Iridomyrmex calvus

Iridomyrmex calvus is known from the southern Australian mainland, Tasmania, Lord Howe Island, Norfolk Island and New Caledonia. This is the most broadly distributed species among its close relatives and one of the most widespread members of the genus.

Identification
This species can be distinguished from close relatives by virtue of its small size (HL less than or equal to 1.15mm), relatively short scape (exceeding posterior margin of the head by a maximum of 1 × its greatest diameter) and paucity of erect setae (fewer than 6) on the mesosoma.

Identification Keys including this Taxon

 * Key to Iridomyrmex species

Distribution based on Regional Taxon Lists
Australasian Region: Australia, New Caledonia , Norfolk Island.

Nomenclature

 * albitarsus. Iridomyrmex albitarsus Wheeler, W.M. 1927i: 147, fig. 9 (w.q.m.) AUSTRALIA. See also: Shattuck, 1993b: 1307. Junior synonym of calvus: Heterick & Shattuck, 2011: 55.
 *  calvus. Iridomyrmex calvus Emery, 1914f: 419, pl. 13, fig. 12 (w.q.) NEW CALEDONIA. Senior synonym of albitarsus, notialis: Heterick & Shattuck, 2011: 55. See also: Shattuck, 1993b: 1313.
 * notialis. Iridomyrmex notialis Shattuck, 1993b: 1318, figs. 16, 17, 36 (w.) AUSTRALIA. Junior synonym of calvus: Heterick & Shattuck, 2011: 55.

Taxonomic Notes
This taxon has previously been considered as three separate but similar species, I. calvus from New Caledonia, I. albitarsus from Lord Howe and Norfolk islands, and I. notialis from Australia. Iridomyrmex calvus has never been clearly separated from the other forms and has been recognised largely on biogeographic grounds, although I. albitarsus has been reported as being smaller than I. calvus, with longer scapes, less erect pilosity and a much more rounded and less angular propodeum (Wheeler, 1927). Iridomyrmex albitarsus was also understood to differ from I. notialis in the lack of erect hairs on the pronotum and first gastral tergite and the generally darker body colour (Shattuck, 1993). However, a re-evaluation of the material belonging to these three forms has shown that while differences are present, there is also considerable variation among populations of these ants, especially within I. notialis, and no clear, consistent diagnostic differences could be found among the formerly recognised taxa. For example, while the pronotum and gaster lack erect hairs on workers from Lord Howe and Norfolk islands and workers generally have erect hairs on the mainland, the Australian populations show variation and glabrous individual are present, these being inseparable from the Lord Howe and Norfolk specimens. Body colour reveals a similar pattern, with populations from Lord Howe, Norfolk and New Caledonia being darker compared to the slightly lighter reddish brown colour of most Australian populations. However, overall body colour varies from light to dark reddish brown in Australia with some individuals being essentially the same colour as those from the islands to the east. Given the broad geographic range occupied by this taxon and the isolated nature of New Caledonia, Lord Howe Island and Norfolk Island relative to Australia, some level of differentiation would be expected, and while this has been noted by earlier studies, the level of this variation is relatively small, especially when compared to other species in the genus such as I. dromus. Because of the lack of clear diagnostic differences among these taxa, all are now considered to belong to a single taxon, I. calvus.

Description
Worker Description. Head. Posterior margin of head weakly concave; erect setae on posterior margin absent; 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 one (a minute ocellus), position of obsolete ocelli indicated by small pits only or pits lacking, or ocelli absent; in full-face view, eyes set at about midpoint of head capsule, or set below 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 concave; antennal scape barely attaining posterior margin of head, or surpassing it by less than 1 x its diameter. Erect setae on scape absent, except at tip; prominence on anteromedial clypeal margin always completely absent; mandible triangular with distinct angle between masticatory and basal margins; long, curved setae on venter of head capsule absent. Mesosoma. Pronotum moderately and evenly curved over its length. Erect pronotal setae sparse (6 or fewer) and bristly. Mesonotum straight. Erect mesonotal setae sparse to absent. Mesothoracic spiracles prominent or inconspicuous; propodeal dorsum protuberant; 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 lacking or very minute (one or two tiny setae present). Petiole. Dorsum of node convex; node thin, scale-like, orientation more-or-less vertical. 'Gaster. 'Non-marginal erect setae of gaster present or absent on first gastral tergite; marginal erect setae of gaster present on first tergite. General characters. Allometric differences between workers of same nest absent. Colour honey- brown, with head a deeper shade. Colour of erect setae pale.

Measurements. Worker (n = 16)—CI 86–97; EI 22–28; EL 0.16–0.23; EW 0.10–0.16; HFL 0.85–0.87; HL 0.77–1.00; HW 0.68–0.96; ML 0.31–0.47; MTL 0.64–0.65; PpH 0.11–0.15; PpL 0.39–0.53; SI 84–99; SL 0.67– 0.84.