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:
Acromyrmex striatus, Argentina
Those of you familiar with South American ants know the distinction between the two great lineages of leafcutters. Acromyrmex comprises the smaller, stubbier, spinier species, while Atta contains the larger, leggier, and less spiny species. This dichotomy is marked enough even to be recognized in the local vernacular. The Guaraní called the former Akêkê and the latter Ysaú, which Brazilians have modified into the Portuguese Quemquem and Sauva. When preliminary phylogenetic work appeared to confirm the duality, I was pleased by the concordance of cultural and phylogenetic classifications.
Alas. Science had to go mess things up again:
Modified from Figure 4. Bayesian inference phylogenetic tree based on the concatenated sequences of four nuclear genes (WG, LW, EF1?F1 and EF1?F2).
A new study in PLoS One by Maykon Passos Cristiano et al added the unusual species Acromyrmex striatus to the phylogenetic framework, and- surprise!- it’s not like the others. Acromyrmex striatus chromosomes appear more similar to those of Atta than to those of its own congeners, and a phylogenetic reconstruction based on several protein-coding genes suggests this species may be a third, distinct lineage.
The authors state that more sampling is needed. Fair enough. But based on these data I would not be surprised if this taxonomic mismatch is eventually resolved by condensing leafcutters into a single monophyletic Atta.
update (3/26): See comment by Terry Nuhn, below.
source: Cristiano MP, Cardoso DC, Fernandes-Salomão TM (2013) Cytogenetic and Molecular Analyses Reveal a Divergence between Acromyrmex striatus (Roger, 1863) and Other Congeneric Species: Taxonomic Implications. PLoS ONE 8(3): e59784. doi:10.1371/journal.pone.0059784
Yesterday I snapped this photograph for the cover of Ant Queen:
Atta texana queen and worker leafcutter ants in the fungus garden (Lab colony at the University of Illinois)
That is, if Ant Queen magazine actually existed. I’m pretty sure I’d subscribe.
The composition of the photograph is intended as a cover, in any case. Vertical layout with space at the top for a title. Next time you publish a Nature paper about eusocial evolution, here’s your shot!
Below are a couple additional images from the session:
Turrets on an Atta vollenweideri nest, northern Argentina
A recent study by Marcela Cosarinsky and Flavio Roces examines turret-building by the chaco leafcutter ant:
…workers do not simply pile clay over sands or sands over clay after replacement of the available materials, and evince some complexity in construction behavior. The micromorphological analysis of the final wall demonstrated that the imported materials were distributed and combined tending toward a kind of microstructural balance that may be related to the maintenance of a porous but mechanically-stable structure.
source: Cosarinsky, M. I., & Roces, F. 2011. The Construction of Turrets for Nest Ventilation in the Grass-Cutting Ant Atta vollenweideri: Import and Assembly of Building Materials. Journal of Insect Behaviour. Online First DOI: 10.1007/s10905-011-9290-8.
Here’s a one-off shot I snapped this afternoon when I had a few minutes. Our lab colony of leafcutter ants is fed oranges, among other things, making for a colorful substrate.
Canon MP-E 65mm 1-5x macro lens on a Canon EOS 7D
ISO 100, f/13, 1/200 sec, diffuse flash.
An army ant raid passes peacefully through a trail of Atta cephalotes leafcutters.
We interrupt Army Ant Week to announce that a paper describing the Atta cephalotes leafcutter ant genome has just been published. I’ll report more on this research next week, once we take leave of the army ants.
Suen G, Teiling C, Li L, Holt C, Abouheif E, et al. (2011) The Genome Sequence of the Leaf-Cutter Ant Atta cephalotes Reveals Insights into Its Obligate Symbiotic Lifestyle. PLoS Genet 7(2): e1002007. doi:10.1371/journal.pgen.1002007
Yellow is all the rage this season among Ecuador’s leafcutters:
Note the must-have mini-ant accessory.
Canon EF100mm f/2.8 Macro USM lens on a Canon EOS 7D
ISO 200, 1/200 sec, f/14, diffuse remote flash
taken at the Jatun Sacha forest in Napo, Ecuador
…they just change jobs.
A study by Schofield et al out this week in Behavioral Ecology and Sociobiology measures the efficiency of leafcutter ant mandibles as they wear with age, noting that individuals with the most worn jaws are less than half as effective at cutting. Instead, these older ants spend more time transporting the leaf fragments sliced by their sharper sisters:
I would also like to nominate this paper for a Most Informative Title award, a category I just created, for encapsulating their entire study in a single sentence: “Leaf-cutter ants with worn mandibles cut half as fast, spend twice the energy, and tend to carry instead of cut.” You almost don’t need to read the paper, except the figures are kind of pretty.
source: Schofield, R. M. S., Emmett, K. D., Niedbala, J. C., Nesson, M. H. 2010. Leaf-cutter ants with worn mandibles cut half as fast, spend twice the energy, and tend to carry instead of cut. Behavioral Ecology and Sociobiology, online early DOI: 10.1007/s00265-010-1098-6.
Tonight’s selection was suggested for your viewing pleasure by Jack Longino.
Leafcutting ants of the genus Atta have perhaps the most complex caste systems of all the social insects. Mature colonies contain millions of workers of varying shapes and sizes. Here are two sisters from opposing ends of the spectrum.
photo details: Canon MP-E 65mm 1-5x macro lens on a Canon EOS 20D
ISO 100, f/13, 1/250 sec, flash diffused through tracing paper