Polyrhachis sexspinosa species-group

The species in this group are members of the subgenus Myrmhopla.

Species

Identification

Key to Australian Polyrhachis (Myrmhopla) species

Overview

Dorow (1995) - Emery (1925) described the workers as: "petiole long, anteriorly with an elevated angle in profile, spines inserting distally, spines relatively short and only little diverging; head long, distally narrowing; sculpture rugose; large species" (own translation). Additional data of this group are: Large slender species (TL: 8-13 mm) with an immarginate thorax. Long slender spines are present on prothorax, propodeum and petiole, only in Polyrhachis calypso the petiolar spines are curved hook-like. The head is elongately oval in frontal view. The long and spider-like legs and the antennae are round in transection, the genae are immarginate. only the neck might wear a "frill". The mat body is usually sculptured rugosely, the shiny gaster is often only finely punctate. Polyrhachis melpomene in contrast has a striate body sculpture except on the gaster. Erect hairs and appressed pubescence are usually numerous. The body colour is black, brownish or reddish.

These species arc polydomous weaver ants of the shrub and tree layer.

This group, which was established by Emery (1925), today comprises 17 species. Polyrhachis melpomene, which was placed by Emery (1925) into the Polyrhachis-dives-group, and Polyrhachis olybrius (=Polyrhachis olybria), which he could not associate, also belong to this species-group. Bolton(1975) and Kohout (1987) (for the Philippines) revised this group.

Kohout (2010) - The Polyrhachis sexspinosa species-group of the subgenus Myrmhopla was established by Emery (1925), who included 12 species and subspecies from New Guinea and south-east Asia. Bolton (1975) revised the world fauna of the group and recognised 12 valid species with all of the included infraspecific taxa considered synonyms. Three new species from the Philippines were later added, one subspecies (Polyrhachis sexspinosa reclinata) raised to specific status (Kohout, 1987) and one species (Polyrhachis barnardi) synonymised (Kohout & Taylor (1990), raising the number of valid species of the group to 15. Dorow (1995) recognised 17 species as constituents of the group, including Polyrhachis melpomene and Polyrhachis olybria. However, these two species were later transferred to different subgenera (P. melpomene and its junior synonym Polyrhachis dolichocephala to subgenus Hedomyrma and Polyrhachis olybria to the nominal subgenus Polyrhachis), and two former subspecies (Polyrhachis arcuspinosa waigeuensis and Polyrhachia sexspinosa esuriens) were raised to specific status by Kohout (1998). At present the P. sexspinosa species-group comprises 19 valid species, including one (P. spinosa) recently elevated to its original specific status (Kohout, 2008) and one described below as new (Polyrhachis dispar). Four species of the group are relevant to the Australian fauna with two (P. dispar and Polyrhachis glabrinotum) considered endemic. They appear to be derived from common ancestral stock, such as the New Guinea-based species Polyrhachis aureovestita and Polyrhachis bubastes Fr. Smith, 1863 (Bolton, 1975) and speciated after loosing connection with the maternal New Guinean populations following the sinking of the continental bridge between New Guinea and Australia. On the other hand, the more robust populations of the relatively common P. sexspinosa (Latreille, 1802) and P. reclinata maintained most of their original characteristics, with specimens from Cape York Peninsula indistinguishable from their New Guinean counterparts.

Diagnosis

Worker

Relatively large ants (HL >2.0), except for the rather small, extralimital Polyrhachis nofra (HL 1.52-1.53), with general characteristics of the genus. Mandibles smooth and polished apart from small piliferous pits towards bases. Anterior clypeal margin arcuate, often obtusely truncate medially or with shallow median emargination. Head contracted posteriorly, distinctly narrower behind than in front of eyes. Eyes with short, erect hairs; strongly convex or protuberant, clearly exceeding lateral cephalic outline in full face view. Occipital margin with more-or-less developed lateral angular prominences which are usually visible with the head in full face view (except in P. dispar and P. glabrinota). Mesosoma immarginate; pronotum strongly convex, humeri armed with rather strong, mostly forward curved spines. Promesonotal suture strongly impressed. Mesonotum flat or weakly convex in profile; mesopleural process present as a simple lobe (as in P. dispar and P. sexspinosa) or a dentiform structure, that can be acute or obtuse (as in P. glabrinota and P. reclinata); metanotal groove usually replaced by a minutely raised ridge. Propodeum armed with a pair of spines that are either vertical or inclined forwards (as in P. sexspinosa), or posteriorly reclined and often curved (as in P. dispar, P. glabrinota and P. reclinata). Petiole nodiform with a pair of lateral spines, without intercalary spines or teeth. Gaster, when contracted, broadly ovate. Sculpturation of head, mesosoma and petiole ranging from weak to rather heavy, gaster usually finely shagreened. All body surfaces, including appendages, with relatively long, erect hairs. Pubescence mostly appressed or suberect, somewhat radiating, present in various densities but usually not completely hiding underlying sculpturation. Pubescence on gastral dorsum organised in a characteristic midline pattern in virtually all species (except P. nofra) or almost completely absent (P. exotica Kohout, 1987). Body mostly black, or partly reddish-brown (as in P. glabrinota), with appendages usually lighter, reddish-brown.

Queen

Very similar to worker with usual characters of full sexuality, including three ocelli, complete thoracic structure and wings. Armament of pronotum, propodeum and petiole somewhat reduced with spines shorter, less curved and usually more stubby. Sculpturation, pilosity, pubescence and colour virtually as in worker.

Pocket nest of Polyrhachis sexspinosa (Latreille) attached to a buttress of a caulifloral rainforest tree at Lockerbie Scrub, Cape York Peninsula. Photo R.J. Kohout.

Distribution and biology

Distribution: Australia. India. Indonesia. Malaysia, New Guinea. New Caledonia, Philippines. Solomons. Singapore. Thailand (new). This group has evolutionary centers in New Guinea and in the Philippines.

Polyrhachis sexspinosa-group species are distributed throughout the Indo-Australian region, with only a few species known from the Oriental region and one from the Solomon Islands. In Australia, the group is limited to the northern part of Cape York Peninsula in Queensland, north of the 14°S parallel. Members of the sexspinosa-group are mostly arboreal nesters, building nests of silk and vegetation debris between the foliage of rainforest trees and shrubs. However, some species deviate from this behaviour and have their own characteristic methods of nesting. Colonies of P. sexspinosa invariably build pocket-like nests of silk, vegetation debris and bark fragments against the trunks of rainforest trees (see figure). In contrast, colonies of P. dispar, P. glabrinota and P. reclinata at Iron Range National Park were frequently found nesting within the hollow internodes of a bamboo, Bambusa forbesii. Only one colony of P. reclinata was located under the bark of a living tree, while most nests of P. glabrinota were constructed between leaves, usually in the lower arboreal zone, about 2-3 m above the ground.

Kohout (2008) - Polyrhachis rugifrons and Polyrhachis spinosa appear to be the only representatives of the sexspinosa-group in Sulawesi. However, while the distribution of P. spinosa is centered on Halmahera, with only a few collections known from elsewhere (Morotai I.), P. rugifrons appears to be endemic to Sulawesi.

Related Pages

Polyrhachis
subgenus Myrmhopla

References