Iridomyrmex bicknelli

Iridomyrmex bicknelli is one of the commonest Australian Iridomyrmex, workers being a familiar component of what is left of the natural environment in all manner of urban areas as well as a wide range of native habitats. Nests can be found in virtually any location, even the seemingly barren footpaths of large cities. It is of a timid disposition, and is one member of the genus that certainly does not deserve Andersen’s (2002) general descriptor for Iridomyrmex spp. of ‘tyrant ants’. The species appears to be most active in warmer temperatures, and this is one ant that seems to thrive in the highly insolated sandy coastal wastes adjacent to beaches.

Identification
Workers can be distinguished from similar, greyish-black ants by the lack of erect setae on the antennal scapes and tibiae, their elongate and rather narrow head capsule that normally has a more-or-less convex posterior margin of the head, the short, bristly setae on the mesosomal and gastral dorsum and their bluish or greenish-yellow iridescence.

This species shows a degree of variability in the phenotype that suggests disconcerting genetic distance among populations; ants in the southwest of Western Australia often possessing a more planar posterior margin of head when seen in full-face view than ants from other parts of Australia. In the latter case, the posterior margin of the head is usually distinctly convex. Ants from the eastern Australian states also tend to be larger on average. However, no clear-cut distinction can be made when phenotypes from all populations are compared. Preliminary molecular data (S. Cameron et al. unpublished) indicate that several populations within I. bicknelli as currently construed display deep genetic divergences, and when appropriately coupled with phenotypic variation may warrant species status. Pending completion of that work, I. bicknelli is tentatively maintained here as a single species.

Identification Keys including this Taxon

 * Key to Iridomyrmex species

Distribution based on Regional Taxon Lists
Australasian Region: Australia. Indo-Australian Region: New Guinea.

Nomenclature

 *  bicknelli. Iridomyrmex bicknelli Emery, 1898b: 236, figs. 6, 7 (w.) AUSTRALIA. Synonym of gracilis Lowne (junior homonym) and hence first available replacement name, splendidus: Heterick & Shattuck, 2011: 49.
 * gracilis. Formica gracilis Lowne, 1865a: 280 (w.) AUSTRALIA. [Junior primary homonym of Formica gracilis Fabricius, 1804: 405 (now in Pseudomyrmex).] Mayr, 1876: 82 (q.m.); Wheeler, G.C. & Wheeler, J. 1966: 729; Crozier, 1969: 248 (k.). Combination in Hypoclinea: Mayr, 1870b: 955; in Iridomyrmex: Dalla Torre, 1893: 169. Synonym of bicknelli, first available replacement name: Heterick & Shattuck, 2011: 49.
 * splendidus. Iridomyrmex bicknelli var. splendidus Forel, 1902h: 468 (w.) AUSTRALIA. Forel, 1907h: 290 (q.). Junior synonym of bicknelli: Heterick & Shattuck, 2011: 49.

Type Material


Because of its ubiquity, I. bicknelli has attracted taxonomic attention from several early researchers. Although it was originally described by Lowne in 1865 (Lowne, 1 865a), his name, (then) Formica gracilis, is preoccupied and thus cannot be used as a valid name. Iridomyrmex bicknelli as the next earliest name is here confirmed (ICZN, article 23.3.5). The subspecies I. bicknelli splendidus has no unique characters and here becomes a junior synonym of I. bicknelli.

Description
Worker Description. Head. Posterior margin of head strongly convex, or weakly convex; erect setae on posterior margin in full-face view set in a row; sides of head convergent anteriad; 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, 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 triangular spur; mandible elongate triangular with oblique basal margin; long, curved setae on venter of head capsule absent. Mesosoma. pronotum weakly undulant or almost straight. Erect pronotal setae moderate in number to numerous (6 or more), short and bristly. Mesonotum sinuous, or straight. Erect mesonotal setae moderate in number (6-12), short and bristly. 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 (6–12), short and bristly. Petiole. Dorsum of node convex, or planar; node thin, scale-like, orientation more-or-less vertical, or thin, scale-like, orientated anteriad, or thick, orientated anteriad. Gaster. Non-marginal erect setae of gaster present on first gastral tergite; marginal erect setae of gaster present on first tergite, or absent on first tergite. General characters. Allometric differences between workers of same nest absent. Colour uniformly brown to black, commonly with greenish-yellow to bluish iridescence. Colour of erect setae pale brown.

Measurements. Worker (n = 10)—CI 74–78; EI 28–34; EL 0.23–0.26; EW 0.14–0.21; HFL 1.23–1.68; HL 0.87–1.15; HW 0.68–0.88; ML 1.36–1.58; MTL 0.89–1.23; PpH 0.12–0.17; PpL 0.49–0.59; SI 127–144; SL 0.87– 1.18.