Flat-backed tyrant ants (species group common name - Andersen 2002)
Synonyms (Not yet formally described. )
According to Shattuck (1999), Iridomyrmex contains 79 described species and subspecies. They are widely distributed from India to China, Australia, and New Caledonia. The one species of Iridomyrmex established in New Zealand is almost certainly of Australian origin. Previously known here as I. anceps (Roger), a tramp species, and possibly the most widespread of the Indo-Australian Iridomyrmex , it has only recently been realized that this is a misidentification based on an initial provisional determination by R. W. Taylor (Faulds 1970). New Zealand specimens recently examined by A. N. Andersen (CSIRO, Berrimah, Northern Territory) apparently match those of an unnamed species widespread in Australia - it is particularly prevalent along the eastern seaboard - which belongs not to the anceps group but to the suchieri group (A. N. Andersen pers. comm.).
The earliest record in this country appears to be a series of workers from Te Puke, Bay of Plenty, collected in 1954 (NZAC) and provisionally described at the time as I. anceps. The first records in the New Zealand literature are of specimens lodged in the Forest Research Institute (Rotorua) collection from Matahina Forest, near Whakatane, 4 November 1961. Further specimens were collected in early 1962 at Otumoetai (Tauranga), and from Tairua Forest, Coromandel Peninsula later the same year (Faulds 1970).
Distribution (see map)
The vast majority of collections are from Auckland City and its surrounds(including offshore islands), where it has become a conspicuous member of the ant fauna. This species is also present in Northland, Coromandel, Waikato, Bay of Plenty, Hawke's Bay and Nelson City.
In Iridomyrmex the compound eyes are placed high and relatively posteriorly on the head. The front margin of the clypeus above the mandibles is highly modified with convex areas towards the sides and a central projection that can vary in size (Shattuck 1999).
Diagnostic features of the worker
Length about 2 mm; antennae 12-segmented; mandibles with 7 teeth; central projection on clypeus obtusely triangular; propodeal angle rounded; the single petiolar node is in the form of a narrow, vertical scale; colour uniformly brown, apart from the gaster, which is darker.
Andersen (2000) states "Iridomyrmex is by far Australia's most ecologically important ant genus." They are highly abundant and aggressive ants, virtually dominating all Australian ant communities. As well as inhabiting remote rural areas, the species is a common "pavement ant" in cities such as Sydney, Melbourne and Adelaide, where it is "characteristic of parks and gardens." (A. Andersen, pers. comm.). Workers are also commonly seen in New Zealand foraging on pavements and areas of bare ground, and entering and exiting their nests below.
Green (1992) notes that Aucklanders often complain that, "its habit of nesting under paving stones displaces quite a large amount of sand, and eventually makes the stones uneven and dangerous." Small mounds of excavated soil or sand mark the location of nest entrances. Most species of the genus are general scavengers (Shattuck 1999), and this species appears to be no exception - Green (1992) notes its habit of invading kitchens where it shows a liking for "sweet and savory foods." She also observes that workers have been known to bite humans on occasions.
While this species is abundant in open, bare ground, it is far less prevalent among vegetation. The typically aggressive nature of this species appears to confer some degree of dominance over other ants. However, there are signs in some areas (on Tiritiri Matangi Island, for example) of this species being displaced by the even more aggressive and invasive Argentine ant, Lithepithema humile (Mayr).
In the field it can readily be distinguished from other ants in New Zealand because of its rapid gait (workers have long legs) and zigzag trailing over the substrate. Specimens have been collected from intertidal situations (for example, Rangitoto Island), a somewhat unexpected habitat for ants. L. humile too has been observed foraging in this zone at low tide.
Likely of nuisance value as it is a highly visible, rapidly moving ant on pavements and areas of bare ground. In summer, ants swarming out of the nest may cause concern. Can enter buildings in search of food, but not as common an occurrence as with some other species. Bites people occassionally. Its abundance and impact in native habitats is unknown.Compiled by Warwick Don & Richard Harris