Iridomyrmex omalonotus

Iridomyrmex omalonotus is a very common species that has a preference for drier habitats. It makes terrestrial nests, denoted by a small mound (Renmark, South Australia: S. O. Shattuck, label data). Sandplain heath-land, Triodia-covered sand dunes and mallee scrubland are typical habitats. The ant frequently ascends trees to forage, and unsurprisingly, label data indicates that a number of specimens have been collected using such methods as malaise traps and honey baits on tree-trunks. Interestingly, foragers have been collected at night (Cocklebiddy, Western Australia), although this ant is also diurnally active. On the basis of its known relationships and attraction to honey, this species is also likely to attend lycaenid caterpillars and honeydew-producing Hemiptera, although label data are lacking for this detail.

Identification
The appearance of many workers of this species is that of their eastern cousins, namely Iridomyrmex mayri and Iridomyrmex obscurior, but in a number of populations the propodeum is peculiarly flattened. However, the propodeum is variable in appearance, and efforts to find discrete characters to differentiate the most distinctive workers representing the two morphs have been unsuccessful. The view taken here is that variable propodeal shape is part of normal intraspecific variation in I. omalonotus. This ant can be distinguished from most others by its long, hairy antennal scapes, erect setae on the hind tibiae and sides of the head, and its relatively small size. Of the few species with which I. omalonotus can be confused, Iridomyrmex spurcus is found in the same habitat, but this species may be recognised by the unusually long setae on its legs and mesosoma (short and bristly in I. omalonotus). Morphological similarities between I. omalonotus and I. obscurior are also discussed under the latter.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  omalonotus. Iridomyrmex omalonotus Heterick & Shattuck, 2011: 118, figs. 60, 90 (w.) AUSTRALIA.

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 noticeably convex; erect genal setae present on sides of head in full-face view. 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 0.2-0.5 x its length. Erect setae on scape present and sparse; 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 moderately and evenly curved over its length, or weakly undulant or almost straight. Erect pronotal setae numerous (12 or more) and longest setae elongate, flexuous and/or curved, or numerous (12 or more), short and bristly. Mesonotum sinuous. Erect mesonotal setae moderate in number to numerous (6 or more), short and bristly. Mesothoracic spiracles always inconspicuous; propodeal dorsum straight and short (equal in length to propodeal declivity), 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 sparse (6 or fewer) to numerous (12 or more), short 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 shades of brown, gaster may be darker than foreparts. Colour of erect setae white, depigmented.

Measurements. Worker (n = 6)—CI 76–89; EI 25–28; EL 0.18–0.22; EW 0.13–0.18; HFL 1.08–1.37; HL 0.82–0.96; HW 0.63–0.86; ML 1.03–1.34; MTL 0.81–0.97; PpH 0.12–0.17; PpL 0.41–0.52; SI 116–141; SL 0.88– 1.04.

Etymology
Greek: omalus—‘flat’ plus Latin: notus—‘back’, referring to the appearance of the worker propodeum seen in profile.

References based on Global Ant Biodiversity Informatics

 * Heterick B. E., and S. Shattuck. 2011. Revision of the ant genus Iridomyrmex (Hymenoptera: Formicidae). Zootaxa 2845: 1-174.