Rhytidoponera Metallica

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'' 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