Monomorium nanum

This tiny species is almost ubiquitous in arid and semi-arid environments in South Australia and Western Australia. The ant is also found in the Northern Territory, New South Wales and Queensland, and may occur in northern Victoria. Despite its broad distribution, this species had long remained undescribed, and is known from the worker caste only. Little is known of the biology of M. nanum. Most specimens are of foragers collected in pitfall traps and additional information is lacking on all labels. Such information is highly desirable, since this species is abundant, and may have an important role as a small, generalist predator and scavenger in the ecosystems in which it occurs.

Identification
Heterick (2001) - A member of the monomorium group. The physical appearance of M. nanum is as varied as that of M. sydneyense, and what is treated here as one species may eventually prove to be a complex of several species.

The variation focuses around two main morphotypes, with some smaller permutations within these forms. The holotype and paratypes represent the larger of the two morphotypes. These ants are generally uniformly brown, with a striolate posterior promesonotum, strongly microreticulate sculpture of the mesopleuron and propodeum, a large, reniform eye with a bottom lobe that is usually larger than the upper lobe, the longitudinal clypeal carinae produced as small spines or denticles, and the anteromedial clypeal margin strongly emarginate. The appearance of the head and the anterior promesonotum is smooth and shining, with sparse, appressed setulae (or, rarely, erect and suberect setae). Several specimens from the remaining material have somewhat smaller, though still reniform eyes. Similar to the above form is a smaller variant, with a head capsule that is darker than the promesonotum in full-face view, elongate eyes reaching to the venter of the head capsule, and reduced propodeal and mesopleural sculpture. Two workers from “Kapunda”, north of Nyngan, New South Wales, are piceous with a smooth alitrunk, elongate eyes, and a straight anteromedial clypeal margin. The longitudinal clypeal carinae in this pair of specimens are not produced as teeth or denticles.

The most common variant of the smaller morphotype is a minute worker with small, slightly reniform eyes, uniform, often pale brown colouration, and a straight anteromedial clypeal margin. Sculpture on the alitrunk is strongly reduced or absent, and the longitudinal clypeal carinae are obsolete or only faintly visible, and not produced as teeth or denticles. A less common worker is gracile in form, with a long promesonotum (like Monomorium stictonotum), many decumbent and subdecumbent setulae on the frons and alitrunk, and a very low petiolar node. Finally, a series from “The Overlander”, in northwestern Western Australia, is smooth and yellowish, with a small, but elongate eye.

As with several other Australian Monomorium, intermediates connect these more conspicuous forms, and the morphotypes cannot be meaningfully separated in a workable key. Oddly, I have not seen a nest series of M. nanum among the many alcohol specimens I have examined. I am therefore unable to gauge the variation within a population, since the pinned material is composed of stray workers, many of them collected in pitfall traps.

Heterick (2009) - Monomorium aithoderum, Monomorium nanum and Monomorium stictonotum are minute, nondescript orange or brownish species of very similar appearance that make up much of the Monomorium biomass in drier areas of southern Australia.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  nanum. Monomorium nanum Heterick, 2001: 407, figs. 38, 132, 133, 140, 141 (w.) AUSTRALIA.

Worker
Holotype. HML 1.25; HL 0.51; HW 0.41; CeI 82; SL 0.34; SI83; PW 0.25. Others. HML 0.88-1.42; HL 0.36-0.57; HW 0.30-0.49; CeI 78-90; SL 0.24-0.36; SI 73- 93; PW 0.16-0.36 (51 measured).

As for the worker of Monomorium sydneyense, but with the following apomorphies.

Head. Frons of head capsule smooth and shining with evenly spaced, appressed setulae, or smooth and shining with combination of appressed setulae and erect and suberect setae. Compound eyes reniform, with posterior surface of eye emarginate, or elongate, much longer than wide; (viewed laterally) compound eyes set posterior of midline of head capsule, or set at midline of head capsule; eye large, eye width greater than 1.5x greatest width of antennal scape. Anteromedial clypeal margin emarginate, median clypeal carinae produced apically as pair of pronounced teeth, or emarginate, median clypeal carinae produced as pair of bluntly rounded denticles, or straight or slightly emarginate, median clypeal carinae not produced as teeth or denticles. Venter of head capsule with elongate, basket-shaped setae in at least some individuals. Mandibles (viewed from front) strap-like with inner and outer edges subparallel, smooth with piliferous punctures.

Alitrunk. Promesonotal sculpture absent, promesonotum smooth and shining, or sculpture present in form of microreticulation and striolae on and around katepisternum, or sculpture present in form of microreticulation, striolae and striae on the mesopleuron, and striolae on posterodorsal promesonotal surface; dorsal promesonotal face evenly convex. Metanotal groove present as feebly impressed furrow between promesonotum and propodeum, or present as distinct and deeply impressed trough between promesonotum and propodeum. Propodeal sculpture absent; propodeum smooth and shining, or present as uniform microreticulation, with few or no striae or costulae; propodeal processes absent (propodeum smoothly rounded in profile or with slight hump at propodeal angle), or present as pronounced lamellae formed by extension of metapleural lobes. Declivitous face of propodeum longitudinally concave between its lateral margins. Propodeal setulae absent.

Petiole and postpetiole. Petiolar node conical, dorsally rounded, or cuneate, dorsally rounded; Ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) near 4:3 to near 1:1. Height ratio of petiole to postpetiole near 4:3.

Gaster. Pilosity of first gastral tergite variable; consisting entirely of well-spaced appressed setulae, or mainly of decumbent and subdecumbent setulae, or of combination of appressed setulae and longer, erect and suberect setae.

General characters. Colour variable: from fulvous through shades of brown to piceous, often with head and gaster slightly darker than alitrunk. Worker caste monomorphic.

Etymology
Latin: “dwarf”.