Iridomyrmex bigi

Iridomyrmex bigi is strictly nocturnal, so much so that the nest entrances are plugged with soil during the day. The nests are small circles of slightly raised, cleared soil with a single entrance. It appears to be a rare species, and apart from one worker (without collection data) supplied by A. Andersen, no additional material has come to light since Shattuck’s (1993b) revision.

Identification
Iridomyrmex bigi is a very odd-looking Iridomyrmex: the combination of the very large eye, pointed node and the general confinement of the erect setae to the anterior margin of the antennal scape make it impossible to confuse this with any other ant in the genus.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  bigi. Iridomyrmex bigi Shattuck, 1993a: 119, fig. 9 (w.) AUSTRALIA. Senior synonym of variscapus: Heterick & Shattuck, 2011: 51.
 * variscapus. Iridomyrmex variscapus Shattuck, 1993a: 141, fig. 9 (w.) AUSTRALIA. Junior synonym of bigi: Heterick & Shattuck, 2011: 51.

Type Material


Clypeal and mesosomal characters and preliminary molecular data place this ant close to the meat ants (I. purpureus species-complex), albeit as a sister to this complex. Shattuck (1993b) separated the form I. variscapus from I. bigi on the basis of the paler ground colour of the head and mesosoma and the colour of the scape in workers. However, when the workers from these two taxa used in the revision of the I. purpureus group were revisited in the current project, the variability in colour was found to be in conformity with intraspecific variation seen in other Iridomyrmex species. The centre of the antennal scape is much lighter than its edges in both taxa, and the flagellum is generally paler than the scape. The workers of I. variscapus, all of which come from a single nest in Sandringham, Queensland, share the same structural and setae patterns as I. bigi, and the morphometric measurements and indices used by Shattuck all overlap with those of I. bigi. Iridomyrmex variscapus therefore becomes a junior synonym of I. bigi (Heterick & Shattuck, 2011).

Description
Worker Description. Head. Posterior margin of head strongly concave; erect setae on posterior margin in full- face view set in a row; 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). Number of ocelli two; in full-face view, eyes set above midpoint of head capsule; in profile, eye set anteriad of head capsule; eye elongate. Frontal carinae convex; antennal scape surpassing posterior margin of head by 1–2 x its diameter. Erect setae on scape present on outside of antennal scape only; prominence on anteromedial clypeal margin projecting as triangular spur; 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 numerous (12 or more), short and bristly. Mesonotum sinuous. Erect mesonotal setae numerous (12 or more), short and bristly. Mesothoracic spiracles always prominent as small, vertical protuberances; propodeal dorsum protuberant; placement of propodeal spiracle mesad, more than its diameter away from 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 numerous (12 or more), short and bristly. Petiole. Dorsum of node acuminate; 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 foreparts light brick-red to dark crimson, gaster brown with weak bluish to purplish iridescence in light-coloured specimens. Colour of erect setae light brown.

Measurements. Worker (n = 16)—CI 94–105; EI 30–37; EL 0.54–0.62; EW 0.28–0.36; HL 1.60–1.91; HW 1.50–1.96; ML 0.82–1.03; PpH 0.23–0.29; PpL 0.85–0.98; SI 78–97; SL 1.38–1.61.

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.
 * Shattuck S. O. 1993. Revision of the Iridomyrmex purpureus species-group (Hymenoptera: Formicidae). Invertebrate Taxonomy 7: 113-149.