Austromorium

Austromorium workers are general scavengers which nest in the soil, often at the base of trees. They occur broadly across southern Australia with most collections being from dry sclerophyll with a limited number from mallee. Two species are currently known, both restricted to Australia.

Identification
Antennae 12 segmented (including the scape) with a 3-segmented club. Antennal scrobes absent. Mandibles with 4 or 5 teeth. Upper surface of the mesosoma forming a uniform arch which is interrupted only by the shallow metanotal groove. Tip of sting broadly flattened and expanded (visible only when the sting is extended).

These ants are superficially similar to workers of Lordomyrma and Tetramorium. They can be separated from Lordomyrma by the lower number of mandibular teeth (4 or 5 in this group, 7 or more in Lordomyrma), the relatively smooth mesosomal dorsum and the broad, expanded tip of the sting (which is sharply pointed in Lordomyrma). They differ from Tetramorium in having the region below the antennal socket rounded rather than ridged, and in lacking a triangular extension on the tip of the sting.

Identification Key

 * World

Description
Mandibles triangular, with 4-5 teeth. Palp formula 2,2. Clypeus projecting slightly forward, bicarinate. Frontal lobes narrow but covering antennal insertions. Eyes with 8-18 ommatidia in greatest diameter, located laterally on head at or just anterior of the midpoint of its length. Antennae 12 segmented with a 3-segmented club.

Mesosoma compact to moderately elongate. Dorsal surface of mesosoma forming a continuous flat to weakly convex surface, the metanotal groove weak or essentially absent. Propodeal spines well developed. Propodeal lobes either large and rounded or developed as sharp spines which are 3/4 the length of the propodeal spines. Propodeal spiracle small, located at or before the base of the propodeal spine and well forward of the posterior propodeal face. Tibial spurs absent from middle and hind legs.

Mandibles smooth but overlain with low carinae. Head, mesosoma, petiole and postpetiole distinctly sculptured, legs and gaster smooth. Entire body with elongate erect or suberect hairs, those on scapes and legs sometimes appressed. Anterior clypeal margin with a row of long, curved setae which extend anteriorly about 1/2 the length of the mandibles. Colour yellowish-red to dull red, antennae and legs sometimes lighter.

Austromorium flavigaster (Clark) was originally described in the genus Xiphomyrmex. When Bolton (1976) synonymised Xiphomyrmex with Tetramorium he questioned the generic placement of flavigaster and, on the advice of R. W. Taylor, transferred it in the genus Chelaner. Bolton was justified in removing A. flavigaster from Tetramorium as this species (1) lacks a dorsal lamellate appendage on the tip of sting shaft, (2) the anterolateral portions of the clypeus are low and rounded, not raised into a shield-wall around the antennal insertion, and (3) the mandibles have four teeth while six or more are found in Tetramorium (Bolton, 2003). Unfortunately, the placement of flavigaster in Chelaner (which was subsequently synonymised with Monomorium by Bolton (1987)) is also unsupportable. As pointed out by Heterick (2001), flavigaster lacks the central clypeal seta found in Monomorium while it possesses elongate, spine-like propodeal lobes, a feature absent from Australian Monomorium. Unfortunately Heterick (2001) could not determine an obvious generic placement for this species and considered it incertae sedis within the subfamily Myrmicinae, suggesting that it will likely need to be placed in a new genus. This follows Shattuck (1999), who similarly could not place this species within an existing genus and treated it as belonging to an undescribed genus, and who subsequently (Shattuck, 2009) established the genus Austromorium for this species and a close relative.

As noted by Heterick (2001, 2009), flavigaster shares a number of similarities with at least some members of the tribe Stenammini as defined by Bolton (2003) and while acknowledging the poor tribal understanding within this subfamily, this species seems best placed within this tribe as we currently understand it. It differs from most members of the tribe as follows: from Adelomyrmex, Baracidris, Lachnomyrmex and Tetheamyrma by the 3-segmented rather than 2-segmented antennal club; from Ancyridris by the lack of elongate spines on the dorsum of the petiolar node; from Calyptomyrmex by the short, rounded rather than forked clypeus; from Cyphoidris, Dacetinops, Indomyrma and Lasiomyrma by the 12-segmented rather than 11-segmented antennae; from Dacatria and Proatta by the smooth rather than protuberant or spined mesosomal dorsum; from Dicroaspis by the short, rounded rather than forked clypeus and the 12-segmented rather than 11-segmented antennae; from Rostromyrmex by the short, rounded rather than rostrum-like clypeus and the 12-segmented rather than 9-segmented antennae; from most Stenamma by the 3-segmented rather than 4-segmented antennal club; and from Vollenhovia by the elongate petiolar peduncle and lack of subpetiolar plate.

Austromorium flavigaster is most similar to species of Lordomyrma or Rogeria, with both of these genera also occurring in the Australian region. Heterick (2009) recognised this similarity and provisionally placed A. flavigaster in Rogeria based on the diagnosis provided by Bolton (2003). However, using the more detailed diagnosis provided by Kugler (1994), flavigaster differs from species of Rogeria in having (1) the anterior margin of the clypeus projecting slightly further anteriorly, (2) a posterior clypeal extension which is broader, causing the frontal lobes to be more broadly separated and (3) a smaller and more anteriorly placed propodeal spiracle which is more than 3x its diameter from the posterior propodeal face. Combined, these characters suggest that flavigaster does not belong in Rogeria.

Finally, flavigaster can be separated from species of Lordomyrma by the (1) reduced number of palp segments (2,2 vs. 4,3, 3,3 or 3,2), (2) broadly spatulate sting tip (at most only slightly expanded in Lordomyrma), (3) fewer mandibular teeth (4-5 vs. 7-9) and (4) absence or at most weak development of a metanotal groove (present to varying degrees in Lordomyrma). The lack of scrobes will separate flavigaster from some but not all species of Lordomyrma as this character is variable within Lordomyrma (Sarnat, 2006; Taylor, 2009).