The green-head ant (Rhytidoponera metallica) is a species of ant that is endemic to Australia. It was described by British entomologist Frederick Smith in 1858 as a member of the genus Rhytidoponera in the subfamily Ectatomminae. These ants measure between . The queens and workers look similar, differing only in size, with the males being the smallest. They are well known for their distinctive metallic appearance, which varies from green to purple or even reddish-violet. Among the most widespread of all insects in Australia, green-head ants are found in almost every Australian state, but are absent in Tasmania. They have also been introduced in New Zealand, where several populations have been established.
This species lives in many habitats, including deserts, forests, woodland and urban areas. They nest underground below logs, stones, twigs, and shrubs, or in decayed wooden stumps, and are sometimes found living in termite mounds. They are among the first insects to be found in burnt-off areas after the embers have stopped smouldering. Rain presents no threat to colonies as long as it is a light shower in continuous sunshine. The green-head ant is diurnal, active throughout the day, preying on arthropods and small insects or collecting sweet substances such as honeydew from sap-sucking insects. They play an important role in seed dispersal, scattering and consuming seeds from a variety of species. Predators include the short-beaked echidna (Tachyglossus aculeatus) and a number of bird species.
Green-head ant workers are gamergates, meaning they can reproduce with winged males. With workers taking over the reproductive role, queens are relatively insignificant and are rarely produced in colonies. Nuptial flight begins in spring, with males mating with one or two females. Queens that establish their own colonies are semi-claustral, going out and foraging to support their young. Another way colonies are formed is through budding, where a subset of the colony leaves the main colony for an alternative nest site. The green-head ant is known for its painful and venomous sting that can cause anaphylactic shock in sensitive humans. However, they can also be beneficial to humans, acting as a form of pest control by preying on agricultural pests such as beetles, moths and termites.
Taxonomy
The green-head ant was first described in 1858 by British entomologist Frederick Smith in his Catalogue of Hymenopterous Insects in the Collection of the British Museum part VI, under the binomial name Ponera metallica based on two syntypes; a worker and a queen he collected in Adelaide, South Australia. These specimens were later reviewed in 1958 with the designation of a lectotype from the syntypes, but it is unclear which specimen was designated. The material is currently housed in the Natural History Museum in London. In 1862 Austrian entomologist Gustav Mayr moved the species from the genus Ponera and placed it in Rhytidoponera as Ectatomma (Rhytidoponera) metallicum, which at the time was a newly erected subgenus of Ectatomma. Mayr's original reclassification was short-lived as in 1863 he moved the ant from Rhytidoponera to Ectatomma. In 1897 Italian entomologist Carlo Emery named the ant Rhytidoponera metallica and in 1911 designated it as the type species of Chalcoponera, a subgenus of Rhytidoponera; the ant was, however, mistakenly identified as the type species of Rhytidoponera by some scientists.
The taxonomy of the green-head ant and other species related to it (forming the R.metallica species group) were a source of confusion due to the high geographic variations in the green-head ant and similar-looking species having been treated as forms (occurrence of multiple morphs). These forms have been described under different names through inadequate characterisations. In 1958 American entomologist William Brown Jr., synonymised Rhytidoponera caeciliae, Rhytidoponera pulchra, Rhytidoponera purpurascens and Rhytidoponera varians with the green-head ant after reviewing the ants in a 1958 journal article. The taxon R.purpurascens was named after its dark-purple appearance by William Morton Wheeler, but R.metallica tends to appear purple around the regions where Wheeler originally collected R.purpurascens. After examining collected specimens, Brown also noted no morphological differences between R.pulchra and R.caeciliae. The species R.varians was described from specimens collected in Darlington, Western Australia. American entomologist Walter Cecil Crawley stated that the subspecies differs from R.metallica by its smaller size and faded metallic colour, varying from yellow-brown in most specimens to a metallic green on the head, thorax and gaster with no evidence of purple colouring. An examination of R.varians showed that the superficial punctures of the gastric dorsum are coarser than usual, but these variants are found not only around the original collection site, but also throughout the southern regions of Australia. Such feature may occur naturally, which disallows R.varians to be considered as a separate population from R.metallica. Under the present classification, the green-head ant is a member of the genus Rhytidoponera in the tribe Ectatommini, subfamily Ectatomminae. It is a member of the family Formicidae, belonging to the order Hymenoptera, an order of insects containing ants, bees, and wasps. The name "green-head ant" is sometimes shortened to "green ant". This may generate confusion, however, since residents of northern Queensland use the name "green ant" for the green-tree ant (Oecophylla smaragdina).
Description
In general, green-head ants are monomorphic (occurs in a single form), measuring in length and varying in colour, ranging from green-blue to green-purple. Their exoskeletons are hard and heavily armoured with a single-segmented waist.
The queens measure with the head, thorax, and abdomen exhibiting various metallic colours. The head is usually green behind the eyes and ferruginous (rust in colour) at the front with a less obvious purple tint between the colours. The antennae are ferruginous and the eyes are ovate (shape resembling an egg). The head is emarginate (having a notched tip or edge) from its posterior view and also rugose, along with the thorax and node (a segment between the mesosoma and gaster); these body parts are covered with large confluent punctures. The basal segment of the abdomen has transversely curved striae (grooves which run across the body). The colour of the thorax is usually greenish, the wings are subhyaline (they have a glassy appearance), and the nervures (the veins of the wings) are testaceous (brick-red colour). The legs and apex are ferruginous, and the abdomen is purple.
The workers and queens closely resemble each other, making the two castes hard to distinguish, but the workers differ in having a compressed and elongated thorax, and an abdomen that is predominately green-tinted. The workers are also slightly smaller than the queens, measuring . The males are smaller than the workers and queens, measuring and appear to be black and fuscous (dark and sombre). The tarsus is fuscous, and the mandibles are rugose. In contrast to the workers and queens, the funicular joints are shorter, the sculpture is denser on the head and thorax, the number of punctures is less than that of the other castes and the postpetiole is coarser. The first segment of the gaster is transversely roughened, and the pilosity (hair) on the legs is less dense. The genitalia of the male is consistent with other formicids, composed of an outer, middle, and inner pair of valves.
The predominant metallic colour is green, but can vary by region, ranging from metallic green to purple. In the Flinders Ranges of South Australia and Alice Springs, the colour of the ants shifts from the typical green to a dark purple colour. In areas with more rainfall such as the New South Wales tablelands and Victorian savannahs, green-head ants are mostly green with purplish tints seen on the sides of the mesosoma. In the northern regions of New South Wales and Queensland, the alitrunk is reddish-violet, which shades into golden around the lower portions of the pleura. The green colour is either limited or completely absent in this case. Most populations have a bright green gaster, except for those living in the central desert. In some areas examined near Brisbane, two different colour forms were discovered within a single colony. One possibility is that the two colour forms may represent two sibling species, but this cannot be confirmed because of lack of evidence. In far north Queensland, populations appear to be dull green and distinct to those living in the south, but it is unknown how the ants in the far north and south hook up at the western and southern regions of the Atherton Tableland. With this said, it is unknown if the far north populations are actually a different species. In addition to colour variation, there are morphological differences among populations. For example, the size and shape of the head and petiole, the length of the appendages, and other sculptural details of the body may all vary.
Sub-mature larvae measure and appear similar to sub-mature R.cristata larvae. They can be distinguished by a less-swollen abdomen and shorter body hairs, measuring . On the thorax and abdomen somite, they measure . On the flagelliform and ventral portion of the abdominal somites, they measure . The hairs on the head have short denticles, and the antennae have three apical sensilla, each containing a somewhat bulky spinule. Young larvae are much smaller than sub-mature larvae at long. They appear similar to sub-mature larvae, but the diameter differs, gradually decreasing from the fifth somite to the anterior end. The hairs measure , with the longest found on the flagelliform and on all somites; the hairs on the somites, however, become sparse. The tips of the hairs on the head are simple or frayed, and overall the hairs measure . Both antennae have a subcone and three apical sensilla that resemble a spinule. The mandibles are sub-triangular with a curved apex. The apical and subapical teeth are sharp and short, but the proximal tooth is blunt. Unlike the mature larvae, the proximal tooth is not divided into two portions.
Distribution and habitat
The green-head ant is among the most widespread of all insects endemic to Australia. The ant is found throughout Victoria, New South Wales, the Australian Capital Territory and South Australia. It can be found in most areas of Western Australia but is less common in the north, and is present in the lower regions of the Northern Territory and east of Queensland. They are not present in Tasmania. The green-head ant is an introduced species to New Zealand and was first seen in 1959. Populations of the ant were probably introduced into the country on cargoes of timber; a number of other Rhytidoponera species were most likely introduced the same way. Populations have been established in Napier, as the ants were collected in the town between 2001 and 2003. Nests were previously found in the suburb of Penrose in Auckland and Mount Maunganui, but no specimens have been collected there since the 1960s.