Colobostruma

These ants can be locally common although they are often overlooked. Most species have small colonies with less than 100 workers, and workers will lie motionless when disturbed. Nests are in soil usually under rocks, in cracks in rocks or in rotten logs. Only a single rainforest species is known to nest arboreally. Foraging is usually on the ground at night but occasionally they are found foraging on mallee. They are also commonly found in leaf litter.

Identification
The antennae have 4 to 6 segments (including the scape). The petiole and postpetiole have thin, wing-like flanges extending outwards from their sides (best viewed from above). Workers of Colobostruma are most similar to workers of Mesostruma. They differ in that Colobostruma has wing-like flanges on both the petiole and postpetiole while the flanges (when present) are limited to the postpetiole in Mesostruma.

Distribution
An Australian genus with one outlier; Colobostruma foliacea occurs in Papua New Guinea and the Solomon Islands.

Biology
Heterick (2009) - Colobostruma species are foragers in litter or vegetation. Although most species have a broad distribution within Australia, the genus is very rare in Western Australia, and colonies or even individual workers are seldom seen.

Nomenclature

 *  COLOBOSTRUMA [Myrmicinae: Dacetini]
 * Colobostruma Wheeler, W.M. 1927f: 32 [as subgenus of Epopostruma]. Type-species: Epopostruma (Colobostruma) leae, by monotypy.
 * Colobostruma raised to genus: Brown, 1948e: 118.
 * Colobostruma senior synonym of Alistruma, Clarkistruma: Brown, 1959c: 1; Brown & Wilson, 1959b: 281.
 * Colobostruma junior synonym of Epopostruma: Baroni Urbani & De Andrade, 2007: 94.
 * ALISTRUMA [junior synonym of Colobostruma]
 * Alistruma Brown, 1948e: 117. Type-species: Epopostruma foliacea, by original designation.
 * Alistruma junior synonym of Colobostruma: Brown, 1959c: 1.
 * CLARKISTRUMA [junior synonym of Colobostruma]
 * Clarkistruma Brown, 1948e: 124. Type-species: Epopostruma alinodis, by original designation.
 * Clarkistruma junior synonym of Colobostruma: Brown, 1959c: 1.

Worker
Shattuck (2000):

With characters of the epopostrumiform genus group......

Palp formula 5, 3.

Labrum large or very large, forming a massive shield in Colobostruma and Mesostruma that can reflex tightly over the labio-maxillary complex and completely cover the buccal cavity; somewhat smaller in Epopostruma where it covers approximately the apical half of the labio-maxillary complex.

Basimandibular gland bulla absent.

Antenna usually with 4 - 6 segments, rarely more.

Scape, when laid back in its normal resting position, passes below the eye or across the ventral margin of the eye; basal part of scape strongly downcurved.

Scrobe usually present, extending below the eye, the latter not located ventrolaterally on side of head.

Femora and tibiae lack gland bullae on their dorsal surfaces.

Pronotal humeri usually armed.

Metapleural gland with apex of bulla close to or abutting the annulus of the propodeal spiracle.

Propodeal spiracle at about the m idheight of the sclerite, separated from margin of declivity.

Tergite of petiole or postpetiole with lateral cuticular laminar outgrowths; extremely rarely (1 species) with traces of spongiform tissue.

Postpetiolar spiracles ventral.

Limbus absent from first gastral tergite.

Suture separating first gastral tergite and stemite angulate laterobasally; horizontal basal margin of stemite with a raised rim or crest adjacent to the tergite margin, this crest usually continues round the laterobasal angle.

Bizarre pilosity never developed.

Wheeler (1927):

With characters of the epopostrumiform genus group and the following.

Mandibles short triangular, serially dentate; preapical and apical teeth enlarged, apical the largest.

Mandibles at full gape open to only 60°-90°; with static pressure mode of action.

Mandible with an inflected prebasal -external angle; in profile mandible abruptly down curved basally.

Basal process of mandible small and truncated, located below the plane of the masticatory margin; at full mandibular closure basal process dorsal to labrum, not fitting into a depression in labral dorsum.

Labrum hypertrophied, forming a massive shield that can reflex tightly over the entire labio-maxillary complex and completely cover the buccal cavity; its apical margin evenly convex to very broadly bilobate.

Trigger hairs on anterior margin of labrum and very reduced, or absent.

Side of head without a vertical preocular groove.

Scrobe usually present below eye but absent or incipient in some species, especially those with abnormally flattened head.

Tergite of petiole with lateral lobes at the node.

Tergite of postpetiole usually with lateral lamellae or lobes.

This is the first synthesising taxonomic study of the genus to be undertaken. Sixteen species are recognised of which 15 are restricted to Australia. The only non-Australian species, and in fact the only non-Australian epopostrumiform ant, is the remarkably flat-headed C. joliacea, found in New Guinea and the Solomon Islands. Another remarkable species, ceromata, is the only epopostrumiform to have spongiform tissue on the waist segments. The convergence of this character with the strumigenyiforms should not cause confusion as the diagnosis of ceromata is otherwise strictly epopostrumiform.

The hypertrophied shield-like labrum in this genus is duplicated in Mesostruma and is a synapomorphy of the two. They are easily separted by the morphological details given in the key and their respective diagnoses.