Diacamma violaceum

Nothing is known about the biology of .

Identification
Laciny et al. (2015) - Large, slender species (TL ca. 16.0–17.9 mm). Trunk black or with weak purple shimmer; parts of frontal lobes and clypeus brown; subpetiolar process and gaster tergites and sternites at posterior margins reddish brown. Standing setae on trunk short. Trunk strongly striate on mesosoma and petiole, slightly finer on head. Head, mesosoma, and petiole with very dense microreticulation, matt. Head elongated, sides posteriorly of eye moderately convex. Striation posterior of eyes meeting narrow, slightly concave occipital margin; occipital margin laterally widened and terminating in short, blunt projections. Eyes small, not protruding. Clypeus finely, very densely punctured, except for anterior margin with larger and sparser puncturation; apex widely rounded. Striae on pronotum transversely elliptical or almost circular. Striation on mesosoma sides slightly oblique, upcurved on propodeum. Petiole strongly compressed, dorsally and dorsolaterally with longitudinal striation, ventrolaterally with reduced striation; petiolar teeth closely spaced, moderately long; subpetiolar process prominent, strongly concave, posterior corner acute, longer than anterior one. Gaster tergite 1 without striation, but with fine puncturation, shiny as the following tergites.

We compared seven specimens from three localities in north-western Thailand with the illustrations of a syntype worker from Myanmar published by Antweb (2015). They largely agree with the type, but have a different pronotum sculpture and do not possess the eponymous purple shimmer. The syntype has distinct transverse rugae on the pronotum surrounded by concentric rugae (Antweb 2015), whereas the rugosity in the Thai specimens is circular. With only these eight specimens available, it is difficult to decide whether the mentioned characters define similar species or show intraspecific variation. In this context it is noteworthy that there are at least two further closely related, undescribed species in Thailand, including one published under the name Diacamma scalpratum by Peeters et al. (2015).

All examined specimens differ clearly from Diacamma scalpratum which is distributed from the Indian subcontinent eastwards to Yangon (Rangoon) in Myanmar. In D. violaceum the dorsal face of the petiole has a longitudinal striation that is similar to the striation of the dorsolateral parts, whereas in D. scalpratum the petiole has a dorsal longitudinal crest with reduced striation. The sculpture of the metapleura is weaker in D. violaceum than in D. scalpratum, and similar to the sculpture of the mesopleura, whereas in D. scalpratum it is coarsely striate, almost as strong as on the propodeum sides. Morphometric analysis showed that the two species also differ in relative length of the antennal scapes (SI 164–174 in D. scalpratum vs. 181–192 in D. violaceum) as well as in the width of the petiole (in relation to head width), which tends to be wider in D. violaceum.

Diacamma violaceum also has strong similarities with Diacamma concentricum but differs distinctly by the structures of the petiole (length, spine length, shape of subpetiolar process).

Distribution based on Regional Taxon Lists
Oriental Region: Myanmar, Thailand.

Nomenclature

 *  violaceum. Diacamma scalpratum var. violaceum Forel, 1900d: 317 (diagnosis in key) (w.) MYANMAR.
 * Subspecies of scalpratum: Emery, 1911d: 65; Chapman & Capco, 1951: 60; Bolton, 1995b: 171.
 * Status as species: Laciny, Pal & Zettel, 2015: 93 (redescription); Zettel, et al. 2016: 167.

Worker
Laciny et al. (2015) - (n= 7): TL 16.04–17.93; HW 2.45–2.63; HL 3.52–3.82; EL 0.66–0.74; SL 4.43–4.96; PH 2.04–2.28; PL 1.57–1.74; PW 1.17–1.35; SpD 0.37–0.51; SpL 0.38–0.51; WL 5.35–5.93; MTL 3.46–3.78. Indices: CI 67–71; SI 181–192; PI 73–79; SpDI 32–40; SpLI 30–39; EI 26–28.

Structures: Head elongated; sides long and parallel in front of eyes, moderately convex behind eyes. Eyes small, not protruding. Striation posterior of eyes divergent, meeting the narrow, slightly concave occipital margin; ridges densely microreticulated, matt, not or hardly narrower than interspaces; occipital margin laterally slightly widened and terminating in very short, blunt projections. Clypeus finely, very densely punctured, except for anterior margin with larger and sparser puncturation; anteromedially with widely rounded apex. Mandible with obliterate fine striation, setae on masticatory margin very long. Mesosoma with coarse striation, especially on pronotum. Striae on pronotum transversely elliptical or almost circular. Striation on mesosoma sides slightly oblique, upcurved on propodeum, reduced on meso- and metapleura. Posterior face of propodeum separated from sides by strong carinae. Petiole strongly compressed, but without dorsal crest, dorsally and dorsolaterally with longitudinal striation approximately parallel to dorsal outline of node, ventrolaterally with reduced striation, almost smooth; at sides the ridges approximately as wide as interspaces; subpetiolar process prominent, with strongly concave ventral outline, anteriorly marginate, posterior corner longer than anterior one. Gaster tergite 1 lacking striation, but with fine puncturation, shiny as the following tergites and sternites.

Pilosity: Standing setae on trunk short, those on head, pronotum, and abdominal apex slightly longer. Short appressed pilosity of trunk well developed, but on matt surfaces less obvious than on the shiny gaster where it is longer and velvety. Standing setae on scape very short, on legs short, slightly longer on flexor sides of femora.

Colour: Trunk black, sometimes with weak purple shimmer. Clypeus and frontal lobes often brown; subpetiolar process and broad stripes at posterior margins of gaster tergites and sternites reddish brown. Mandibles and tibiae more or less extended red, other leg parts strongly infuscated. Colour of antennae variable.

References based on Global Ant Biodiversity Informatics

 * Forel A. 1900. Les Formicides de l'Empire des Indes et de Ceylan. Part VII. J. Bombay Nat. Hist. Soc. 13: 303-332.
 * Laciny A., A. Pal, and H. Zettel. 2015. Taxonomic notes on the ant genus Diacamma Mayr, 1862 (Hymenoptera: Formicidae), part 1. Zeitschrift der Arbeitsgemeinschaft Österreichischer Entomologen 67: 83-136.
 * Zettel H., A. Pal, and A. Laciny. 2016. Taxonomic notes on the ant genus Diacamma Mayr, 1862 (Hymenoptera: Formicidae), part 2. Zeitschrift der Arbeitsgemeinschaft Osterreichischer Entomologen 68: 129–168.