Iridomyrmex

Iridomyrmex is one of the largest and most frequently encountered groups of ants in Australia. They are also one of the most ecologically important groups as they interact strongly with many other invertebrates as well as many plants. Iridomyrmex species frequently form large nests which are patrolled by aggressive workers. This can significantly reduce the number of other species which can nest or forage in the area. Sometimes the only species which can co-exist with Iridomyrmex are those which forage at different times of the day or differ in size compared with Iridomyrmex species, and thus "escape" interactions with the Iridomyrmex workers.

The aggressive actions of Iridomyrmex species are not just limited to other species of ants. Individual colonies of the same and closely related meat ants (Iridomyrmex purpureus group) form discrete, non-overlapping territories with well-defined boundaries. These boundaries are patrolled regularly and when disputes arise, ritualised fighting can occur. During these fights, large numbers of workers from each colony come together, stand upright on the tips of their legs, and kick each other with their hind legs. These confrontations can go on for hours or even days with little or no mortality among the combatants. Once the boundary dispute has been resolved, the workers return to their nests and little interaction is seen in the former battlefield.

Nests are located in soil, with or without covering, and range in size from a few hundred to over 300,000 workers. The above-ground structure of nests varies from large mounds decorated with small pebbles and having many entrances to single, cryptic holes just large enough for individual workers to squeeze through. Several species in southern Western Australia alternate between two distinct nest types. In the cool winter months they construct above ground twig nests in open areas, while in the hot summer months they move to below ground nests in shaded areas. Colonies of meat ants (Iridomyrmex purpureus group) are often spread over wide areas with many individual nests connected by well defined paths. In some cases these "super nests" can stretch up to 650 metres.

Most species of Iridomyrmex are general scavengers. They also tend aphids and coccids and will collect nectar when available. Workers of some forage in large, well defined columns to the same feeding sites for extended periods of time, while others forage singly. Some Iridomyrmex associate closely with the caterpillars of certain butterflies. In extreme cases, the caterpillars live in the nests of Iridomyrmex and are carried to feeding areas and protected by the ants. The caterpillars have special glands that produce secretions which are very attractive to these ants.

A number of invertebrates have taken advantage of the large nests of Iridomyrmex by becoming specialist predators on these ants. Some spiders prey largely on Iridomyrmex workers, and have even developed the ability to use the ants' communication chemicals to determine which individuals to attack. The ants release a special chemical when injured to alert other ants of potential danger. The spiders detect this chemical and preferentially selects these injured workers as potential prey. Some predacious ground beetles establish their burrows in soil near the ants' nests. From the relative safety of their burrows, they grab passing ants and kill them, feeding on their body fluids.

Many plants produce seeds with special food bodies (elaiosomes) that are attractive to ants and other insects. Iridomyrmex foragers are often attracted to these seeds and carry them into their nests. Once the food bodies are taken from the seeds, the seeds are discarded. Being in or near the ants' nests provides protection to the young seedlings and may increase the survival of the plants.

Fossil species of Iridomyrmex are known from the Eocene (60-35 million years before present) and Oligocene (35-25 million years before present).

Identification
The front margin of the clypeus above the mandibles is highly modified with convex areas towards the sides and a central projection (this central projection varies from strongly to weakly developed). The compound eyes are placed relatively high on the head and away from the mandibles. Most other genera in this subfamily have the front margin of the clypeus weakly convex, straight or weakly concave. Only Froggattella and Philidris share the central projection with Iridomyrmex, but these have the eyes low on the head, nearer to the clypeus and Froggattella has the propodeal spiracle higher and on the propodeal spines.

Extended diagnosis

Worker: Compound eyes placed relatively posteriorly on head; anterolateral clypeal margin posterior to the mediolateral region and separated from it by a shoulder; anteromedial clypeal margin with a central projection, either pointed or rounded (sometimes only feebly projecting).

Queen: Compound eyes relatively posterior on head; anterolateral clypeal margin posterior to the mediolateral areas and separated from it by a shoulder; anteromedial clypeal margin with a central projection, either pointed or rounded (sometimes only feebly projecting); apical tooth of mandible elongate and much longer than subapical.

Male: Mandible with 1 tooth (the apical) and no denticles (rarely with up to about 25 denticles); first gastral segment vertical and not concealing the petiole in dorsal view.

Identification Keys
World species

Description
WORKER

HEAD. Vertex convex to weakly concave. Compound eyes present, approximately round; relatively posterior on head. Ocelli absent (rarely present). Antennae 12 segmented. Scape relatively short, at most surpassing the vertex by less than one-half (often less than one-third) its length. Anterolateral clypeal margin posterior to the mediolateral region and separated from it by a shoulder. Anteromedial clypeal margin with a central projection, either pointed or rounded (sometimes only feebly projecting). Anterior clypeal setae 6-15; short, less than twice the maximum scape diameter; straight. Posterior clypeal margin between the anterior and posterior surfaces of the antennal socket cavities. Anterior tentorial pit nearer the antennal socket than the mandibular insertion. Frontal carina present. Anterolateral hypostoma reduced to a thin sclerite. Medial hypostoma entire. Psammophore absent (gula rarely with numerous short, erect, randomly placed hairs). MOUTHPARTS. Palp formula 6:4. Third maxillary palp segment subequal in length to segment 4. Fifth maxillary palp segment at the apical extreme of segment 4. Mandible with about 7-10 teeth and about 2-5 denticles. Apical tooth varying from slightly to much longer than the subapical tooth. Basal angle varying from distinct (with a well developed tooth or angle separating the masticatory and basal margins) to indistinct (with a relatively uninterrupted curve between the two margins). Basal margin denticulate distally, smooth proximally. MESOSOMA. Posteroventral pronotum lateral, rounded or angled. Mesopleural process absent. Anteromedial mesosternum even with the lateral regions. Declivitous face of propodeum convex to flat; dorsal face convex, varying from longer than, to subequal in length to, the declivitous face. Propodeal angle distinct to indistinct (rarely expanded dorsally into a cone-like projection). Mesosomal spines and tooth absent. Erect pronotal hairs 0-80; when present, varying from short (about as long as maximum scape width) to elongate (much longer than the maximum scape width). Dorsal pro-mesonotal junction with the pronotum and mesonotum even. Metanotal groove forming a distinct angle between the mesonotum and propodeum. Metanotal spiracle lateral and ventral of the dorsal surface, or dorsal and lying on the dorsal surface, when viewed in lateral profile. Propodeal spiracle lateral and ventral of the propodeal dorsum. Hind tibial spur with well developed barbules along entire inner surface (except extreme base). PETIOLE. Scale present; rounded and forming an even arch dorsally, or ridged and with a distinct angle dorsally; vertical and not inclined anteriorly to moderately inclined anteriorly but with the anterior and posterior faces approximately the same length (rarely strongly inclined anteriorly and with the anterior face much shorter than the posterior face). Venter with or without a well developed lobe. GASTER. First tergite vertical and not concealing the petiole in dorsal view and with a groove or indentation for the reception of the basal portion of the petiole. Anterior tergosternal suture of the first segment extending laterally from the helcium in a distinct arch which extends dorsal of the dorsal helcial surface. Fifth tergite ventral, gaster with 4 apparent tergites. Gastral compression absent (gaster circular in cross section) or weakly lateral. Fourth sternite flat across entire posterior border. GENERAL CHARACTERS. Worker caste monomorphic or rarely polymorphic. Chromosome number 9 (2n=18, I. purpureus-group, blue form, black form, Imai et al. 1977; 2n=18, I. gracilis-group, sp. ANIC-13, ANIC-14, ANIC-15, ANIC-16, ANIC-17, Imai et al. 1977; n=9, 2n=18, I. gracilis(A), gracilis(B), gracilis(C), Crozier 1968a; 2n=18, I. anceps, Imai et al. 1984; 2n=48 (probably in error), I. anceps, Imai et al. 1985b; n=9, 2n=18, I. detectus-group (=purpureus-group), Crozier 1968a, Crozier 1968b; n=9, 2n=18, I. mattiroloi, Crozier 1968a). Integument thin and flexible, weakly sculptured. PROVENTRICULUS. Cupola much broader than bulb; round; with short pile; smooth, without sculpture; and with narrow phragma. Bulb completely hidden by cupola in lateral view. Longitudinal muscle No. 1 absent. Occlusory tract absent.

QUEEN

HEAD. Vertex weakly convex to flat. Compound eyes relatively posterior on head. Antennae 12 segmented. Scape short, surpassing the vertex by less than one-half scape length. Anterolateral clypeal margin posterior to the mediolateral region and separated from it by a shoulder. Anteromedial clypeal margin with a central projection, either pointed or rounded (sometimes only feebly projecting). Anterior clypeal setae 8-16; short, less than twice the maximum scape diameter; straight. Posterior clypeal margin posterior to the anterior surfaces of the antennal socket cavities. Anterior tentorial pit nearer the antennal socket than the mandibular insertion. Anterolateral hypostoma reduced to a thin sclerite. Medial hypostoma entire. Psammophore absent. MOUTHPARTS. Palp formula 6:4. Third maxillary palp segment subequal in length to segment 4. Fifth maxillary palp segment at the apical extreme of segment 4. Mandible with 7-9 teeth and 4-7 denticles. Apical tooth elongate and much longer than the subapical tooth. Basal angle distinct, with a well developed tooth or angle separating the masticatory and basal margins. Basal margin varying from denticulate distally, smooth proximally to denticulate along entire surface. MESOSOMA. Posteroventral pronotum lateral, rounded or angled. Episternal suture weak, nearly absent. Mesopleural process absent. Anteromedial mesosternum even with the lateral regions. Axilla parallel, and entire or with a suture medially. Anterior axillar suture straight or angular medially. Declivitous and dorsal faces of propodeum convex; dorsal face subequal in length to the declivitous face. Propodeal angle indistinct. Propodeal suture absent. Mesosomal spines and tooth absent. Erect mesoscutal hairs 6-60; short, less than twice the maximum scape diameter. Propodeal spiracle lateral and ventral of the propodeal dorsum. Hind tibial spur with well developed barbules along entire inner surface (except extreme base). WINGS. Radial cell closed. Fore wing with 1-2 cubital and 1 discoidal cell. Hind wing with 2 cells. PETIOLE. Scale present; ridged and with a distinct angle dorsally; varying from vertical and not inclined anteriorly to moderately inclined anteriorly but with the anterior and posterior faces approximately the same length. Venter with a well developed to slight or weakly developed lobe. GASTER. First segment vertical and not concealing the petiole in dorsal view and with a groove or indentation for the reception of either the basal portion or the entire height of the petiole. Fifth tergite vertical and with the distal terminus of the gaster not well defined. Gastral compression absent (gaster circular in cross section). Fourth sternite flat across entire posterior border.

MALE

HEAD. Inner margin of eye entire, flat. Scape length shorter than the length of funicular segments 2+3. First funicular segment cylindrical (or cone-shaped) or barrel-shaped. Second funicular segment cylindrical, straight. Funicular segments 2 and 3 at most twice as long as broad. Third and fourth funicular segments straight. Anteromedial clypeal margin entire, without a central notch or concavity of any type. Anterior clypeal setae 6-8; with both short (about as long as the maximum diameter of the scape) and long (about the same length as the closed mandibles) hairs (sometimes only short hairs present); straight. Posterior clypeal margin even with or anterior to the anterior surfaces of the antennal socket cavities. Anterior tentorial pit nearer the antennal socket than the mandibular insertion. Anterolateral hypostoma reduced to a thin sclerite. Medial hypostoma entire. MOUTHPARTS. Palp formula 6:4. Third maxillary palp segment subequal in length to segment 4. Fifth maxillary palp at the apical extreme of segment 4. Mandible with 1 tooth and no denticles (rarely about 25 denticles are present). Apical tooth distinct. Basal angle indistinct, with a relatively uninterrupted curve between the two margins and without a distinct tooth or angle. Basal margin smooth and without teeth or denticles (rarely denticulate along entire margin). MESOSOMA. Posteroventral pronotum lateral, rounded or angled. Episternal suture varying from present and complete to reduced and incomplete. Anteromedial mesosternum even with the lateral regions. Axilla parallel and entire. Anterior axillar suture straight. Declivitous and dorsal faces of propodeum convex; dorsal face longer than the declivitous face. Propodeal angle distinct or indistinct. WINGS. Radial cell closed. Fore wing with 1 cubital and 1 discoidal cell. Pterostigmal appendage absent. Hind wing with 2 cells. PETIOLE. Scale present; rounded and forming an even arch dorsally, or ridged and with a distinct angle dorsally; vertical and not inclined anteriorly. Venter with or without a slight or weakly developed lobe. Attachment to gaster broad to intermediate. GASTER. First segment vertical and not concealing the petiole in dorsal view, smooth and without a groove or indentation. GENITALIA. Pygostyles present. Posterior margin of subgenital plate with a "V"-shaped notch. Paramere divided by a membranous region. Digitus linear, with a slight ventral arch. Cuspis parallel with digitus. Ventral lobe of volsella absent. Aedeagus with ventral teeth.

LARVA Shape dolichoderoid. Protuberances present as a single boss located mid-dorsally on abdominal tergite 2. Body hairs sparse; simple; short. 9 spiracular pairs. Antennae short.

Nomenclature

 *  IRIDOMYRMEX [Dolichoderinae: Leptomyrmecini]
 * Iridomyrmex Mayr, 1862: 702. Type-species: Formica detecta (junior synonym of Formica purpurea), by subsequent designation of Bingham, 1903: 297.

The genus Iridomyrmex has proven to be one of the most taxonomically challenging in the subfamily. It had long been suspected of being an assemblage of unrelated species (Brown 1958, Crozier 1968a, Brown 1977, Snelling and Hunt 1975). However, few suggestions have been put forth to resolve this problem even though there is no lack of available material; most larger museums have hundreds of specimens representing dozens of species. Moreover, there is no lack of taxonomically useful characters, as detailed examination revealed numerous character systems which vary among species. Shattuck (1992) examined this problem group and found that the 152 described species and subspecies previously placed in Iridomyrmex actually belonged to seven separate genera, with the species being scattered among Anonychomyrma, Doleromymra, Iridomyrmex, Linepithema, Ochetellus, Papyrius and Philidris.

Additional References

 * Heterick, B.E. & Shattuck, S.O. 2011. Revision of the ant genus Iridomyrmex (Hymenoptera: Formicidae). Zootaxa 2845: 1–174.