Iridomyrmex dromus

Iridomyrmex dromus is mainly nocturnal, and has been collected in a bewildering variety of habitats across Australia, ranging from desert and dry sclerophyll to rainforest litter (Tasmania). However, this species is absent from the northern tropics, where its niche is probably filled by Iridomyrmex pallidus. Although some workers have been collected from vegetation, most foraging is terrestrial, and pitfall trapping has been the main source of specimens. Workers move over the soil in loose columns, and are generally timorous.

Identification
Iridomyrmex dromus is one of a small number of Iridomyrmex taxa that are genuinely problematic for the alpha taxonomist. The taxon is very widespread throughout Australia, and morphological examination reveals an uncomfortable array of variation, if this is to be considered a single species. Limited DNA analysis also reveals wide variation in several samples examined. Aside from the fact that the colour of the body can vary from very pale, depigmented yellow all the way through to coal-black, there are considerable differences in the size of the eyes (which can vary from medium-sized and slightly asymmetrical to enormous and protuberant), the length of the antennal scape (from extending to just beyond the posterior margin of the head to extending by at least half its length beyond that margin) and the shape of the propodeum (which varies from broadly protuberant to long and dorsally flattened). Relatively distinctive forms can be found in certain regions; for example, workers collected in the south-west of the continent tend to be pale and very small with narrow head and small eyes, whereas many taken from the mid-west of Western Australia are brown and large-eyed. Also brown and with short antennal scapes are specimens collected in the Simpson Desert and its environs in Queensland, northern New South Wales and South Australia. Perhaps the most spectacular morph is one collected from a variety of sites, but generally inland. This ant is honey-coloured, with very elongate scapes, large eyes and a flattened propodeum. However, none of these forms possesses discrete characters that enable it to be recognised as a unique taxon when all populations are considered: that is, intermediate forms connect the clusters of the more distinctive workers, and hours of examination have failed to identify diagnostic features unique to a particular morph. As a consequence, the position taken here is that I. dromus is genuinely polymorphic with respect to phenotype and also genetically variable. This position could change, if further molecular analysis reveals unacceptable genetic distance between given populations. However, if such is the case, then workers of the constituent species would have to be regarded as truly cryptic and possibly impervious to separation using morphological characters. For the present, the distinctly asymmetrical eyes placed near the margin of the head in full-face view, the oblique angle between the dorsal and declivitous propodeal surfaces, and the lack of erect or subdecumbent setae on the hind tibiae, sides of head and antennal scape serve to identify the species as understood here. Morphologically similar species include Iridomyrmex cupreus (erect setae on tibiae), Iridomyrmex exsanguis (sharp to blunt right angle between propodeal dorsum and propodeal declivity), Iridomyrmex macrops (erect setae on the tibiae and often the antennal scape) and Iridomyrmex pallidus (eyes placed away from sides of head in full-face view and subdecumbent setae on tibiae). Iridomyrmex hartmeyeri may also be confused with I. dromus, but the latter always has some erect pilosity on the mesosomal dorsum, whereas this is usually lacking in the larger I. hartmeyeri.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  dromus. Iridomyrmex dromus Clark, 1938: 374, fig. 12 (w.) AUSTRALIA. See also: Heterick & Shattuck, 2011: 76.

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, or present in small aggregations on one or both sides of posterior margin of head, or present singly or as a couple of setae on either side of posterior margin of head; 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). 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 its outer margin and anterior sector of eye distinctly broader than its posterior sector. Frontal carinae concave, or straight; 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 present as an indistinct swelling or undulation; mandible elongate triangular with oblique basal margin; long, curved setae on venter of head capsule present in some workers. Mesosoma. Pronotum moderately and evenly curved over its length. Erect pronotal setae moderate in number (6–12), short and bristly, or sparse to absent. Mesonotum sinuous, or straight, or evenly curved. Erect mesonotal setae sparse to absent. 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 present as a bluntly defined right angle, the dorsal and declivitous propodeal faces never separated by a carina, or weakly present or absent, the confluence of the dorsal and declivitous propodeal faces indicated, if at all, by an undulation. Erect propodeal setae sparse to absent. 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, or absent on first tergite. General characters. Allometric differences between workers of same nest present. Colour of most workers depigmented yellow with darker head, but quite often mottled brown, while workers of other populations may be dark brown and even black. Colour of erect setae light brown.

Measurements. Worker (n = 46)—CI 79–92; EI 29–42; EL 0.18–0.35; EW 0.14–0.27; HFL 0.8 1–1.57; HL 0.61–1.03; HW 0.49–0.95; ML 0.80–1.44; MTL 0.54–1.12; PpH 0.09–0.20; PpL 0.31–0.58; SI 107–145; SL 0.63– 1.20.