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.



Worker of I. purpureus from New South Wales.



Worker of I. rufoniger from New South Wales.

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.

Distribution and Habitats
Australian distribution

Regional Species Lists

 * Australia

Keys to Species

 * Australia