How to tell the difference between the trap-jaw ants Anochetus and Odontomachus

I recently posted a photograph of the trap-jaw ant Anochetus micans to facebook and G+, prompting one commentator to ask about the difference between Anochetus and the related genus Odontomachus. The easy diagnostic answer is this:

images from Antweb.org

The ridge along the back margin of the head in Anochetus is simple, while that of Odontomachus folds inward to become a crease down the center line of the head. The trait should be relatively easy to spot.

Odontomachus chelifer

Anochetus faurei

A more complicated question is if the two groups really ought to be classified as separate genera, rather than lumped into a singular Odontomachus. Both are genealogically related, and the lineage doesn’t contain any non-trap-jaw species, so our classification troubles are more a semantic problem than a biological one.

A few years ago my labmate Chris Schmidt attempted to determine the evolutionary relationships of various species using molecular markers. The data were ambiguous, providing some support for Anochetus as a daughter lineage emerging from within Odontomachus, and some support for two sister lineages. From an empirical standpoint, it’s still not clear whether dividing these ants into different genera will render a classification more in line with ancestry.

source: Schmidt, Chris A. 2009. Molecular Phylogenetics and Taxonomic Revision of Ponerine Ants (Hymenoptera: Formicidae: Ponerinae). Ph.D. Thesis, University of Arizona. online at http://books.google.com/books?id=Sby7w0Hec6EC&lpg