Iridomyrmex anceps is a dull, light to dark brown polymorphic species with large inset eyes high on the head, dense pubescence, relatively long limbs, and distinctive anterior clypeal margin with three convexities. While most Iridomyrmex species are restricted to Australia and New Guinea, I. anceps occurs more broadly. The species is found in the Old World from India, China and Malaysia through the Philippines and Indonesia, across northern Australia and throughout Melanesia, Micronesia and into Polynesia. It is not known whether I. anceps has increased its range with the assistance of human-mediated dispersal, or if its current range was already established prior to contact with humans. The species is present on Tokelau, which is the easternmost Pacific Island with a native ant fauna, but has not been reported from Hawaii. The species is not considered a significant pest across any of its range.
The confusion surrounding what part, if any, of the range is introduced is driven by the taxonomic uncertainty of the I. anceps group. Heterick & Shattuck (2011)recently synonymized six names with I. anceps, lumping the group across its entire range into a single species. Arguments for treating the taxa involved instead as a group or complex of species were made by Hoffman et al. (2011). It is the opinion of the authors that a more comprehensive study of the entire group across its entire range, possibly with the aid of molecular tools, is required to answer the question.
Iridomyrmex anceps had long been considered to have established an introduced population in New Zealand (e.g. (Faulds, 1970; Lester, 2005)), but the ants were later confirmed to be a species distantly related to I. anceps (Don, 2007). Iridomyrmex anceps is reported from the United Arab Emirates, but the record is not discussed in either Heterick & Shattuck (2011)or Hoffman et al. (2011). Wilson & Taylor (1967)claimed that the specimen of I. anceps collected at the Nadi airport in Fiji was evidence of the species’ eastward expansion into the Pacific. That claim is obviated, or at least pushed back a half century, by the recent synonymy of I. anceps ignobilis (Mann) which Mann described from Fiji in 1921. The distribution of I. anceps in Fiji is limited to the mostly disturbed habitat on the western (leeward) side of Fiji’s largest island, Viti Levu, suggesting perhaps that it is a relatively recent arrival that has not dispersed to the full extent of its capacity (Sarnat & Economo, In Press). The species was found by Ward (2007)to be aggressive at baits, but such behavior was not observed by the less rigorous observations reported in Sarnat & Economo (In Press). Given the success of I. anceps on Pacific islands, it is somewhat surprising that it has not yet been found in Hawaii.
Diagnosis of worker among Antkey species. Worker caste weakly polymorphic, but lack distinct major caste. Antenna 12-segmented. Antennal scape length less than 1.5x head length. Eyes medium to large (greater than 6 facets); placed at or above midline of face. Eyes do not break outline of head. Antennal sockets and posterior clypeal margin separated by a distance less than the minimum width of antennal scape. Anterior clypeal margin with one median and two lateral rounded projections. Mandible with distinct basal angle. Dorsum of mesosoma lacking a deep and broad concavity, with erect hairs that are thin and flexuous. Promesonotum separated from propodeum by metanotal groove. Propodeum and petiolar node both lacking a pair of short teeth. Propodeum lacking posteriorly projecting protrusion; dorsal surface not distinctly shorter than posterior face. Waist 1-segmented. Petiole upright and not appearing flattened. Gaster armed with ventral slit. Distinct constriction not visible between abdominal segments 3+4. Color uniformly light to dark brown.
Among the species treated here, I. anceps is most likely confused with dolichoderines with upright petiolar nodes and inset eyes that do not break the outline of the head, namely Dolichoderus, Linepithema and Ochetellus species. The low propodeal profile and convex propodeal declivity of I. anceps separates it from Dolichoderus and Ochetellus species. Iridomyrmex anceps is separated from Linepithema species by (1) eyes that are situated at or above the midline of the head; (2) mandibles with distinct basal angles; and (3) anterior clypeal border with one median and two lateral convexities.
Dolichoderus thoracicus, Linepithema humile, Ochetellus glaber
Native range includes: Malaysia, India, China, Philippines, Indonesia, Australia, Micronesia, Papua New Guinea, Fiji, Tokelau. United Arab Emirates population may be introduced. It is unknown what extent of the southwestern Pacific island populations are introduced vs. native.