army ants

The Evolution of Swarm-Raiding Army Ants

Today is not only the final day of Army Ant Week, but Charles Darwin‘s 202nd birthday.

So I close Army Ant Week with a bit of speculation about evolution, and what army ants suggest about the nature of the evolutionary process.

Neivamyrmex californicus workers attack a pavement ant (California)

The 300 or so army ant species vary in behavior, but most are specialized ant predators that hunt in soil or leaf litter. Their limbs are short and stubby, ideal for subterranean maneuvering, and the workers are largely to completely blind.

The “classic” army ants, the surface-foraging swarm raiders familiar to most people, are in fact a minority of species (Eciton burchellii, Labidus praedator, and several Dorylus). Where did these spectacular swarm raiders come from?


Eciton enjoying a light snack

Along the margins of a raid, Eciton hamatum workers create a cache of captured ant brood

In the comments, myrmecologist James Trager notes:

A serious question, though, have you ever seen these soldiers eat? I have not and always wondered how they go about it.

Come to think of it, I don’t have much recollection of seeing any army ants eating.

I have never seen a soldier or a queen army ant actively feeding. But last month I did catch some hungry Eciton hamatum workers taking a snack from a cache of pilfered ant brood:


Friday Beetle Blogging: Army Ant Associates

See the little rove beetle? (Jatun Sacha, Ecuador)

Last year army ant guru Carl Rettenmeyer posthumously published a paper documenting the tremendous diversity of animals associated with Eciton burchellii. Over 500, in fact. Eciton burchellii has a larger known entourage than any other species of animal.

Although Eciton‘s associates are the best documented, all army ant species have them. Ant colonies represent a tremendous concentration of resources, and animals that have figured out how to subvert the ants’ communication systems gain access to rich stores of food.

This week’s Friday beetle features a few of the coleopterous army ant associates I encountered on my recent trip to Ecuador. I haven’t yet had the time to identify them beyond family (they’re all Staphylinidiae), but feel free to share your knowledge in the comments.

A rove beetle with Labidus praedator.
The same species (at right) sneaks onto the prey of the ants to steal a meal.
A different species of rove beetle running in a Labidus column.

Finally, let’s play Spot-That-Beetle:

Did you see it?


Eciton hamatum
Eciton burchellii

It’s hard to look at Eciton soldiers and not wonder what’s up with those evil tusks.

So here it is: those hooked jaws are designed to inflict pain on vertebrates.

I don’t post this for shock value but as a statement of fact. The ant uses them to snare offending animals and anchor herself to their skin. Once hooked, she uses her stinger (a sharp lance at the butt end of the insect) to inject venom. It’s an ingenious trick. Removing a stinging insect attached with fishing hooks is not easy.

Army ant bivouacs conceal piles of tasty larvae, and army ant raids gather concentrated stores of valuable rainforest protein. Many a mammal or bird could feast well off the army ants, and it is the job of the soldier to make such attempts as unpleasant as possible.

Outside their defensive post, soldiers are useless. Those tusks do not permit them to capture or carry prey, nor are they suitable for tending to brood.

Enough biology, though. Which species’ sting hurts more?

No question. Eciton burchelli has both a nastier sting and less trepidation about using it.

In the grand scheme of insect-induced agony, surprisingly, army ants rank low. I’d rate them as somewhat less painful than the common honey bee. Makes for a dramatic photo, though, even if I am shamelessly ripping off Mark Moffett.

Ant Death Spiral

A while back this video was making the rounds:

It’s a self-reinforcing circular mill composed of- now here’s a change of pace! – army ants. I thought I’d reintroduce it in honor of this week’s festivities.

Labidus praedator

The species is Labidus praedator, a swarm-raiding army ant from Central and South America, and these circular mills are a common byproduct of army ant navigation. Labidus is completely blind, so ants in this genus get about by following the insect in front and laying down a chemical trail. The system works well enough in a straight line.

The trouble begins when the ants loop around and intersect their own path. The poor insects end up on a mobius strip of their own making, circling around and around until some either chance to leave the mill and the circle is broken, or they run of steam and perish. Thus, the Ant Death Spiral.