Anterior part of the head of an Australian Onychomyrmex doddi worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Among the many interesting features found in members of the subfamily Amblyoponinae is the presence of unique teeth-like structures at the anterior margin of the ant’s cranium. They are arranged in one or two parallel rows, right above the opening of the oral cavity, in a plate called clypeus.

Dentiform clypeal setae on a Onychomyrmex doddi worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Close examination reveals that the teeth-like structures are overgrown modified setae, that is, insect hairs or bristles with a socket at their bases rather than spiny projections of the cuticle (as is the case of the teeth in the ant’s mandible). The longitudinal ridges or striations are a common feature of most setae, formed during development as they elongate. Similar setae are sometimes found also in the labrum, the lid that closes the oral cavity from above.

Nobody really knows the function of these stout dentiform setae, but given their location one hypothesis is that they help secure prey or nestmates when transported between the mandibles: the mandibles in this group are elongated and slender, without a clear masticatory margin. The objects being carried are “hugged” against the frontal margin of the clypeus.

Though presence of these specialized setae is a synapomorphy of ambyoponines, they have been secondarily lost in some derived lineages within the group. I surveyed these structures as part of my morphological work on ant phylogeny, and found them to be quite diverse. The setal apex can be acute, blunt, truncated or slender and oblique, and each seta can arise from a tubercle-like process or right from the flat cuticle as in the Onychomyrmex pictured above. They vary in a way that seems to hold good phylogenetic potential, but I am not using that information right now because my taxon sampling is too broad within this group.

Lastly, it is interesting to note that some lower cretaceous ant fossils not belonging to the ambyoponines display similar dentiform setae in the clypeus. Whether they are homologous to the ones found in this subfamily remains to be tested.

Below, a few more images of these fascinating setae for you enjoyment.

Dentiform clypeal setae on a Concoctio concenta worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Dentiform clypeal setae on a Amblyopone armigera worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Hypertrophied dentiform clypeal setae on a Amblyopone pluto worker (Scanning Electron Micrograph, Roberto Keller/AMNH)