Monomorium exiguum

The tiny yellow workers of Monomorium exiguum can be found throughout Madagascar but are most abundant in Toliara Province, where they are often among the most commonly collected Monomorium in sifted litter samples (Heterick 2006). Collingwood et al. (2004) found that this species apparently inhabits leaf litter or topsoil. In Yemen it was found nesting under a stone next to a large fig tree and inside the galleries of a termite nest. The soil was dry with ample dry seeds. Another nest series was found in leaf litter next to a date palm tree where the soil was rich in decaying organic material (Sharaf et al., 2017).

Identification
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 by 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. mictile 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 Santschi 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 mictile 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. mictile and long and erect in M. exiguum. In fact the setae are present, but short and appressed in M. mictile. 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. mictile 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.

Interestingly, the paler ‘flavescens’ and ‘bulawayense’ forms also occur in Madagascar, along with M. exiguum sensu stricto. In the taxon faurei the head is darker than the mesosoma, and the gaster is pale with darker infuscation on its basal half, as well as on the sides of the first gastral tergite. This variant, too, is quite common on Madagascar. Monomorium exiguum workers from Madagascar tend to have a slightly more tapered petiolar node than their counterparts on the African mainland.

Distribution based on Regional Taxon Lists
Afrotropical Region: Cameroun, Central African Republic, Democratic Republic of Congo, Gabon, Ghana, Guinea, Ivory Coast, Kenya, Nigeria, Saudi Arabia, United Arab Emirates, United Republic of Tanzania, Yemen, Zimbabwe. Palaearctic Region: Balearic Islands, Ethiopia, Oman.

Nomenclature

 *  exiguum. Monomorium exiguum Forel, 1894b: 85 (w.) ETHIOPIA.
 * Heterick, 2006: 118 (q.m.).
 * Combination in M. (Martia): Forel, 1913h: 351.
 * Combination in M. (Lampromyrmex): Wheeler, W.M. 1922a: 876.
 * Senior synonym of bulawayensis, faurei, flavescens: Bolton, 1987: 388.
 * Senior synonym of minutissimum: Heterick, 2006: 116.
 * Senior synonym of exiguum: Sharaf, Collingwood, et al., 2015: 52.
 * Senior synonym of baushare, garahe: Sharaf et al., 2017: 343.
 * bulawayensis. Monomorium (Mitara) exiguum var. bulawayensis Forel, 1913j: 217 (w.) ZIMBABWE.
 * Combination in M. (Lampromyrmex): Wheeler, W.M. 1922a: 876.
 * Junior synonym of exiguum: Bolton, 1987: 388.
 * desertorum. Monomorium desertorum Collingwood & Agosti, 1996: 344 (w.) SAUDI ARABIA.
 * Junior synonym of exiguum: Sharaf, Collingwood, et al., 2015: 52.
 * faurei. Monomorium (Mitara) faurei Santschi, 1915c: 260, fig. 10 (w.) GABON.
 * Combination in M. (Lampromyrmex): Wheeler, W.M. 1922a: 876.
 * Junior synonym of exiguum: Bolton, 1987: 388.
 * flavescens. Monomorium (Mitara) exiguum r. flavescens Forel, 1916: 418 (w.) DEMOCRATIC REPUBLIC OF CONGO.
 * Junior synonym of exiguum: Bolton, 1987: 388.
 * minutissimum. Monomorium (Lampromyrmex) minutissimum Santschi, 1937d: 227, figs. 27, 28 (w.) ANGOLA.
 * Junior synonym of mictile: Bolton, 1987: 401; of exiguum: Heterick, 2006: 116.
 * baushare. Monomorium baushare Collingwood & Agosti, 1996: 342 (w.) OMAN.
 * Junior synonym of exiguum: Sharaf et al., 2017: 343.
 * qarahe. Monomorium qarahe Collingwood & Agosti, 1996: 353 (w.) SAUDI ARABIA.
 * Junior synonym of exiguum: Sharaf et al., 2017: 343.

Worker
Heterick (2006) - Lectotype (M. exiguum): HML 0.93 HL 0.36 HW 0.28 CeI 78 SL 0.22 SI 79 PW 0.18. Lectotype (Monomorium exiguum bulawayense): HML 1.00 HL 0.39 HW 0.32 CeI 81 SL 0.26 SI 83 PW 0.20. Lectotype (Monomorium faurei): HML 0.88 HL 0.35 HW 0.29 CeI 83 SL 0.22 SI 76 PW 0.18. Lectotype (Monomorium exiguum flavescens): HML 0.89 HL 0.36 HW 0.30 CeI 82 SL 0.22 SI 75 PW 0.18. Lectotype (Monomorium minutissimum): No measurements taken as lectotype is headless.

HML 0.79–0.97; HL 0.33–0.39 HW 0.26–0.29 CeI 73–81 SL 0.22–0.26 SI 81–93 PW 0.16–0.20 (n=20).

HEAD: Head rectangular, vertex planar or weakly concave; frons shining and smooth except for piliferous pits; pilosity of frons a mixture of well-spaced, distinctly longer erect and semi-erect setae interspersed with shorter decumbent setae or setulae. Eye moderate, eye width 1–1.5× greatest width of antennal scape; (in full-face view) eyes set below midpoint of head capsule; (viewed in profile) eyes set around midline of head capsule; eye elliptical, curvature of inner eye margin may be more pronounced than that of its outer margin. Antennal segments 11; antennal club three-segmented. Clypeal carinae weakly to strongly defined; anteromedian clypeal margin straight; paraclypeal setae moderately long and fine, curved; posteromedian clypeal margin approximately level with antennal fossae. Anterior tentorial pits situated nearer antennal fossae than mandibular insertions. Frontal lobes straight, parallel. Psammophore absent. Palp formula 2,2. Mandibular teeth three, plus minute, basal denticle or angle; mandibles with sub-parallel inner and outer margins, smooth (except for piliferous pits); masticatory margin of mandibles approximately vertical or weakly oblique; basal tooth a small to minute denticle or angle, much smaller than t3 (four teeth present).

MESOSOMA: Promesonotum shining and mainly smooth, vestigial striolae, if present, confined to lower anterior mesopleuron; (viewed in profile) anterior promesonotum smoothly rounded, thereafter more-or-less flattened; promesonotum on same plane as propodeum, promesonotal setae seven to twelve; standing promesonotal setae a mixture of well-spaced, distinctly longer, erect and semi-erect setae which are curved distally and often paired, interspersed with much shorter, incurved, decumbent setae; appressed promesonotal setulae few, mainly on sides of promesonotum. Metanotal groove strongly impressed, with distinct transverse costulae, or, metanotal groove vestigial. Propodeum shining and smooth, with a few weak striolae on metapleuron; propodeal dorsum convex; propodeum always smoothly rounded; standing propodeal setae consisting of one prominent pair anteriad, with other shorter setae very sparse or absent; appressed propodeal setulae very sparse or absent; propodeal spiracle equidistant from metanotal groove and declivitous face of propodeum. Vestibule of propodeal spiracle absent or not visible. Propodeal lobes present as vestigial flanges or small strips of cuticle only.

PETIOLE AND POSTPETIOLE: Petiolar spiracle lateral and situated within anterior sector of petiolar node; node (viewed in profile) conical, vertex tapered; appearance of node shining and smooth throughout; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) about 1:1; anteroventral petiolar process absent or vestigial; ventral petiolar lobe present; height ratio of petiole to postpetiole between 3:2 and 4:3; height–length ratio of postpetiole about 1:1; postpetiole shining and smooth; postpetiolar sternite without anterior lip or carina, or this structure vestigial.

GASTER: Pilosity of first gastral tergite consisting of well-spaced, erect and semi-erect setae interspersed with a few appressed setulae.

GENERAL CHARACTERS: Color variable, pale yellow to brown. Worker caste monomorphic.

Queen
Heterick (2006) - HML 1.55–2.00 HL 0.45–0.55 HW 0.39–0.54 CeI 83–98 SL 0.34–0.42 SI 75–90 PW 0.28–0.55 (n=19).

HEAD: Head rectangular; vertex weakly concave or planar; frons shining and smooth except for piliferous pits; pilosity of frons a mixture of well-spaced, distinctly longer erect and semi-erect setae interspersed with shorter setae or setulae, which are decumbent or appressed, longer setae thickest on vertex. Eye elliptical, margin sometimes shallowly concave; (in full-face view) eyes set at about midpoint of head capsule; (viewed in profile) eyes set posteriad of midline of head capsule.

MESOSOMA: Anterior mesoscutum smoothly rounded, thereafter more-or-less flattened; pronotum, mesoscutum and mesopleuron shining and mainly smooth, vestigial striolae, if present, confined to anterior katepisternum; length-width ratio of mesoscutum and scutellum combined between 2:1 and 3:2; axillae contiguous, or nearly so; standing pronotal/mesoscutal setae a mixture of well-spaced, distinctly longer, erect and semi-erect setae which are curved distally, interspersed with much shorter, incurved, decumbent setae; appressed pronotal, mescoscutal and mesopleural setulae few, mainly on sides of pronotum and mesopleuron. Propodeum shining and smooth, with a few weak striolae on metapleuron; propodeum smoothly rounded or with indistinct angle; propodeal dorsum convex; standing propodeal setae consisting of up to a dozen or more longer erect and shorter sub-erect setae; appressed propodeal setulae very sparse or absent; propodeal spiracle nearer metanotal groove than declivitous face of propodeum; propodeal lobes present as vestigial flanges only, or absent.

WING: Wing not seen (queens dealated).

PETIOLE AND POSTPETIOLE: Petiolar spiracle lateral and situated slightly anteriad of petiolar node; node (viewed in profile) cuneate, vertex tapered; appearance of node shining, with vestigial sculpture; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) between 2:1 and 1:1; anteroventral petiolar process present as a thin flange tapering posteriad; height ratio of petiole to postpetiole between 4:3 and 1:1; height–length ratio of postpetiole between 4:3 and 1:1; postpetiole shining and weakly striolate posteriad; postpetiole shining and smooth; postpetiolar sternite without anterior lip or carina, or this structure vestigial (NB. May be confused with rear margin of postpetiolar sternite, which does project as a spur, length of sternite being much reduced).

GASTER: Pilosity of first gastral tergite consisting of well-spaced, erect and semi-erect setae interspersed with a few appressed setulae.

GENERAL CHARACTERS: Color yellowish-brown. Brachypterous alates not seen. Ergatoid or worker-female intercastes seen.

Male
Heterick (2006) - HML 1.37–1.96 HL 0.41–0.51 HW 0.42–0.48 CeI 102–116 SL 0.10–0.19 SI 24–34 PW 0.44–0.66 (n=11).

HEAD: (In full-face view) head width-mesosoma width ratio between 1:1 and 3:4; frons finely micropunctate. Compound eyes protuberant and elliptical tending to elongate; margin of compound eye clearly separated from posterior margin of clypeus. Ocelli turreted. Ratio of length of first funicular segment of antenna to second funicular segment between 1:1 and 3:4. Maximum number of mandibular teeth and denticles three.

MESOSOMA: Mesoscutum broadly convex; pronotum and mesoscutum shining and mainly smooth, vestigial striolae, if present, confined to lower anterior mesopleuron; parapsidal furrows vestigial or absent; notauli absent; axillae widely separated (i.e., by width of at least one axilla), axilla fused with scutellum to narrowly separated (i.e., less than width of one axilla).

WING: Wing veins predominantly depigmented, with distal segments reduced to vestigial lines; vein m–cu absent; vein cu–a absent.

PETIOLE AND POSTPETIOLE: Petiolar spiracle lateral and situated slightly anteriad of petiolar node; node (viewed in profile) conical, vertex tapered, appearance of node shining and smooth; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) between 4:3 and 1:1; anteroventral petiolar process absent or vestigial; height ratio of petiole to postpetiole between 1:1 and 3:4; height–length ratio of postpetiole between 2:1 and 4:3; postpetiole shining, with vestigial sculpture.

GASTER: Pilosity of first gastral tergite consisting of well-spaced, semi-erect setae interspersed with a few appressed setulae.

GENERAL CHARACTERS: Color light to medium brown, appendages pure brown to off-white

Type Material
Heterick (2006) - (lectotype here designated), ETHIOPIA: (locality on label now illegible, but given as ‘Südabessinien’ in Bolton 1987).

LECTOTYPE: worker, Ethiopia, Ilg. In view of the variability, particularly in color, that led researchers to erect new ‘varieties’ for this species, a lectotype has been chosen for M. exiguum that fixes the species name for populations with dark, longer-headed specimens that have the characteristics mentioned in the original description. This particular specimen has been glued in the normal way, dorsal surface uppermost. PARALECTOTYPE: worker, same data as lectotype, here designated for a specimen that has been pointed on the same pin as the lectotype and glued on its back. This specimen has been repinned with photocopies of the original labels (MHNG).