Three pairs of promesonotal spines indicates Acromyrmex, while two pairs indicates Atta (photos from antweb.org).

The iconic leafcutter ants of the New World tropics and subtropics are currently split into two similar genera: Acromyrmex and Atta. What’s the difference?

In an evolutionary sense, the answer isn’t clear. A recent molecular study suggests Atta may be no more than a derived lineage within a larger Acromyrmex, and that our distinction is artificial.

But what if you just want to key a specimen to one or the other? That’s easier. Count the spines on the front of the thorax- the promesonotum- you’ll find that Acromyrmex sports three pairs, while Atta has just two.

As an exercise, see if you can identify the ants in the following images: