Monomorium sydneyense

Possibly the most abundant ant in Australia.  Monomorium sydneyense can be found in virtually all Australian habitats. (Heterick 2009)

Identification
Heterick (2001) - More variable in its worker caste than any other Australian Monomorium, M. sydneyense can be found in a welter of related forms. The external appearance varies from almost uniformly smooth and yellow in colour to pitchy dark chocolate, with a uniformly shagreenate or otherwise sculptured head, alitrunk and nodes. The common features uniting most of these workers are the absence of erect and suberect setae on the dorsum of the major body segments, the presence of a cuboidal (and usually more-or-less microreticulate) propodeum, and a rectangular head capsule. Among 55 workers measured, the CeI varied only from 79-88. Erect and suberect setae may be found in a few populations, but with the exception of a tiny minority of individuals such workers closely resemble the type specimens of M. sydneyense and M. sydneyense nigellum.

Variability in colour and cuticular appearance in many M. sydneyense populations is clinal. Unfortunately, only a few queens (only one alate) and males can be assigned with confidence to this taxon, so the variation described here is that of the worker caste.

Specimens from the type locality (Sydney) have a shining promesonotum, with punctation confined to the mesopleuron in the region of the katepisternum. This is the pattern for most of New South Wales, though some populations on the north coast have a uniformly shagreenate promesonotum, while others have shining humeral patches amid the shagreenation. Many specimens have been taken at Cooloola, just north of Brisbane, and these reveal both shagreenate and smooth, dark-coloured individuals in about the same ratio. Workers from Mt Coot-tha (Brisbane), with shagreenate exoskeleton and a strongly truncated and oblique propodeum, are the smallest Australian Monomorium. Workers from some samples are only 1 mm in total length.

Series from the north of Australia are always shagreenate, and sometimes almost black in colour with russet legs. In Broome, Western Australia, the ant shows reduced sculpture and is orange with a black gaster. This morph has been found in a kitchen, and foraging on wooden structures in the backyard of a house.

The greatest variation appears to be in the Adelaide hills, South Australia, where P. J. M. Greenslade made many collections in the vicinity of Belair. A number of pins hold both brown and paler, yellowish morphs. Further south, on Kangaroo Island, some specimens have a truncated propodeum with reduced sculpturing, and in a few cases converge on Monomorium fieldi morphology. The many workers collected by Greenslade east of Adelaide tend to be pale in colour, as are specimens from both the north and south Flinders Ranges.

In Western Australia, populations sampled in the extreme southwest are very similar to the type specimens, but the colour gradually lightens as one moves towards the mid-west coast. However, most ants have a darker head capsule than promesonotum when viewed full-face, even in lighter-coloured workers. Around Perth and in the Darling Ranges, many nests produce minute, often uniformly yellow workers, most of which have a truncated and mainly smooth propodeum. While these ants generally exhibit the usual cephalic features for M. sydneyense, the longitudinal clypeal carinae of some workers are produced as minute spines. I have seen two pins of workers taken by the Rev. B. B. Lowery in Kings Park, Perth, on the same day. The workers on one pin have this feature, whilst it is lacking in workers on the other pin. Those workers possessing clypeal spines have a slightly darker colouration, but are otherwise indistinguishable from the remaining specimens. This phenomenon appears to be localised in scattered populations on the southwest coast, and without taxonomic significance. Monomorium sydneyense workers taken at various Tasmanian localities are invariably dark brown.

Apart from the variation described above, two distinctive morphs in M. sydneyense are retained under that species epithet, though further investigation may reveal them to be sibling species. A uniformly pale yellow form with large eyes occurs in New South Wales, The Northern Territory, South Australia and Western Australia. The ant has a distinctly truncated and high propodeum with obsolescent metapleural lobes (unlike typical sydneyense). The morphology is consistent throughout the range of this ant. A number of such pale workers from Manbulloo, near Katherine in the Northern Territory, are sympatric with the usual dark, shagreenate northern M. sydneyense, heightening suspicions that they do belong to a separate species. In the Adelaide Hills, however, where this form also occurs I cannot separate it from pale M. sydneyense, with which it seems to converge: a “ring species”?

The other morph is known from a few samples taken at Emmettvale, New South Wales and Glen Osmond, South Australia. Workers are very large, and light brown in colour, with large eyes, a rounded and truncated propodeum, and a broad petiolar node. However, specimens with intermediate morphology between this form and typical M. sydneyense do occur. The taxon M. sydneyense poses a number of interesting problems, including the reason for the plasticity evident among its populations when compared with, say, Monomorium sordidum, which varies very little throughout its range. Comprehensive nest series with reproductives are badly needed.

Heterick (2009) - The appearance of workers of what is currently known as M. sydneyense varies enormously, from uniformly dark chocolate with a shagreenate exoskeleton, through orange-and-brown to light yellow with a smooth, shiny appearance, along with many specimens of intermediate facies. The propodeum (and often, the promesonotum and mesopleuron) can be long and reticulate-punctate or shorter and relatively unsculptured. A minute, small-eyed pale form with a darker head is most common in the .SWBP. In workers of coastal populations of this form the clypeal carinae are often extended beyond the clypeus as small spines, and the antennal count in at least some populations is 10. Since workers of these ants are all but morphologically indistinguishable from other workers with the normal count of 11 segments, molecular techniques are probably necessary to determine whether these populations represent a separate species. What appears to be a similar form with the same number of antennal segments has been identified in the Monarto region of South Australia. However, workers of the latter are darker in colour without extended clypeal carinae.

Distribution based on Regional Taxon Lists
Australasian Region: Australia, New Zealand.

Biology
Heterick (2001) - Monomorium sydneyense is found in a wide range of habitats. Workers often forage arboreally as well as terrestrially. This species can be quite common in urban areas. In Perth the minute yellow form, along with M. fieldi, is frequently found in sandy suburban backyards, even those overrun by the coastal brown ant (Pheidole megacephala).

Nomenclature

 *  sydneyense. Monomorium sydneyense Forel, 1902h: 442 (w.) AUSTRALIA. Heterick, 2001: 416 (q.m.). Combination in M. (Mitara): Emery, 1914b: 184; in M. (Lampromyrmex): Emery, 1922e: 184. Senior synonym of barretti, nigella; Heterick, 2001: 415.
 * nigella. Monomorium (Mitara) sydneyense subsp. nigella Emery, 1914b: 184, fig. 3 (w.) AUSTRALIA. Viehmeyer, 1925a: 28 (q.). Combination in M. (Lampromyrmex): Emery, 1922e: 184. Junior synonym of sydneyense: Heterick, 2001: 415.
 * barretti. Monomorium (Lampromyrmex) fraterculus var. barretti Santschi, 1928e: 467 (w.) AUSTRALIA. Subspecies of fraterculum: Taylor & Brown, D.R. 1985: 70. Junior synonym of sydneyense: Heterick, 2001: 415.

Worker
Heterick (2001) - HML 0.80-1.33; HL 0.35-0.55; HW 0.27-0.46; CeI 79-88; SL 0.24-0.38; SI 75-90; PW 0.16-0.27 (55 measured).

Head. Head square or rectangular; vertex slightly concave; frons smooth and shining with evenly spaced, appressed setulae, or smooth and shining with combination of appressed setulae and erect and suberect setae, or mat in appearance with evenly spaced, appressed setulae. Compound eyes elliptical; (viewed from front) compound eyes set in anterior half of head capsule; (viewed laterally) compound eyes set posterior of midline of head capsule; eye moderate, eye width 0.5-J.5x greatest width of antennal scape to large, eye width greater than 1.5 x greatest width of antennal scape. Antennal segments II; club three-segmented. Anteromedial clypeal margin emarginate, median clypeal carinae produced apically as pair of pronounced teeth, or 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. Venter of head capsule without elongate, basket-shaped setae. 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 or striate 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, or present in form of uniform microreticulation with few mesopleural striolae; dorsal promesonotal face evenly convex, or convex anteriad, otherwise flattened; erect and suberect promesonotal setae absent, or 5-10 setae present; 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 present as pronounced lamellae formed by extension of metapleural lobes. Propodeal angle absent; declivitous face ofpropodeum flat to longitudinally concave between its lateral margins. Erect and suberect propodeal setae absent or very sparse to >5; propodeal setulae absent, or appressed. Propodeal spiracle lateral and nearer metanotal groove than declivitous face of propodeum; vestibule absent or undeveloped.

Petiole and postpetiole. Petiolar spiracle lateral and in anterior sector of petiolar node. Petiolar node conical, dorsally rounded, or cuneate, dorsally rounded, or cuneate, sharply tapered; 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 2:1 to near 1:1. Anteroventral process distinct in some individuals as slender carina that tapers posteriad. Ventral lobe present in some individuals. Height ratio of petiole to postpetiole near 4:3 to near 2:1; height-length ratio of postpetiole near 2:1. 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 entirely of well-spaced appressed setulae, or consisting of combination of appressed setulae and longer, erect and suberect setae.

General characters. Colour from yellow to chocolate brown, many individuals bicoloured, with gaster, appendages and often head brown or fulvous, and alitrunk of a lighter colour. Worker caste monomorphic.

Queen
Heterick (2001) - HML 2.00-2.52; HL 0.62-0.69; HW 0.57-0.64; Cel 93-96; SL 0.44-0.51; Sl 75-96; PW 0.44-0.67 (4 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, or striolate with a 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 posterior of the midline of head capsule, or set at midline of head capsule; eye large, eye width greater than 1.5 x greatest width of antennal scape.

Alitrunk. Mesoscutum in profile evenly flattened. Mesoscutal pilosity consisting of dense in curved 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 contiguous or nearly so. Propodeal sculpture present as uniform microreticulation, with few or no striae or costulae; 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 5-1 0; propodeal setulae decumbent and subdecumbent. Propodeal spiracle lateral and nearer metanotal groove than 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. Ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) near 4:3 to near 1:1. Anteroventral process always absent or vestigial. 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; ventral process present and distinct.

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

General characters. Colour russet. Brachypterous alates not seen. Ergatoid or worker-female intercastes not seen.

Male
Heterick (2001) - HML 2.08-2.39; HL 0.57-0.62; HW 0.62-0.74; Cel 108-119; SL 0.20-0.21; SI 28-31; PW 0.75-0.85 (3 measured).

Head. Head width-mesoscutal width ratio 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. Maximum number of mandibular teeth and denticles: three.

Alitrunk. Mesoscutum in profile evenly convex; dorsal appearance of mesoscutum striolate and microreticulate; mesoscutal pilosity consisting of long, dense setae, or consisting of numerous short setae, incurved medially. Parapsidal furrows vestigial; notauli absent. Axillae separated by distance of about 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 to near 2:1; sculpture present in form of microreticulation; ventral process absent or vestigial.

Gaster. Pilosity of first gastral tergite consisting entirely of well-spaced appressed setulae.

General characters. Colour chocolate.