Monomorium sordidum

Despite an absence of records from Tasmania, M. sordidum is possibly the most abundant of the Australian Monomorium. This species is ubiquitous in all habitats, from desert to tropical rainforest, and from the remotest regions to suburban backyards in capital cities. Despite its abundance, the biology of M. sordidum has not been studied, doubtless because of the taxonomic confusion surrounding this and other small Monomorium species. Presumably this ant is a generalist with catholic tastes. (Heterick 2001)

Identification
Heterick (2001) - This ant is far more conservative in its morphology than are most other small Australian Monomorium. The form known as “sordidum” represents the morph most commonly seen in the vicinity of Canberra. The exoskeleton is relatively thinly sclerotised and the worker is a uniform light brown in colour with only faint microreticulation on the propodeum. In comparison, the head and gaster (chocolate) contrast with the alitrunk (russet or brown) in M. sordidum nigriventris, Monomorium foreli, and M. rothsteini tostum. The propodeum is also more sculptured, particularly in M. foreli and M. rothsteini tostum. Monomorium rothsteini squamigena resembles M. sordidum, but is darker, while Monomorium micron is a small-bodied M. sordidum with a rather yellowish-brown alitrunk in the worker. This latter form tends to occur commonly in Western Australia, where it is the most abundant morph found in the eastern goldfields. Monomorium “micron” is also not uncommon in the Darling Range and around Perth, together with the larger, darker and more sculptured morphs. Whether these variations represent populations or individuals within a nest is not known.

The above morphological and colour permutations, however, are quite minor, and are part of a rather small variation in habitus for such a widespread animal.

Monomorium sordidum shares with Monomorium rothsteini a number of apparent synapomorphies, including the shape and sculpture of the propodeum, the shape of the petiole and postpetiole, the reduction in the number of mandibular teeth in the worker (the fourth tooth in M. sordidum is reduced to a minute offset denticle, while the other teeth are large and prominent), and the broad anteromedial sector of the clypeus. Brachyptery is also common among M. sordidum queens. An analysis of the venom constituents in M. sordidum may be instructive for phylogeny if compared with the same analysis already performed for M. rothsteini.

Heterick (2009) - Shares many morphological similarities with the rothsteini species complex in both reproductive and worker castes. Moreover, these species and Monomorium megalops are the only Australian members of the M. monomorium group with 12-segmented antennae, the remaining members possessing 11-segmented antennae.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  sordidum. Monomorium sordidum Forel, 1902h: 443 (w.) AUSTRALIA. Heterick, 2001: 413 (q.m.). Combination in Chelaner: Ettershank, 1966: 97; in Monomorium: Taylor, 1987b: 4. Senior synonym of foreli, micron, nigriventris, squamigena, tostum: Heterick, 2001: 412.
 * nigriventris. Monomorium sordidum var. nigriventris Forel, 1910b: 29 (w.q.) AUSTRALIA. Combination in Chelaner: Ettershank, 1966: 97; in Monomorium: Taylor, 1987b: 4. Subspecies of sordidum: Taylor & Brown, D.R. 1985: 59. Junior synonym of sordidum: Heterick, 2001: 412.
 * foreli. Monomorium (Holcomyrmex) foreli Viehmeyer, 1914b: 32 (w.) AUSTRALIA. Combination in Chelaner: Ettershank, 1966: 97; in Monomorium: Taylor, 1987b: 2. Junior synonym of sordidum: Heterick, 2001: 412.
 * tostum. Monomorium rothsteini var. tostum Wheeler, W.M. 1915g: 806 (w.) AUSTRALIA. Combination in Chelaner: Ettershank, 1966: 97; in Monomorium: Taylor, 1987b: 3. Subspecies of rothsteini: Taylor & Brown, D.R. 1985: 58. Junior synonym of sordidum: Heterick, 2001: 412.
 * micron. Monomorium micron Crawley, 1925b: 593 (w.q.) AUSTRALIA. Junior synonym of sordidum: Heterick, 2001: 412.
 * squamigena. Monomorium rothsteini var. squamigena Viehmeyer, 1925a: 28 (w.) AUSTRALIA. Combination in Chelaner: Ettershank, 1966: 97; in Monomorium: Taylor, 1987b: 3. Subspecies of rothsteini: Taylor & Brown, D.R. 1985: 58. Junior synonym of sordidum: Heterick, 2001: 412.

Worker
Heterick (2001) - HML 1.25-1.75; HL 0.52-0.71; HW 0.42-0.58; Cel 82-90; SL 0.36-0.50; Sl 81-98; PW 0.25-0.34 (40 measured).

Head. Head square or rectangular; vertex slightly concave; frons smooth and shining with combination of appressed setulae and erect and suberect setae. Compound eyes elliptical; (viewed from front) compound eyes set in anterior half of head capsule; (viewed laterally) compound eyes set at midline of head capsule; eye moderate, eye width 0.5-1.5x greatest width of antennal scape. Antennal segments 12; club three-segmented. Anteromedial clypeal margin straight or slightly emarginate, median clypeal carinae not produced as teeth or denticles. Longest lateral anterior clypeal setae long, extending beyond dorsal margin of closed mandibles. Posteromedial clypeal margin level with posterior surface of antennal fossae. Anterior tentorial pits situated nearer antennal fossae than mandibular insertions. Frontal lobes parallel straight, or parallel, sinuate. Venter of head capsule with elongate, basket-shaped setae in at least some individuals. Palp formula 1,2. Maximum number of mandibular teeth and denticles: four; mandibles (viewed from front) strap-like with inner and outer edges subparallel, smooth with piliferous punctures; basal tooth not enlarged; basal angle indistinct; apical and basal mandibular margins meeting in tooth or denticle.

Alitrunk. Promesonotal sculpture present in form of microreticulation and striolae on and around katepisternum, otherwise promesonotum smooth and shining; dorsal promesonotal face evenly convex; erect and suberect promesonotal setae greater than 10; setulae appressed. Mesonotal suture absent. Metanotal groove present as distinct and deeply impressed trough between promesonotum and propodeum. Propodeal sculpture present as faint microreticulation with few striae, mainly on lower lateral surface, or present as uniform microreticulation, with few or no striae or costulae; dorsal propodeal face sloping posteriad, with wedge-shaped flattening or shallow depression that is widest between propodeal angles; processes absent (propodeum smoothly rounded in profile or with slight hump at propodeal angle); lobes present as blunt flanges. Propodeal angle absent; declivitous face of propodeum flat. Erect and suberect propodeal setae 5-10; propodeal setulae absent. Propodeal spiracle lateral and nearer metanotal groove than declivitous face of propodeum, or lateral and about midway between metanotal groove and declivitous face of propodeum; vestibule absent or undeveloped.

Petiole and postpetiole. Petiolar spiracle lateral and in anterior sector of petiolar node. Petiolar node cuneate, dorsally rounded; sculpture absent, petiolar node smooth and shining. Ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) near 4:3 to near 1:1. Anteroventral process distinct in some individuals as slender carina that tapers posteriad. Ventral lobe always absent. Height ratio of petiole to postpetiole near 4:3; height-length ratio of postpetiole near 2:1. Sculpture absent on dorsum, at least: postpetiole smooth and shining. Ventral process absent or vestigial.

Gaster. Pilosity of first gastral tergite consisting of combination of appressed setulae and longer, erect and suberect setae.

General characters. Colour either uniformly brown, or with alitrunk, petiole and postpetiole a tawny yellow, other body parts of a deeper brown colouration, mandibles tawny yellow. Worker caste monomorphic.

Queen
Heterick (2001) - HML 2.72-3.93; HL 0.83-1.10; HW 0.79-1.07; Cel 96-114; SL 0.55-0.76; SI 63-77; PW 0.62-1.03 (12 measured).

Head. Head square or rectangular; vertex planar; frons smooth and shining with combination of incurved decumbent and subdecumbent setulae and erect and suberect setae. Compound eyes circular or subcircular, or elliptical; (viewed from front) compound eyes set at midpoint of each side of head capsule; (viewed laterally) compound eyes set at midline of head capsule; eye large, eye width greater than 1.5 x greatest width of antennal scape.

Alitrunk. Mesoscutum in profile convex anteriad; thereafter flattened. Mesoscutal pilosity consisting of dense incurved setulae and setae; dorsal appearance of mesoscutum smooth and shining; length-width ratio of mesoscutum and scutellum combined near 2:1 to near 4:3. Axillae separated by distance less than half greatest width of scutellum. Propodeal sculpture minutely and transversely striate; dorsal propodeal face flattened. Propodeal processes absent (propodeum smoothly rounded in profile or with slight hump at propodeal angle); lobes present as blunt flanges. Propodeal angle absent. Erect and suberect propodeal setae greater than 10; propodeal setulae decumbent and subdecumbent. Propodeal spiracle lateral and about midway between metanotal groove and declivitous face of propodeum.

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 in anterior sector of petiolar node. Petiolar node cuneate, dorsally rounded; sculpture absent, petiolar node smooth and shining, or present in form of microreticulation. Ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) near 4:3 to near 1:1. Anteroventral process distinct in some individuals as slender carina that tapers posteriad. Height ratio of petiole to postpetiole near 1:1 to near 4:3; height-length ratio of postpetiole near 3:1. Sculpture absent on dorsum, at least: postpetiole smooth and shining, or present in form of microreticulation; ventral process absent or vestigial.

Gaster. Pilosity of first gastral tergite consisting of combination of appressed setulae and longer, erect and suberect setae.

General characters. Colour brown. Brachypterous alates seen and examined. Ergatoid or worker-female intercastes not seen.

Male
Heterick (2001) - HML 3.17-3.48; HL 0.76-0.86; HW 0.90-1.07; CeI 112-124; SL 0.21-0.28; SI 22-28; PW 0.90-1.34 (3 measured).

Head. Head width-mesoscutal width ratio near 1:1 to near 3:4. Compound eyes protuberant and elliptical; (viewed laterally) compound eyes set at midline of head capsule; ocelli not turreted. Ratio of length of first funicular segment of antenna to length of second funicular segment near 2:5 to near 1:2. Maximum number of mandibular teeth and denticles: three.

Alitrunk. Mesoscutum in profile evenly convex; dorsal appearance of mesoscutum finely microreticulate; mesoscutal pilosity consisting of numerous short setae, incurved medially. Parapsidal furrows present and distinct; notauli absent. Axillae separated by distance more than half greatest width of scutellum.

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 in anterior sector of petiolar node. Sculpture present in form of microreticulation; ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) near 1:1 to near 3:4. Anteroventral process always absent or vestigial; ventral lobe always absent. Height-length ratio of postpetiole near 3:1; sculpture present in form of microreticulation; ventral process absent or vestigial.

Gaster. Pilosity of first gastral tergite consisting of combination of appressed setulae and longer, erect and suberect setae.

General characters. Colour dark brown to black.