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Genus Iridomyrmex
(Subfamily Dolichoderinae)

Iridomyrmex head

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 vicinity.  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.

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 select 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.

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 (in the Iridomyrmex conifer species group) 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 may 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.

The literature concerning these ants is extensive.  A few of the more significant papers include those by Andersen and Patel (1994), Briese and Macauley (1981), Clayton-Green and Ashton (1990), Ettershank (1971), Ettershank and Ettershank (1982), Greenslade (1974, 1979), Greenslade and Halliday (1982), Moore (1974) and Shattuck (1992, 1993a, 1993b, 1996).

Distribution of Iridomyrmex

Distribution and Habitats
The 79 described species and subspecies of Iridomyrmex are distributed from India east to China and south to Australia and New Caledonia.  An additional seven species are known from fossil records.  Within Australia, 63 species and subspecies are described.  They occur in all areas (see map) and all major habitats, often in large numbers and with many species at any given site.

Identification Notes
Members of this genus can be identified by having the front margin of the clypeus above the mandibles highly modified with convex areas towards the sides and a central projection (this central projection varies from strongly to weakly developed) (Figs 1a, 2).  In addition, the compound eyes are placed relatively high on the head and away from the mandibles (Figs 1b, 2).  Most other genera in the subfamily Dolichoderinae 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.

Iridomyrmex head

Iridomyrmex head Iridomyrmex body

Many species of Iridomyrmex form "highways" between their nests and favorite feeding areas. These highways are warn smooth by the action of thousands of tarsii marching over them day after day, month after month. In this case the roadway runs from an Iridomyrmex purpureus nest (upper right) towards a food source in a Eucalyptus tree located some 9m away (to the lower left). (Photo by Steve Shattuck.)

Iridomyrmex are highly efficient predators/scavangers, taking a wide range of prey as food for their larvae. These workers have found a queen Aphaenogaster which is being returned intact to their nest (upper right). It will then be broken into sections small enough to fit through the nest entrance before being take to the brood chambers. (Photo by Steve Shattuck.)

General Literature

Crozier 1968 (karyotypes of the I. purpureus group), Ettershank 1971 (ecology), Greenslade 1974 (ecology), Moore 1974 (predation by Coleoptera), Briese and Macauley 1981 (biology), Ettershank and Ettershank 1982 (behaviour), Greenslade and Halliday 1982 (taxonomy and speciation in the I. purpureus group), Brophy et al. 1983 (biochemistry), Greenslade and Halliday 1983 (ecology of the I. purpureus group), Pierce 1984 (association with Lepidoptera), Fox et al. 1985 (ecology), Pierce and Elgar 1985 (association with Lepidoptera), Greenslade 1987 (ecology of the I. purpureus group), Pierce et al. 1987 (association with Lepidoptera), Haering and Fox 1987 (ecology), Elgar and Pierce 1988 (association with Lepidoptera), Clayton-Greene and Ashton 1990 (ecology), Shattuck 1992 (generic status), Shattuck 1993a (revision of the I. purpureus group), Shattuck 1993b (revision of the I.calvus group), Andersen and Patel 1994 (ecology), Shattuck 1996 (revision of the I. discors group), Shattuck and McMillan 1998 (revision of the I. conifer group).

Genus Nomenclature

Iridomyrmex Mayr 1862:702. Type species: Formica purpurea F. Smith, designated by Bingham 1903:297.

Taxonomic Catalogue

Go to the Taxonomic Catalogue of Species.


Literature Cited / Additional Reading

Andersen, A. N., Patel, A. D. 1994. Meat ants as dominant members of Australian ant communities: an experimental test of their influence on the foraging success and forager abundance of other species. Oecologia (Berlin). 98 : 15–24.
Briese, D. T., Macauley, B. J. 1981. Food collection within an ant community in semi-arid Australia, with special reference to seed harvesters. Australian Journal of Ecology. 6 : 1–19.
Brophy, J. J., Cavill, G. W. K., Davies, N. W., Gilbert, T. D., Philp, R. P., Plant, W. D. 1983. Hydrocarbon constituents of three species of dolichoderine ants. Insect Biochemistry. 13 : 381–390.
Clayton-Greene, K. A., Ashton, D. H. 1990. The dynamics of Callitris columellaris/Eucalyptus albens communities along the Snowy River and its tributaries in South-eastern Australia. Australian Journal of Botany. 38 : 403–432.
Crozier, R. H. 1968. Interpopulation karyotype differences in Australian Iridomyrmex of the "detectus" group (Hymenoptera: Formicidae: Dolichoderinae). Journal of the Australian Entomological Society. 7 : 25–27.
Elgar, M. A., Pierce, N. E. 1988. Mating success and fecundity in an ant-tended lycaenid butterfly, pp. 59-75. In T. H. Clutton-Brock (ed.) : studies of individual variation in contrasting breeding systems. University of Chicago Press. Chicago. pp.
Ettershank, G. 1971. Some aspects of the ecology and nest microclimatology of the meat ant, Iridomyrmex purpureus (Sm.). Proc. R. Soc. Victoria. 84 : 137–151.
Ettershank, G., Ettershank, J. A. 1982. Ritualised fighting in the meat ant Iridomyrmex purpureus (Smith) (Hymenoptera: Formicidae). Journal of the Australian Entomological Society. 21 : 97–102.
Fox, B. J., Fox, M. D., Archer, E. 1985. Experimental confirmation of competition between two dominant species of Iridomyrmex (Hymenoptera:Formicidae). Australian Journal of Ecology. 10 : 105–110.
Greenslade, P. J. M. 1974. Some relations of the meat ant, Iridomyrmex purpureus (Hymenoptera: Formicidae) with soil in South Australia. Soil Biology and Biochemistry. 6 : 7–14.
Greenslade, P. J. M. 1987. Environment and competition as determinants of local geographical distribution of five meat ants, Iridomyrmex purpureus and allied species (Hymenoptera: Formicidae). Australian Journal of Zoology. 35 : 259–273.
Greenslade, P. J. M., Halliday, R. B. 1982. Distribution and speciation in meat ants, Iridomyrmex purpureus and related species (Hymenoptera: Formicidae). pp. 249–255 in Barker, W. R., Greenslade, P. J. M. Evolution of the flora and fauna of arid Australia. Frewville, South Australia : Peacock Publications. 392 pp. Greenslade, P. J. M., Halliday, R. B. (1983). Colony dispersion and relationships of meat ants Iridomyrmex purpureus and allies in an arid locality in South Australia. Insectes Sociaux. 30 : 82–99.
Haering, R., Fox, B. J. 1987. Short-term coexistence and long-term competitive displacement of two dominant species of Iridomyrmex: The successional response of ants to regenerating habitats. Journal of Animal Ecology. 56 : 495–507.
Moore, B. P. 1974. The larval habits of two species of Sphallomorpha Westwood (Coleoptera: Carabidae, Pseudomorphinae). Journal of the Australian Entomological Society. 13 : 179–183.
Pierce, N. E. 1984. Amplified species diversity: a case study of an Australian lycaenid butterfly and its attendant ants. Pp. 197-200 in R. I. Vane-Wright and P. Ackery (eds.) The Biology of Butterflies. xxv + 429 pp. Academic Press, London.
Pierce, N. E., Elgar, M. A. 1985. The influence of ants on host plant selection by Jalmenus evagoras, a myrmecophilous lycaenid butterfly. Behavioral Ecology and Sociobiology. 16 : 209–222.
Pierce, N. E., Kitching, R. L., Buckley, R. C., Taylor, M. F. J., Benbow, K. F. 1987. The costs and benefits of cooperation between the Australian lycaenid butterfly, Jalmenus evagoras, and its attendant ants. Behavioral Ecology and Sociobiology. 21 : 237–248.
Shattuck, S. O. 1992. Review of the dolichoderine ant genus Iridomyrmex Mayr with descriptions of three new genera (Hymenoptera: Formicidae). Journal of the Australian Entomological Society. 31 : 13–18.
Shattuck, S. O. 1993a. Revision of the Iridomyrmex purpureus species group (Hymenoptera: Formicidae). Invert. Tax. 7: 113-149.
Shattuck, S. O. 1993b. Revision of the Iridomyrmex calvus species-group (Hymenoptera: Formicidae). Invertebrate Taxonomy. 7 : 1303–1325 .
Shattuck, S. O. 1996. Revision of the Iridomyrmex discors species-group (Hymenoptera: Formicidae). Aust. J. Entomol. 35 : 37–42.
Shattuck, S.O. & McMillan, P. 1998. Revision of the species of the Iridomyrmex conifer group (Hymenoptera : Formicidae), with notes on their biology. Australian Journal of Zoology. 46 : 301–315.



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