Eciton burchellii, the swarm raider

ORIGINAL: Myrmecos.net

Feb 7th, 2011

by myrmecos. 

Eciton burchellii is, according to Wikipedia, “the archetypal species of army ant“. Insofar as this is the most-studied species, and the ant that dominates the nature documentaries, I suppose the moniker is true. 

Yet, the biology of E. burchellii is not terribly representative of army ants. It is an outlier, an ant whose behavior has diverged in significant ways from its relatives, even from its congeners. 

media and minor workers stream towards the front 

The primary difference is gustatory. Most army ants boast a fine-tuned pallate, favoring the brood of particular ant genera. E. burchellii is vulgar by comparison. It’ll eat just about any sort of animal protein. Spiders? Katydids? Lizards? Termites? Ants? They’re all good. 

a submajor worker carries a spider 

media workers pull a termite from a rotting log 

Corresponding to the broad diet, Eciton burchellii‘s foraging behavior is a radical departure from the army ant norm. For being an “archetypal” army ant, its raids ironically lack the military precision of other species. 

An overhead depiction of an Eciton burchellii swarm raid (from Rettenmeyer 1963) 

Instead of tight, focused columns that concentrate the ants’ efforts at a single point at the raid front, E. burchellii’s raids are messy, diffuse affairs. Foragers spread out in a swarm, casting a vicious stinging, biting net for animals too slow to escape. The strategy isn’t great for capturing concentrated food sources like another ant colony, but it works well for a generalist’s lunch. 

Raids set out in the morning with a swarm front near the overnight bivuoac. As the day progresses, the swarm moves through the forest for a distance about that of a football field. The back end of the swarm organizes into a series of converging trails to funnel captured prey back to the bivouac, so that by mid-day the swarm from above looks rather like the figure at left. Last night’s video featured one such trail. By nightfall the ants are back in the bivuoac, where, depending on the development of their larvae, they either regroup to march to a new bivouac site by morning, or they stay put and launch a new raid in a different direction the following day. 

Eciton burchellii raids are fascinating to watch, but they aren’t indicative of how most army ants behave. The fame of this one species is partly an artifact of the degree to which its unique biology intersects with the cognitive quirks of our own species. 

These ants are large, they forage above ground, and their sprawling raids cover extensive areas. And they raid primarily during the day, when we humans tend to walk about in the forest. That they catch our attention may be due as much to our biology as to theirs. 

[photos 1,2,4 taken at Jatun Sacha; 3,5 at Maquipucuna]