Iridomyrmex trigonoceps

This ant appears to be relatively uncommon, but is broadly distributed in Western Australia, South Australia and the Northern Territory. All but one of the records for the species come from desert or semi-arid areas in the northern zone, but a single series, taken from Renmark, South Australia, suggests this ant may occur across a wider range than the sparse collection data suggest. The only ecological note accompanies the Renmark specimens: they were collected in a sand dune with mallee and Triodia vegetative cover. The nest was indicated by a small mound.

Identification
Next to Iridomyrmex mirabilis, this species is unquestionably the most aberrant Iridomyrmex: in full-face view, the severely triangular shape of the head, the high, lateral placement of the small eyes on the head capsule and the large mandibles ensure that this species cannot be mistaken for any other Iridomyrmex. However, it is the unique, elongate second and third maxillary palps that make this species difficult to place in a taxonomic context.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  trigonoceps. Iridomyrmex trigonoceps Heterick & Shattuck, 2011: 150, fig. 80 (w.) AUSTRALIA.

Types. Holotype worker from 32km N Renmark, 33°52’52”S 140°43’54”E, South Australia, 3.v.1995, S. O. Shattuck, mallee and Triodia on sand dune, ground nest with small mound (, ANIC32-038597). Paratypes: 2 workers, same data as holotype ; 3 workers from 18km E ‘The Granites’, Tanami Desert, Northern Territory, 19 March 1987, P. J. M. Greenslade, Ants Vial 60.15 (, ANIC32-0326 13); 3 workers from Tanami, Northern Territory, 25-27 March 1986; P. J. M. Greenslade (, ANIC32-032809); 1 worker from Tanami Desert, Northern Territory, 20 May 1986, P. J. M. Greenslade (, ANIC32-037606).

Description
Worker Description. Head. Posterior margin of head 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. 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, or surpassing posterior margin of head by approximately 3 x its diameter. Erect setae on scape absent, except at tip; prominence on anteromedial clypeal margin always completely absent; mandible regularly triangular with oblique basal margin; long, curved setae on venter of head capsule present. Mesosoma. Pronotum weakly undulant or almost straight. Erect pronotal setae numerous (12 or more), short and bristly. Mesonotum evenly curved. Erect mesonotal setae moderate in number to numerous (6 or more), short and bristly. Mesothoracic spiracles always inconspicuous; propodeal dorsum smoothly and evenly convex; 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 numerous (12 or more), short and bristly. Petiole. Dorsum of node convex; node thin, scale-like, orientation moreor-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 brown. Colour of erect setae pale yellow.

Measurements. Worker (n = 4)—CI 91–94; EI 24–25; EL 0.21–0.24; EW 0.16–0.17; HFL 1.28–1.43; HL 0.93–1.05; HW 0.87–0.99; ML 1.28–1.54; MTL 0.89–1.02; PpH 0.15–0.19; PpL 0.48–0.57; SI 103–110; SL 0.92– 1.01.

Etymology
Greek: ‘trigon ’—‘triangular’ plus Latin: ‘ceps’ derivative of ‘caput’—‘head’.

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.