the pronotum is flattened with accentuated angular humeri; features not seen in the other four. All species occur in western and central Africa and morphologically appear intermediate between the schultzei- and rhopalocerum-complexes and the malatu-complex. They are particularly close to the latter but lack its characteristic petiolar structure. Males are not known for any species in this complex but females of egens and draxocum have been examined.

Clypeal carinae sharply developed, widely separated and only feebly divergent anteriorly. The points at which the clypeal carinae meet the anterior clypeal margin are not marked with prominent angles or denticles. Antennae with 11 or 12 segments and when laid straight back the scapes surpass the occipital margin (except in egens). With the head in full-face view the posterior margins of the eyes are approximately at the midlength of the sides. In profile the eyes are round to subcircular (rather than elongate-oval). Head capsule in profile distinctly biconvex, deepest just behind the level of the eye or at the midlength. Occipital margin usually convex in full-face view. Petiole with a long anterior peduncle, subtended by a minute anteroventral process. Postpetiole low and rounded.

Notes

This is the largest species-group currently recognized within Monomorium, containing 69 known Afrotropical species and an unknown but quite large number of extralimital forms. Members of the group occur in all zoogeographical regions but the group is predominantly Afrotropical. At least one member, Monomorium floricola, is an accomplished and very widespread tramp-species in the tropics and subtropics. On occasion floricola also occurs in the temperate zones in hothouses and other constantly heated buildings.

Most species of the monomorium-group are small to minute and are only poorly represented in collections. Their biologies are mostly utterly unknown but the vast majority of species inhabit the leaf litter or topsoil layer. Several nest and forage arboreally or subarboreally and some have only been found in trees. As elsewhere in this publication the species-group is defined on a world-wide basis, and fundamentally the group contains all those species whose monomorphic workers have mostly or entirely unsculptured head and body, reasonably large eyes, fewer than 5 mandibular teeth with unsculptured mandibular blades, PF 2,2 or less, a rounded propodeum, and conspicuous dorsal pilosity. As elsewhere in the genus the definitions based on females and males are somewhat less certain as so few are known, but males always lack cross-vein m-cu in the forewing and lack all those characters diagnostic of the scabriceps-group and the destructor-group. Further study will almost certainly detect ways to subdivide what is here termed the monomorium-group into smaller groups. I have attempted here to define meaningful species-complexes as they occur in the Afrotropical region but have not carried this investigation over to the extralimital species. Shortage of material is a limiting factor in this survey and it is freely admitted that some of the Afrotropical species-complexes discussed below are for convenience only, whilst others certainly constitute holophyletic assemblages.

The monomorium-group diagnosis includes all the species previously placed in the subgenus Monomorium s.str. or given as related to M. minutum (now monomorium) in the catalogues of Wheeler (1922) and Emery (1922), with the exception of those species excluded in this study. Forms previously placed elsewhere but now added to the monomorium-group include the inquiline species formerly constituting Epoecus and Corynomyrmex, and the disparate forms with 11-segmented antennae originally grouped together on the strength of this spurious character under the subgenus Lampromyrmex (= Mitara).

Additional Resources

References