These exploding ants take defending their colony to the next level

Weaver ants are a mean bunch. They’re not very big, but they’re highly aggressive and extremely territorial. Their main weapons are a pair of ferocious jaws; after they bite they also like to spray formic acid into the wound, just to be jerks. They go after other insects with particular vigor, and are so good at murdering their fellow minibeasts that weaver ant colonies are used as a form of biological pest control across the tropics.

So if you’re another kind of ant who happens to share the treetops with weavers, what are you to do? As it turns out, the answer is simple: explode.

Ok, sure, that sounds dramatic. But it’s also literally true. When dealing with a threat like the weaver ants, workers from some ant species actually blow themselves up, in a process known as autothysis. Basically, when these ants are in enough trouble, they squeeze their internal organs so hard that they burst their exoskeletons, coating the surroundings with their innards.

Ant innards on their own aren’t enough to discourage weaver ants. Indeed, weavers are positively in favor of innards. But our exploding ants — to give them a name, they’re colobopsis explodens, which is Latin for “hey, these ants explode!” — aren’t just blowing up for the hell of it or as a quick escape from impending doom. These ants, in fact, are packed to the petiole with goopy poison, and when they explode, guess where that all goes? Yep. Everywhere.

Blowing themselves up in little poison bombs is an effective way for colobopsis ants to deal with threats which might otherwise be overwhelming. But autothysis is also a little bit drastic. What makes these little critters sacrifice themselves in the face of the enemy? Surely their instincts would prevent them doing something like that? What of evolution? What of survival of the fittest?

This sort of behavior troubled Charles Darwin, back when he was writing On the Origin of Species. Not the behavior of the colobopsis ants themselves — as far as I know he knew nothing of autothysis, which I imagine he’d have enjoyed — but of ants more generally. Ants (and bees, wasps, termites, etc.) practice a set of behaviors called eusociality, which essentially involves cooperative care of young and a sharp division between reproductive and non-reproductive individuals.

It’s that last part which really messed with Darwin. In chapter eight of Origin, he writes:

I … will confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect communities: for these neuters often differ widely in instinct and in structure from both the male and fertile females, and yet from being sterile they cannot propagate their kind.

Darwin struggles to explain away the problem, for very good reason: despite evolving several times across various animal groups (my favorite is a kind of sponge-dwelling snapping shrimp from the Caribbean) eusocial behavior is extremely difficult to explain simply. That doesn’t mean it’s impossible, just that it’s complicated, and research is still ongoing to sharpen the theories around the edges.

I bring this up, however, because while Darwin doesn’t explain the evolutionary mechanics behind eusociality, his objection does reframe the way we should be thinking about our friends the exploding ants. Instead of being concerned with their suicidal tendencies, we need to understand a simple fact: in evolutionary terms, a sterile worker is already dead. They are a resource to be expended by the breeding animals at the heart of the colony, not an autonomous agent.

When I was a kid I always felt guilty playing games like Starcraft or Age of Empires. I knew that the best way to play them was to build units solely so they could fight and die on my behalf, but I never could bring myself to be that ruthless, even to the little people on my computer screen. Ant queens have no such worries. They lay their eggs, they hatch their little minions, and they expend those minions for the greater good of the colony. And if that greater good requires them blowing themselves up in a smear of poisonous goo, who cares? Not the queen. Thousands more where those came from.

Once you move from the individual level to the organizational level, the evolutionary behavior of these ants become clearer: they are expendable parts of a whole, specially adapted for a specific job. Autothysis costs a colony resources, of course, but they are resources that can be replaced, a little like the human body sacrificing white blood cells to fight off infection. Thinking on this scale, with the colony as its own genetic unit, eusociality becomes even more fascinating.

Incidentally, the explosive workers aren’t the only specialists available to colobopsis ants. When fighting doesn’t work and when their poison squads fail, the ants sometimes have to retreat back to their holes. And they refer the rearguard action to a special type of soldier:

Yeah, they plug the gaps with their faces. Very normal ants.

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