Iridomyrmex minor

Iridomyrmex minor is primarily a northern and central Australian species: in Western Australia the most southerly populations have been found near Billabong Roadhouse in the Shark Bay region, and in South Australia populations can be found as far south as the Gawler Ranges. The ant is also common in the Northern Territory and Queensland, but so far has not been collected in New South Wales, and is unlikely to occur in Victoria or Tasmania. This species occurs in a variety of habitats, although sandy areas may be preferred, judging from collections. At least one nest has been found under bark, but soil is a more usual nest substrate. Along with Iridomyrmex anceps and several meat ants, I. minor is included among the Iridomyrmex species most likely to be encountered by the general public in northern parts of Australia.

Identification
This very variable species is probably the most difficult of all the Iridomyrmex species to characterise in terms of its external morphology, and also the most likely to contain cryptic taxa. The ant was originally described as a subspecies of Iridomyrmex gracilis (now Iridomyrmex bicknelli). The populations in different parts of Australia include strikingly distinctive phenotypes. The population from the Kimberley region represents the form most commonly seen in the drier areas of the Torresian zone. This is a broad-headed, large-eyed ant. Seen in full-face view, the appressed genal setae confer a partially silvery appearance to the head capsule. The ant is generally a medium brown, with some vaguely mottled orange-brown sectors on the head and mesosoma in most workers. The antennal scapes are moderately long, exceeding the posterior margin of head by about half their length. Erect setae on the outline of the head capsule are restricted to the posterior margin of head, and there are no erect setae on the hind tibiae. In the wetter northern fringes is a much more gracile morph, with a flattened mesosomal outline, in which the legs and antennal scape are typically noticeably more elongate, and the head is visibly narrower. The antennal scape exceeds the posterior margin of head by more than half its length. The appressed setae on the mesosoma are relatively long and fine, giving the ant a ‘furry’ appearance. In a few rare instances erect setae extend along the sides of the head and can be found on the hind tibiae. Similarly elongate, but virtually glabrous, is a morph from inland areas of mid-west Western Australia. The appressed setae in this form are very short and virtually invisible except in certain lights. In both this and the far northern phenotype the hind tibiae are as long as or even longer than the mesosoma. Workers from coastal Western Australia present yet another distinct phenotype: here the ant also has a rectangular head capsule, along which extend short, erect setae, the hind tibiae are likewise setose, and the antennae are also long. However, the head and mesosoma are a bright brick red. This phenotype and a form intermediate between this and the typical northern phenotype (i.e., reddish but broad-headed and with erect setae confined to the posterior margin of head and absent from the hind tibiae) can be found on Barrow Island. Finally, a distinct form occurs in inland South Australia. Workers of this phenotype have orange heads and bodies with contrasting brown antennal scapes, legs and gaster. The antennal scape is quite short, extending by only about quarter of its length beyond the head capsule. Erect setae on the posterior margin of head do not extend to the sides of the head.

Despite these distinctive morphs, there are very many intermediate forms, making a hypothesis that this is a single innately morphologically highly variable species the hypothesis of choice for the present time. There are no molecular data as yet, and this would be highly desirable, in the light of the huge variation seen for I. minor. As it is constituted here, this taxon can be separated from the similar Iridomyrmex anceps by its longer pronotal setae, and (in the glabrous form) its non-uniform colouration.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  minor. Iridomyrmex gracilis var. minor Forel, 1915b: 80 (w.q.) AUSTRALIA. Raised to species: Heterick & Shattuck, 2011: 104.

Type Material


The following specimens were included in the syntypes of Iridomyrmex gracilis minor but are actually Iridomyrmex suchieri:

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 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 above 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 convex; antennal scape surpassing posterior margin of head by at least 0.5 x its length, or 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, or projecting as blunt but distinct protuberance; 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, or weakly undulant or almost straight. Erect pronotal setae moderate in number (6–12), longest setae elongate, flexuous and/or curved, or sparse (6 or fewer) and bristly. Mesonotum sinuous, or straight. Erect mesonotal setae moderate in number (6–12), short and bristly, or sparse to absent. Mesothoracic spiracles prominent or 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, or sparse to absent. 'Petiole. 'Dorsum of node convex; node thin, scale-like, orientation more-or-less vertical, or thick, 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 head and body brown to orange, usually variegated, with at least the clypeal region distinctly lighter in colour than the upper vertex in mostly brown specimens, gaster light to dark chocolate brown. Colour of erect setae pale yellow-brown.

Measurements. Worker (n = 12)—CI 78–93; EI 23–30; EL 0.21–0.30; EW 0.16–0.23; HFL 1.29–2.08; HL 0.87–1.34; HW 0.74–1.21; ML 1.2 1–2.03; MTL 0.94–1.65; PpH 0.15–0.24; PpL 0.47–0.74; SI 105–173; SL 0.95– 1.6 1.

References based on Global Ant Biodiversity Informatics

 * CSIRO Collection
 * Chong C-S., L. J. Thomson, and A. A. Hoffmann. 2011. High diversity of ants in Australian vineyards. Australian Journal of Entomology 50: 7-21.
 * Forel A. 1915. Results of Dr. E. Mjöbergs Swedish Scientific Expeditions to Australia 1910-13. 2. Ameisen. Ark. Zool. 9(16): 1-119
 * Heterick B. E., and S. Shattuck. 2011. Revision of the ant genus Iridomyrmex (Hymenoptera: Formicidae). Zootaxa 2845: 1-174.
 * Shattuck S. O. 1994. Taxonomic catalog of the ant subfamilies Aneuretinae and Dolichoderinae (Hymenoptera: Formicidae). University of California Publications in Entomology 112: i-xix, 1-241.