Iridomyrmex brunneus

Iridomyrmex brunneus is common and widespread throughout Australia. At least in Western Australia, workers are often seen foraging on the trunks of eucalypts or fallen timber. Populations of the ant can persist in settled areas, and label data indicates it will enter dwellings, mainly in country towns (in Alice Springs it has been recorded as a ‘house pest’).

Identification
Workers of I. brunneus are medium-sized brown ants that may be easily confused with several other species, most notably Iridomyrmex minor and Iridomyrmex anceps. The easiest way to distinguish workers of this ant is to check the anteromedial clypeal process: in I. brunneus this is very weak to vestigial, whereas the process is a prominent triangle in both I. minor and I. anceps. Most I. brunneus workers also have erect genal setae (absent from many I. minor workers and almost all I. anceps workers). Characteristically, when a worker of I. brunneus is viewed in profile, the pronotum and mesonotum form an even curve, whereas the mesosomal outline in I. minor and I. anceps is less regular, being either more domed at the junction of the pronotum and mesonotum, or elongate and flattened (some populations of I. minor).

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  brunneus. Iridomyrmex bicknelli r. brunneus Forel, 1902h: 469 (w.) AUSTRALIA. Raised to species: Heterick & Shattuck, 2011: 54. Senior synonym of fusciventris: Heterick & Shattuck, 2011: 54.
 * fusciventris. Iridomyrmex gracilis var. fusciventris Forel, 1913g: 188 (w.) AUSTRALIA. Junior synonym of brunneus: Heterick & Shattuck, 2011: 54.

Description
Worker Description. Head. Posterior margin of head weakly convex to planar, or planar to weakly concave; erect setae on posterior margin in full-face view set in a row; sides of head straight or weakly convex; erect genal setae present on sides of head in full-face view, or absent from sides of head in full-face view (one to a few small setae may be present near mandibular insertion). Ocelli absent; in full-face view, eyes set above midpoint of head capsule, or set at about midpoint of head capsule; in profile, eye set anteriad of head capsule; eye semi-circular, or asymmetrical, curvature of inner eye margin more pronounced than that of its outer margin. Frontal carinae straight, or convex; antennal scape surpassing posterior margin of head by 0.2–0.5 x its length. Erect setae on scape absent, except at tip; prominence on anteromedial clypeal margin projecting as blunt but distinct protuberance, or present as an indistinct swelling or undulation; mandible regularly triangular with oblique basal margin; long, curved setae on venter of head capsule absent. Mesosoma. pronotum moderately and evenly curved over its length. Erect pronotal setae moderate in number to numerous (6 or more) and longest setae elongate, flexuous and/or curved. Mesonotum straight, or evenly curved. Mesothoracic spiracles always inconspicuous; propodeal dorsum smoothly and evenly convex, or straight and long (half as long again as length of propodeal declivity); placement of propodeal spiracle mesad, more than its diameter away from propodeal declivity; propodeal angle weakly present or absent, the confluence of the dorsal and declivitous propodeal faces indicated, if at all, by an undulation. Erect propodeal setae moderate in number to numerous (6 or more), short and bristly. Petiole. Dorsum of node convex; node thin, scale-like, orientated anteriad. Gaster. Non-marginal erect setae of gaster present on first gastral tergite; marginal erect setae of gaster present on first tergite. General characters. Allometric differences between workers of same nest present. Colour foreparts tawny to dark reddish brown, upper vertex of head often darker than clypeal region and also mesosoma, legs same colour as mesosoma, but generally of a deeper shade, gaster dark brown. Colour of erect setae pale, whitish.

Measurements. Worker (n = 6)—CI 85–88; EI 27–28; EL 0.21–0.26; EW 0.16–0.20; HFL 1.26–1.56; HL 0.90–1.07; HW 0.76–0.94; ML 1.26–1.60; MTL 0.90–1.08; PpH 0.15–0.19; PpL 0.50–0.61; SI 122–133; SL 1.02– 1.20.

References based on Global Ant Biodiversity Informatics

 * CSIRO Collection
 * Heterick B. E., and S. Shattuck. 2011. Revision of the ant genus Iridomyrmex (Hymenoptera: Formicidae). Zootaxa 2845: 1-174.
 * Taylor R. W. 1987. A checklist of the ants of Australia, New Caledonia and New Zealand (Hymenoptera: Formicidae). CSIRO (Commonwealth Scientific and Industrial Research Organization) Division of Entomology Report 41: 1-92.