Rhytidoponera violacea

Rhytidoponera violacea is a keystone seed disperser in many Western Australia habitats.

Identification
Heterick (2009) - A small population of this species on the northern coast around Geraldton, WA lacks the usual iridescence, while specimens of R. violacea from more arid eastern and northern areas have a greenish-yellow rather than blue-purple iridescence on the head and mesosoma. Molecular or karyotype analysis of representatives of these populations may reveal that they are genetically distinct.

Distribution
Occurs in Western Australia and Northern Territory.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Habitat
This species occurs in a wide range of habitats. The images below are of a WA Iron Ore Range hillside and Kwongan Heathland from the Enneaba Sand Plains.

Biology
In areas where R. violacea is found, it appears to be the dominant ant responsible for seed dispersal (Majer 1984; Gove et al. 2007; McCoy 2008).

Heterick (2009) - Abundant throughout WA, this is an important taxon for those using ants as bioindicators for environmental management purposes.

Lubertazzi et al. (2010) studied the biology of this species in two multi-hectare plots located just north of Eneabba and on opposite sides of the Brand Highway. Both areas contained native Kwongan Heathland. Findings from this study include:

Foraging/Diet
The average number of successful foragers returning to the nest over a 30-minute period was 5 ± 0.5 (range = 1 - 14, n = 36). There there were typically between 10 to 15 return trips to the nest during 30 minutes of sampling, suggesting that approximately 30 – 50% of foraging bouts are successful. The majority of items captured (65%) were insects (either whole individuals or insect parts). Plant parts were more commonly collected (17%) than seeds (5%). In a few cases, foragers were observed subduing live insect prey that was then brought back to the nest. In addition, some insects (e.g., non-conspecific ants, small beetles, and termites) were often found to be alive when they were collected from the foragers.

The average foraging distance from the nest was 3.5 ± 2.2 m, with a maximum distance of 10.4 m. These distances are similar to those previously observed in the same study region for R. violacea carrying seeds back to the nest (Gove et al. 2007; McCoy 2008).

Majer et al. (2010) monitored 5 nests in southwestern WA for a full year and found 91% of the collected forage was invertebrates. The remainder were seeds (6%) and plant parts (3%). Seeds were more or less prevelant in their diet during different times of the year. The ants do not appear to have a dependance on seeds, gathering them opportunistically and not storing them for long periods, i.e., large seed caches were not found. Foraging occurred throughout the year with much greater activity in the summer months. During the warmer parts of the year foraging activity peaks in the morning and in cooler times is restricted to the warmest part of the day.

Colony Attributes
The average number of workers in a colony was 190 ± 23.5 (range = 47 - 474, n = 22). All of the colonies excavated contained pupae (mean = 83 ± 13.4, range = 2 - 293) and many contained larvae (mean = 22 ± 3.8, range = 0 - 85). Small larvae (< 3 mm) and eggs were either uncommon or entirely absent. A total of seven males were collected from two colonies.

Majer et al. (2010) found an average of 173 workers per nest (n=5) in Karragullen in southwestern WA.

Nesting Biology
All the excavated nests were located under plants. The nests were either supported in part by a large mass of roots from a plant or, less commonly, incorporated a larger root of a shrub into their structure. The mean (n = 14) size of the nest entrance was 4.0 ± 0.58 cm (longest axis) by 2.1 ± 0.29 cm (perpendicular to widest axis). The number of nest openings varied (9 nests with 1 opening, 3 nests with 2 openings, 1 nest with 3 openings, and 1 nest with 4 openings). When present, multiple nest openings were located within a few centimeters of one another and coalesced into a single chamber or tunnel 1 - 2 cm below ground. A mound of nest spoil was found around most, but not all, nest entrances.

Mounds were typically oval in shape, approximately centered on the nest opening, and often obscured by the stems and shoots of the vegetation of the overlaying plant. The average longest axis width of a mound was 21.0 ± 1 cm, the width perpendicular to the longest axis averaged 15.7 ± 1.4 cm, and the average mound height was 5.4 ± 0.7 cm.

Nests were centered under nest entrances and had an average depth of 23.0 ± 1.6 cm (N = 19 nests). The upper portion of the nest (the first 4 to 5 cm below ground) was a collection of small chambers, side by side, that were often supported by many fine roots. These chambers were between 1 and 2 cm deep and collectively filled an area from 5 to 10 cm in diameter.

Majer et al. (2010), sampling in laterite soils in southwestern WA, found ground nests that averaged 22 cm in depth. The nest structure was typically a broad vertical shaft, often associated with and spiralling around buried woody material.

Nomenclature

 * gemma. Ectatomma (Rhytidoponera) convexum var. gemma Forel, 1907h: 269 (w.m.) AUSTRALIA. Combination in Rhytidoponera: Emery, 1911d: 37. Junior synonym of violacea: Clark, 1936: 87.
 *  violacea. Ectatomma (Rhytidoponera) convexum subsp. violaceum Forel, 1907h: 269 (w.m.) AUSTRALIA. Combination in Rhytidoponera: Emery, 1911d: 38. Raised to species: Brown, 1958g: 205. Senior synonym of gemma: Clark, 1936: 87; of opacior: Brown, 1958g: 205.
 * opacior. Rhytidoponera convexa subsp. opacior Clark, 1936: 86 (w.) AUSTRALIA. [First available use of Rhytidoponera convexa subsp. violacea var. opacior Crawley, 1925b: 583; unavailable name (Taylor, 1986: 34).] Junior synonym of violacea: Brown, 1958g: 205.

Worker
Crawley (1925) - Length 6 mm.

Dark ferruginous; gaster with a faint metallic-blue tint, mandibles dark castaneous, apical joints of funiculus castaneous.

Scapes and legs less pilose than in the type.

The node of the petiole in profile is thinner, especially at the top, and the eyes slightly less prominent. The sculpture differs from that of convexa, s.sp. violacea and var. subumbrata, as follows: —the punctures are decidedly less shilling, giving the whole insect a duller appearance. On the head the punctures are smaller, shallower, and further apart, showing more striation and reticulation; similarly on dorsum of thorax. On the pronotum there is a larger space in front which is transversely striate and not punctate, and on the epinotum: the transverse striae are more distinct and not broken by punctures; a good deal of the meso- and epinotum is transversely striate. Striation on postpetiole and gaster finer.