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:
hi alex, I sent an email about it. Now it’s easier to identify.
first image: Atta
second image: Acromyrmex (this was not so easy to identify)
Third image: Acromyrmex
Correct?
Correct!
Thanks for the excuse to write this post, Igor.
How about a post to tell the difference between Acromyrmex and Trachymyrmex?
And between all those other miserable M-genera of attine ants?
What a coincidence! I was just having trouble identifying one of those! I still don’t know what species is it though.
http://www.inaturalist.org/observations/446674
Thanks for your comment, Luis. identifying Atta to species without a major worker can be difficult, as they are all quite similar. I suspect yours might be A. cephalotes, although I am not confident in that ID.
Are there any exceptions to this rule, that you know of?
None that I have seen. Although, on very small Acromyrmex workers, the third spine can be more of a bump.
While photographing both Atta and Acromyrmex in lab colonies I’ve noticed that Attas seem to have a glossy or shiny exoskeleton while the Acromyrmex ants seem to be not as glossy. Is this also an identifier?
That’s an astute observation for the species you have been photographing, Don, but it doesn’t apply across all species. Some Atta are quite dull (Like A. sexdens and A. texana), while a few Acros are shiny (A. striatus).
I looked at my images of A. sexdens sure enough they are not as shiny as A. cephalotes. Thanks again. Has anyone developed an online dichotomous key to id ants? I’ve seen some for bees but not ants.
There’s antkey.org – though with a coverage of 100 species (of over 15000) its obviously not going to get you everything. It requires java to work. Last time I tried it, it was fairly slow to load.
Do leaf cutter ants exist in PA, CA, USA?
Are there distinct societal differences between the two genera?
From Ants of North America: “Atta may be distinguished from the closely related genus Acromyrmex by the presence of three pairs of spines on the promesonotum (two pairs in Acromyrmex).” vs “Three pairs of promesonotal spines indicates Acromyrmex, while two pairs indicates Atta” Based on photos I believe the former is correct.
From Ants of North America: Acromyrmex is separated from Atta by the presence of three pairs of spines or teeth on the promesonotum (Atta has two pairs), … (p.60)