Iridomyrmex gibbus

Nothing is known of the biology of this species apart from the fact that it is a ground forager, but its colonies do not seem to be as populous as those of Iridomyrmex chasei, judging from the relative proportions of workers of both species occurring in pitfall traps.

Identification
Iridomyrmex gibbus is morphologically close to the ‘concolor’ morph of Iridomyrmex chasei, and very easily confused with I. chasei as a consequence. The distinguishing features are subtle, but in the Pilbara region of Western Australia, where the two species are sympatric, both are able to be recognised among pitfall-trapped material by parataxonomists. These distinguishing features include the anterior pronotal angle (60° in I. chasei, versus < 60° in I. gibbus, whose mesosoma has a symmetrical appearance in profile); a more obviously concave posterior margin of the head when seen in full-face view in I. chasei (most workers) compared to I. gibbus, and, often, more erect pronotal setae (usually 10 in non-abraded specimens) in I. chasei than in I. gibbus (commonly ca. 6). As with I. chasei, the propodeum is truncate. Finally, I. gibbus is always uniformly dull brown in colour, whereas I. chasei populations of all phenotypes often include lighter coloured morphs.

Distribution
Because of the ease with which I. chasei and I. gibbus can be confused, the distribution of the latter is uncertain. Most records have come from northern Western Australia, including Barrow Island, and there are two records from the Northern Territory and one from Queensland.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  gibbus. Iridomyrmex gibbus Heterick & Shattuck, 2011: 84, fig. 38 (w.) AUSTRALIA.

Description
Worker Description. Head. Posterior margin of head weakly concave; erect setae on posterior margin in full- face view, present in small aggregations on one or both sides of posterior margin of head; sides of head noticeably convex; erect genal setae 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; 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 convex; antennal scape surpassing posterior margin of head by 1-2 x its diameter. Erect setae on scape absent, except at tip; prominence on anteromedial clypeal margin projecting as blunt but distinct protuberance; mandible regularly triangular with oblique basal margin; long, curved setae on venter of head capsule absent. Mesosoma. pronotum strongly inclined anteriorly, or moderately and evenly curved over its length. Erect pronotal setae moderate in number (6-12), longest setae elongate, flexuous and/or curved. Mesonotum evenly curved. Erect mesonotal setae sparse (6 or fewer) and bristly. Mesothoracic spiracles always inconspicuous; propodeal dorsum protuberant; placement of propodeal spiracle posteriad and near propodeal declivity; propodeal angle present as a bluntly defined right angle, the dorsal and declivitous propodeal faces never separated by a carina. Erect propodeal setae sparse (6 or fewer) and bristly. Petiole. Dorsum of node convex; node thin, scale- like, orientation more-or-less vertical. 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 absent. Colour uniformly brown. Colour of erect setae yellowish-brown.

Measurements. Worker (n = 6)—CI 90–92; EI 25–27; EL 0.16–0.17; EW 0.13–0.14; HFL 0.72–0.79; HL 0.67–0.72; HW 0.60–0.66; ML 0.75–0.85; MTL 0.54–0.58; PpH 0.13–0.15; PpL 0.36–0.39; SI 97–102; SL 0.61– 0.65.

Etymology
Latin: ‘humpbacked’.