Iridomyrmex pallidus

Within its range, consisting mainly of tropical rainforest and tropical eucalypt forest, I. pallidus probably fills a similar niche to that filled by Iridomyrmex hartmeyeri in more xeric regions. This ant nests in soil and is mainly nocturnal. However, it may forage diurnally in suitable conditions: the Rev. Bede Lowery collected workers from tree-trunks at 3 pm in the afternoon, 160 km SW of Mackay. Furthermore, other ants collected by Rev. Lowery next to a PNG hotel were active on a garden tree at 9 am in the morning. Soil nests uncovered near CSIRO Tropical Ecosystems Research Centre, Darwin, were identifiable by small mounds of topsoil on the ground surface.

Identification
Iridomyrmex pallidus is a distinctive pale yellow to brownish-yellow ant that is largely confined to the Torresian biogeographic zone. Two forms have been described (one with an invalid quadrinomial). Type material of the first, I. rufoniger pallidus, lacks genuine erect setae on the hind tibiae (although the occasional sub- decumbent seta may be artificially bent in a vertical plane), whereas type material of the second, I. rufoniger pallidus ‘pallidior’, possesses genuine erect setae on the hind tibiae. The phenotypes meld together in a definite clinal pattern throughout the range of the genotype, with the hairiest workers being found in Papua New Guinea, and the most glabrous in the more southerly parts of the range on the Australian mainland. Intermediate forms (with a few erect setae on the hind tibiae) occur around the centre of the species’ range, mainly on islands just off the Australian mainland, although populations also occur on the very far north of the Australian mainland. The species itself can be separated from other pale, large-eyed taxa by virtue of the position of the asymmetrical eyes and the abundant, small, subdecumbent and suberect setae that give most workers a shaggy appearance.

Identification Keys including this Taxon

 * Key to Iridomyrmex species

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

Biology
This species is most closely related to Iridomyrmex hartmeyeri, and recent molecular data has also linked it, more surprisingly, with the stick-nest ants (Iridomyrmex conifer and relatives) from the south-western part of the Australian continent, and with Iridomyrmex alpinus from the cool montane regions of New South Wales, Victoria and Tasmania (unpubl. data).

Nomenclature

 *  pallidus. Iridomyrmex rufoniger r. pallidus Forel, 1901b: 22 (footnote) (w.) AUSTRALIA. Forel, 1902h: 466 (q.). Raised to species: Heterick & Shattuck, 2011: 120. Senior synonym of incertus, wingi: Heterick & Shattuck, 2011: 120.
 * incertus. Iridomyrmex rufoniger var. incertus Forel, 1902h: 466 (w.) NEW GUINEA (Bismarck Archipelago). [First available use of Iridomyrmex rufoniger r. pallidus var. incertus Forel, 1901b: 22; unavailable name.] Junior synonym of pallidus: Heterick & Shattuck, 2011: 120.
 * wingi. Iridomyrmex wingi Donisthorpe, 1949c: 754 (w.) NEW GUINEA. Junior synonym of pallidus: Heterick & Shattuck, 2011: 120.

Type Material


While type material of I. rufoniger incertus Forel could not be located by Heterick & Shattuck (2011), it seems highly likely that this taxon is synonymous with I. pallidus. Forel’s (1901) description matches I. pallidus, he compared incertus with pallidus at several points in the description, and he established this name as a variety of pallidus. Iridomyrmex pallidus is distinct from the other known Papuan species of the genus (Iridomyrmex anceps, Iridomyrmex coeruleus, Iridomyrmex mjobergi and Iridomyrmex angusticeps) and it is unlikely that Forel would have confused this taxon with any of these species. The possibility exists that this name represents a taxon not seen during this study. However, in the unlikely event that this proves to be the case then this name can be resurrected and used for this additional species.

Donisthorpe’s I. wingi matches the concept of I. pallidus developed here, and there is nothing to suggest that it represents a separate species. Donisthorpe (1949) provides no indication of how his species might differ from any other species in the genus as no other species are mentioned. Based on the material examined this synonymy is straightforward.

Description
Worker Description. Head. Posterior margin of head planar to weakly concave, or weakly concave, or strongly 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, or absent from sides of head in full-face view (one to a few small setae may be present near mandibular insertion). Ocelli absent; in full-face view, eyes set at about midpoint of head capsule; in profile, eye set anteriad of head capsule; eye asymmetrical, curvature of inner eye margin more pronounced than that of its outer margin, or asymmetrical, curvature of inner eye margin more pronounced than its outer margin and anterior sector of eye distinctly broader than its posterior sector. Frontal carinae straight, or convex; antennal scape surpassing posterior margin of head by 1–2 x its diameter. Erect setae on scape absent, except at tip; prominence on anteromedial clypeal margin projecting as blunt but distinct protuberance, or present as an indistinct swelling or undulation; mandible elongate 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 straight. Erect mesonotal setae moderate in number (6–12), short and bristly, or sparse (6 or fewer) 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 moderate in number (6– 12), 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 present. Colour Tawny yellow-orange. Colour of erect setae yellow.

Measurements. Worker (n = 9)—CI 90–104; EI 24–30; EL 0.23–0.29; EW 0.18–0.22; HFL 1.18–1.42; HL 0.88–1.14; HW 0.80–1.16; ML 1.17–1.49; MTL 0.80–0.97; PpH 0.16–0.22; PpL 0.46–0.61; SI 88–109; SL 0.87– 1.04.