Trap-jaw Ants of Australia (and a couple from Africa)

Foraging worker of Epopostruma frosti, copyright Alex Wild.

Anyone who finds themselves travelling through regional Australia will soon find themselves convinced that this is a continent ruled by ants. During the course of the day, while the hot Australian sun drives other animals to seek shelter and seclusion, ants are often the only living things (other than plants) to be seen. To match this abundance, Australia's ants also come in a variety of distinctive forms, many of them unique to this country.

One distinctively Australian group of ants are the 'epopostrumiforms'. This is a small group of genera belonging to the tribe Dacetonini of the subfamily Myrmicinae (in the past the epopostrumiforms have been formally recognised as the subtribe Epopostrumiti, though Bolton eschewed the use of formal subtribes in his 1999 review of the Dacetonini). The Dacetonini are all predatory ants, with a distinctive large process inside the base of the mandibles that helps to lock them closed when holding struggling prey. The mandibles may be particularly long and slender, sometimes with only a few teeth present at the end. Where their habits are known, epopostrumiforms are predators of springtails; these are the most typical prey for the Dacetonini as a whole though some species of the tribe are more catholic in their tastes. Dacetonins live in small colonies, commonly in secluded habitats such as leaf litter; the epopostrumiforms include species that nest and forage either above or below ground (Brown & Wilson 1959). Dacetonins hunt their prey by stealthily sneaking up to it with the mandibles held open, followed by a quick lunge combined with snapping the mandibles shut. Once the prey has been successfully grabbed, those dacetonins with shorter mandibles rapidly bring the sting forward to quell it. Even after using the sting, however, hunters of springtails may find themselves flung into the air by flicks of the springtail's furca a couple of times before the venom takes full effect (hence the need for a firm mandibular lock). Dacetonins with longer mandibles may also deploy their sting or they may simply lift the prey above their heads until it gives up the ghost.

The African Microdaceton tanyspinosum, copyright April Nobile.

As already indicated, the majority of epopostrumiforms are endemic to Australia (one genus, Colobostruma, includes a few species found in New Guinea and the Solomon Islands). The only non-Australasian taxon to be assigned to the Epopostrumiti is an African long-mandibulate genus Microdaceton. Features uniting Microdaceton with the Australasian epopostrumiforms include the presence of lateral outgrowths on the petiole and postpetiole (the first two nodular segments of the metasoma) and the position of the petiolar spiracle (Bolton 1999) but some authors have suggested a closer relationship of Microdaceton to other dacetonin genera. Even if correctly positioned, Microdaceton is at most the sister taxon to the Australasian clade, members of which are united by features such as reduced antennae and an enlarged labrum.

Face of Colobostruma alinodis, copyright Estella Ortega.

Bolton (1999) divided the Australasian epopostrumiforms between three genera: Colobostruma, Mesostruma and Epopostruma. Less than fifty species of this clade have been described to date though others probably remain to be named. Even among the known species, many are rare and/or cryptic and some are known from only a very few specimens. Epopostruma resembles Microdaceton in having elongate mandibles with only a small number of interlocking teeth at the end (two in Epopostruma, three in Microdaceton). When hunting, Epopostruma may open their mandibles to a full 170°. Colobostruma has much shorter, triangular mandibles with numerous teeth; Mesostruma has triangular mandibles somewhat intermediate between the other two genera. The mandibles of both Colobostruma and Mesostruma cannot be opened to the same degree as those of Epopostruma; rather, species of these two genera will open their mandibles to a maximum angle of 90° when hunting. Whether the Dacetonini involved long mandibles on a single occasion, with a number of sub-lineages reverting to shorter mandibles afterwards, or whether the short-mandibled Dacetonini retain the ancestral morphology and long mandibles evolved on multiple occasions within the tribe, remains a question occasioning some debate.

REFERENCES

Bolton, B. 1999. Ant genera of the tribe Dacetonini (Hymenoptera: Formicidae). Journal of Natural History 33: 1639–1689.

Brown, W. L., Jr & E. O. Wilson. 1959. The evolution of the dacetine ants. Quarterly Review of Biology 34 (4): 278–294.