Iridomyrmex is a genus of ants called rainbow ants (referring to their blue-green iridescent sheen) first described by Austrian entomologist Gustav Mayr in 1862. He placed the genus in the subfamily Dolichoderinae of the family Formicidae. It has 79 described species and five fossil species. Most of these ants are native to Australia; others are found in Asia and Oceania, and they have been introduced to Brazil, New Zealand, and the United Arab Emirates. Fossil species are known from China, France, and the United States.
These ants are known to be an ecologically dominant and important group of ants, but they are sometimes regarded as pests because they disturb soil and enter human houses. Farmers in rural Australia place animal carcasses on meat ant (I. purpureus) mounds as a method of disposing of them; meat ants consume the carcass and reduce it to bones in a matter of weeks. Meat ants also engage in ritualised fighting, which helps prevent casualties and solve territorial disputes between neighbouring colonies. The largest members of this genus are those of the I. purpureus species group, measuring .
After their nuptial flight, queen ants may establish a colony by themselves, by budding, or cooperatively, where a subset of the colony migrates to a new location or when multiple queens help find a suitable nesting spot, but they display intolerance to each other when workers are present. The eggs take 44 to 61 days to fully develop into adults. Ants of the genus live in a wide variety of habitats and nest in soil in numbers that range from a few hundred individuals to over 300,000 in a single colony. Depending on the species, nests are large mounds covered in pebbles with multiple entrances, while others live above ground in twig nests. In some cases, ants dwell in several nest sites connected by paths; some of these nests can extend to in length. Some species associate with caterpillars and butterflies that provide the ants with secretions and honeydew, and I. bicknelli pollinates orchids. These ants are predators and scavengers; they hunt for prey to feed their young. Notably, these ants are immune to the toxins of the cane toad and feed on the juveniles. Predators such as spiders, birds, lizards, and other ants prey on Iridomyrmex ants.
Phylogeny The separation of the genus Iridomyrmex from its most recent common ancestor began around 12 million years ago. Its sister group, Froggattella, has only two species, even though both genera are the same age. In comparison to other Indo-Australian genera in the subfamily Dolichoderinae, the clade of which it is a part, is 23 million years old, meaning that it is rather young. However, the fossil species that have been found are from the Eocene and Oligocene. The following cladogram shows the phylogenetic position of Iridomyrmex among the Indo-Australian genera:
Taxonomy The genus Iridomyrmex was first described by Austrian entomologist Gustav Mayr in 1862, but it had no designated type species until 1903. By then, the type species was designated as Formica detecta, a synonym of Iridomyrmex purpureus. The genus was placed in the subfamily Dolichoderinae in 1878, the same year the subfamily was established. Since the establishment of the genus, it suffered taxonomic misunderstanding due to the gradual development of unrelated ants being designated into the genus by early researchers who failed to identify features that would make them distinct from the genus Iridomyrmex.
Myrmecologist William Brown Jr., was perhaps the first person to question the monophyly of the genus, mentioning that the Argentine ant, a former member of Iridomyrmex, differed from the rest of the genus. The first proper revisions of the genus began in the 1990s, and 91 species were transferred to six genera; only 62 species remained after these revisions. These species were placed into the genera Anonychomyrma, Doleromyrma, Linepithema, Ochetellus, Papyrius and Philidris. In 2011, the genus was revised again, and 79 species and five extinct species were now present, 31 of which were newly described. Four species were placed in different genera while 25 species and subspecies were classified as synonyms. According to one source, 350 Australian Iridomyrmex species may be present.
The scientific name of the genus, Iridomyrmex, meaning "rainbow ant", refers to their blue-green iridescent sheen. The word Irido, meaning "rainbow", derives from Ancient Greek, and myrmex, another Greek word, means "ant".
Species [extinct species are marked with †]
Iridomyrmex adstringatus Heterick & Shattuck, 2011 Iridomyrmex agilis Forel, 1907 Iridomyrmex alpinus Heterick & Shattuck, 2011 Iridomyrmex anceps (Roger, 1863) Iridomyrmex anderseni Shattuck, 1993 Iridomyrmex angusticeps Forel, 1901 Iridomyrmex anteroinclinus Shattuck, 1993 Iridomyrmex atypicus Heterick & Shattuck, 2011 Iridomyrmex azureus Viehmeyer, 1914 Iridomyrmex bicknelli Emery, 1898 Iridomyrmex bigi Shattuck, 1993 Iridomyrmex brennani Heterick & Shattuck, 2011 †Iridomyrmex breviantennis Théobald, 1937 Iridomyrmex brunneus Forel, 1902 Iridomyrmex calvus Emery, 1914 Iridomyrmex cappoinclinus Shattuck, 1993 Iridomyrmex cephaloinclinus Shattuck, 1993 Iridomyrmex chasei Forel, 1902 Iridomyrmex coeruleus Heterick & Shattuck, 2011 Iridomyrmex conifer Forel, 1902 Iridomyrmex continentis Forel, 1907 Iridomyrmex cuneiceps Heterick & Shattuck, 2011 Iridomyrmex cupreus Heterick & Shattuck, 2011 Iridomyrmex curvifrons Heterick & Shattuck, 2011 Iridomyrmex cyaneus Wheeler, 1915 Iridomyrmex difficilis Heterick & Shattuck, 2011 Iridomyrmex discors Forel, 1902 Iridomyrmex dromus Clark, 1938 Iridomyrmex elongatus Heterick & Shattuck, 2011 Iridomyrmex exsanguis Forel, 1907 †Iridomyrmex florissantius Carpenter, 1930 Iridomyrmex fulgens Heterick & Shattuck, 2011 Iridomyrmex galbanus Shattuck, 1993 Iridomyrmex gibbus Heterick & Shattuck, 2011 Iridomyrmex gumnos Heterick & Shattuck, 2011 Iridomyrmex hartmeyeri Forel, 1907 Iridomyrmex hertogi Heterick & Shattuck, 2011 Iridomyrmex hesperus Shattuck, 1993 Iridomyrmex infuscus Heterick & Shattuck, 2011 Iridomyrmex innocens Forel, 1907 Iridomyrmex lividus Shattuck, 1993 Iridomyrmex longisoma Heterick & Shattuck, 2011 Iridomyrmex luteoclypeatus Heterick & Shattuck, 2011 Iridomyrmex macrops Heterick & Shattuck, 2011 †Iridomyrmex mapesi Wilson, 1985 Iridomyrmex mattiroloi Emery, 1898 Iridomyrmex mayri Forel, 1915 Iridomyrmex meridianus Heterick & Shattuck, 2011 Iridomyrmex minor Forel, 1915 Iridomyrmex mirabilis Heterick & Shattuck, 2011 Iridomyrmex mjobergi Forel, 1915 Iridomyrmex neocaledonica Heterick & Shattuck, 2011 Iridomyrmex niger Heterick & Shattuck, 2011 Iridomyrmex nudipes Heterick & Shattuck, 2011 †Iridomyrmex obscurans Carpenter, 1930 Iridomyrmex obscurior Forel, 1902 Iridomyrmex obsidianus Emery, 1914 Iridomyrmex omalonotus Heterick & Shattuck, 2011 Iridomyrmex pallidus Forel, 1901 Iridomyrmex phillipensis Heterick & Shattuck, 2011 Iridomyrmex prismatis Shattuck, 1993 Iridomyrmex purpureus (Smith, 1858) Iridomyrmex reburrus Shattuck, 1993 Iridomyrmex roseatus Heterick & Shattuck, 2011 Iridomyrmex rubriceps Forel, 1902 Iridomyrmex rufoinclinus Shattuck, 1993 Iridomyrmex rufoniger (Lowne, 1865) Iridomyrmex sanguineus Forel, 1910 Iridomyrmex setoconus Shattuck & McMillan, 1998 †Iridomyrmex shandongicus Zhang, 1989 Iridomyrmex spadius Shattuck, 1993 Iridomyrmex splendens Forel, 1907 Iridomyrmex spodipilus Shattuck, 1993 Iridomyrmex spurcus Wheeler, 1915 Iridomyrmex suchieri Forel, 1907 Iridomyrmex suchieroides Heterick & Shattuck, 2011 Iridomyrmex tenebrans Heterick & Shattuck, 2011 Iridomyrmex tenuiceps Heterick & Shattuck, 2011 Iridomyrmex trigonoceps Heterick & Shattuck, 2011 Iridomyrmex turbineus Shattuck & McMillan, 1998 Iridomyrmex victorianus Forel, 1902 Iridomyrmex viridiaeneus Viehmeyer, 1914 Iridomyrmex viridigaster Clark, 1941 Iridomyrmex xanthocoxa Heterick & Shattuck, 2011
Description Unlike other genera in Dolichoderinae, the front margin of the clypeus is not below the mandibles; instead, it is located above them. The eyes are located on top of the ants' head, and they are noticeably distant from the mandibles. These ants range from small to medium; members of the I. conifer species group measure , being reddish-brown in colour with a distinctive propodeum, while members of the I. purpureus species group are the largest at . Worker castes are rarely polymorphic.
Based on observations, Iridomyrmex workers can be deformed when they are born. For example, a captured specimen had its propodeum and gaster fused together, and the petiole was still present but enlarged. Despite the deformity, the worker was still able to forage with little trouble.
Distribution and habitat
Ants of this genus are found in a range spanning several continents. In the Oceanian region, these ants are found in Fiji, Indonesia, New Caledonia, Papua New Guinea and the Solomon Islands. In Australia, they are found in every state and territory. In New Zealand, this genus has been introduced to the country, establishing themselves in both the North Island and South Island. In Asia, they are found in Burma, East Timor, India, Malaysia and the Philippines. I. anceps is the only known ant of this genus present in the Middle East, having been introduced to the United Arab Emirates. I. rufoniger was introduced to Brazil through human activity. Populations are present on the islands of Norfolk Island and Phillip Island in Oceania. Extinct species were native to a range spanning beyond the modern distribution of the genus; fossils have been found in China, France and the United States.
The genus Iridomyrmex inhabits rural, arid, and semi-arid areas. Nests are found in Acacia, Agathis, Banksia, Casuarina and Eucalyptus woodland, in pastures, Callitris forests, city parks, farmlands, grasslands, heath, sand dunes, savanna woodland, swamps, urban gardens, wet rain-forests, wet and dry sclerophyll woodland, and even in buildings such as hotels and houses.
Most of these ants nest in soil, but some prefer to live under rocks, rotten wood, and in some cases in sandy beaches and footpaths. Certain species will avoid nesting in sandy soil. Nests will vary in appearance; most species live in small mounds with a single entrance, while other species create large mounds covered in pebbles with multiple entrances. Some colonies are known to create "super-nests": workers construct many nests connected through established paths, extending up to in length. In one extreme case, a single colony was found to occupy over 10 hectares of land with 85 individual nests and 1,500 entrance holes. While meat ants are never aggressive to their nest mates, they will be aggressive to those who live in different nests within the same colony. Nests may be above or below ground, with some species such as I. conifer alternating between the two; during winter, nests will be constructed above in twigs, with the ants moving back underground during the warmer months. Underground nests are located near areas where food sources are present. Colonies will relocate their nests to areas where there are reasonable levels of sunlight during winter.
Behaviour and ecology