The Wasp That Brainwashed the Caterpillar

by Matt Simon

  PUTTING DEAD SNAKES IN LIQUOR AND OTHER BORNEAN PARTY TRICKS

  Darwin and Wallace may have stumbled upon the same superb idea, and they may have been good friends despite Darwin taking the limelight, but the two men couldn’t have lived more different lives if they’d tried. Darwin came from wealthy stock, which allowed him to study and write without worrying about making a living, while Wallace had no such privilege.

  The voyages that led them to their discoveries couldn’t have been more different either. Compared to Wallace, Darwin had a downright comfortable trip around the world aboard the Beagle, finding danger and toils all along, sure, but also good lodgings among wealthy expat Europeans while wandering on land. Wallace—a lanky, bespectacled man—rotted in the jungles of Southeast Asia, earning what you’d generously call a living by shipping specimens back to European collectors.

  Even preserving the specimens in the first place was a tall order for Wallace. On Borneo, he pickled them in a local liquor called arrack, which unsurprisingly was a favorite of the local people. “To prevent the natives from drinking it,” Wallace writes in The Malay Archipelago, a fantastic account of his travels, “I let several of them see me put in a number of snakes and lizards; but I rather think this did not prevent them from tasting it.” Whether it ever crossed his mind to give up in despair and just drink all the liquor himself is sadly lost to history.

  When it comes to explosive populations to rival humanity, there are few species more successful than the ants. Conservatively estimated, they number 10,000 trillion. But as the old adage goes, haters gonna hate, and there are a lot of ant haters out there willing to keep the ants in check. Of them, perhaps the most creative and brutal is the ant-decapitating fly. Yes, that’s what it’s called. And yes, that’s exactly what it does: decapitate ants . . . from the inside out.

  Getting laid is all well and good, but it’d be nice to be able to protect your young as they develop, since you don’t want that fancy jousting and humming and having sex until you die to be for naught. So instead of leaving its larvae to the mercy of predators and the elements, the ant-decapitating fly goes out in search of babysitters, specifically the fire ants of South America. Female ant-decapitating flies descend upon a colony, hovering over their targets—which are many times their size—waiting for the right moment to strike. And it happens in an instant: The female fly dives and slams her ovipositor into the membrane between an ant’s legs, injecting a tiny egg. (Think of the ovipositor as a needle that instead of delivering drugs delivers a baby. Interestingly, bee stingers are modified ovipositors that instead of delivering babies deliver venom. When a bee stings you, the offender is a female, never a male.)

  This does not suit the ants one bit. They sprint around like maniacs, curling up into balls in a pathetic attempt to protect themselves, all the while releasing alarm pheromones, which only makes things worse for themselves. Other ant-decapitating flies are attracted to these scents, and reinforcements zoom in. Soon enough, a swarm of flies is hovering over a battlefield that’s littered with wounded ants, still very much alive, yet totally unprepared for what’s about to happen to their bodies.

  After a few days the egg in each ant hatches into a maggot, which worms its way through the body and into the head. Here it feeds on juices, all the while making sure not to damage its host’s brain. The whole time the ant is behaving normally, but in another few weeks it becomes clear why the fly has left the brain alone: This isn’t just a host—it’s a vehicle, and a vehicle is no good without an engine. The larva releases a chemical concoction that gives it control over the ant’s mind, guiding the host away from the colony and down into the leaf litter. It’s nice and humid here, the perfect environment for the next stage of the fly’s development. (A quick note here on some insect life cycle terminology. The egg hatches into the larva, or maggot, a kind of squishy wormlike thing. It feeds and grows for a while before turning into an inactive pupa, which would be the cocoon stage of something like a butterfly. From the cocoon the insect emerges as an adult.)

  Once the ant reaches an ideal location, the maggot releases a chemical that dissolves the membranes holding the ant’s various parts together, including the bit that keeps the head on. The noggin pops right off the body with the larva inside, and only then does the fly eat the brain to hollow out the head. When it has had its fill, it clears away the ant’s mouthparts, leaving an escape route that it plugs up with a hardened end of its body (remove the pupa at this point and it’ll be shaped like the ant’s head, a bit like a kernel of corn). The ant-decapitating fly will develop for a few more weeks, all snug in its crib, before emerging from the head and flying away to mate and start the whole weird circus all over again.

  All of this makes the ant-decapitating fly a kind of folk hero in the United States. Not because we’re impressed by its work in South America, but because we’re dependent on its work here at home: Fire ants from down south have invaded the United States and grown enormously successful. Their economic impact each year tallies in the billions, ranging from crop damage to the medical costs for humans unfortunate enough to tango with them (stings result in painful pustules and the occasional severe allergic reaction). The ants seem to have found an unassailable niche in the ecosystem, or so they thought. The government has taken the somewhat desperate step of importing the ant-decapitating fly to wage biological warfare with the fire ant. And it appears to be working, where things like insecticides have largely failed.

  WHAT’S IN A NAME? WELL, SOMETIMES “ZAPPA”

  I’ve written “ant-decapitating fly” so many times at this point that I feel it needs to be addressed: It’s one of my favorite names in the animal kingdom. Other honorable mentions are the scrotum water frog, which looks like you’d imagine; the Rasberry crazy ant (“Rasberry” without the “p” because it’s named after some dude who found it to be crazy), which behaves like you’d imagine; and Phialella zappai, a jellyfish named after Frank Zappa. It seems that a biologist was really keen on meeting Zappa and figured naming a species after him would do the trick. And of course it did. I mean, we’re talking about Frank Zappa here.

  Fighting nature with nature is a strange and precarious technique. We’ve grown so used to dousing our problems in chemicals that it’s easy to forget the greatest weapon of all: Every creature has its foe—even apex predators like bears or lions have parasites to worry about. True, introducing an invasive species to combat an invasive species is a risky maneuver, what with them both being invasive species and all. But scientists with the USDA determined that the ant-decapitating fly is so specialized, so intent on ruining the fire ant’s day and only the fire ant’s day, that it was safe to bring to the United States. And with the fire ant laying waste to agriculture and livestock and sending the ecosystem into chaos, not to mention attacking humans, I find it a wee bit hard to feel sorry for the invaders. So welcome to America, little ant-decapitating fly. And good hunting.

  Glyptapanteles Wasp

  PROBLEM: I wasn’t kidding when I said maggots are helpless.

  SOLUTION: A momma Glyptapanteles wasp injects her eggs into living caterpillars. After the larvae feed on the juices and burst out of their host, they mind-control the poor beast to defend them as a bodyguard/babysitter.

  All of this invading other creatures’ bodies and mind-controlling them and dispatching them in horrible ways is relatively rare for a fly. But not the wasps. No, some of nature’s most sophisticated parasites are in fact wasps, such as the Ichneumonidae variety, insects capable of brutality like you wouldn’t believe.

  Indeed, parasitic wasps that inject their young into caterpillars presented Darwin with quite the conundrum: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars, or that a cat should play with mice,” he once wrote in a letter to Asa Gray (whom, if you’ll remember, he also
complained to about the peacock’s tail). Darwin’s theory of natural selection posited that a whole lot of creatures must die so others could live, which didn’t go over so well at the time, particularly with the religious establishment. These were people with an idyllic conception of nature, who thought that predator and prey could hang tight in Noah’s boat and somehow not devour each other before the flood subsided.

  The Ichneumonidae wasps have some cousins, though, that aren’t about to let caterpillars off so easy: the Glyptapanteles. These species don’t kill caterpillars outright. Instead, as they erupt out of their host, the larvae bestow the victim with an extra indignity by mind-controlling it into becoming a bodyguard. Thus, under such devoted care, the young wasps are ferried safely into the world.

  The female wasp begins by seeking out a caterpillar. And like the ant-decapitating fly, she drills into her victim. Unlike the fly, though, the wasp deposits as many as eighty eggs into the poor critter. These will hatch into larvae and feed on bodily juices, growing fatter and fatter until their host looks like an overfilled water balloon. All the while the caterpillar goes about its life, meandering around and eating—after all, it has eighty-one mouths to feed. But there comes a time in every parasitized caterpillar’s life when it must bid farewell to the maggots it has housed, fed, and shared so many memories with. So the larvae release chemicals to paralyze their host and erupt en masse from its body for a full hour, all writhing and wiggling right through the caterpillar’s skin. It’s not what you’d call a pleasant scene, as almost every square inch of the caterpillar’s body is bursting with squirming maggots.

  But this isn’t the end of the unwitting host. As the larvae grow, each must periodically shed its exoskeleton and grow a new one. This happens several times inside the caterpillar, but most significant is the last molt, which is timed with the larvae’s exit (an event known formally and ever so measuredly as an “egress”). As they squirm out, the larvae shed their exoskeletons, which plug up the wounds they leave behind, as it’s in their best interest to keep their host alive, at least for the time being. But not all the larvae exit the caterpillar. One or two will stay behind, and these stragglers seem to be responsible for what happens next. By remaining in the caterpillar, those larvae may release chemicals that mess with their host’s brain, transforming the caterpillar into an ultraviolent goon that protects the rest of the brood.

  AS IF YOU NEEDED ONE MORE REASON TO DESPISE MOSQUITOES

  Oh, thank goodness this can’t happen to me, you may be thinking. Well, it won’t be these wasps, but there’s another insect that targets mammals, including human beings: the infamous botfly. It begins by tackling a mosquito and laying eggs on its belly. When the mosquito lands and feeds on a human, the eggs hatch and the larvae drop off into the wound and burrow under the skin. Each larva, which is covered in spines to make its removal not just difficult but downright excruciating, grows to over an inch long feeding on its host, always poking its all-purpose arse out of the hole in the skin so it can breathe and go to the bathroom. Leave it alone and after three months it’ll retract the spines and back out on its own, nearly pain free. But opt to remove the larva prematurely and you’re in for agony. I, for one, would let it develop in the name of science—and I’d probably make it a week before I cut off whatever part of my body it occupied. Even if it were my head. Actually, especially if it were my head.

  So as the caterpillar shakes off its paralysis, it doesn’t attack its assailants, and it doesn’t flee. It doesn’t even have any desire to eat. It stays right where it is as the larvae around it start spinning their cocoons. In fact, it even chips in, spinning a covering over the cocoons with its own silk. And woe to any predators that try to attack the young wasps, including the parasitic wasps that in turn attack these parasitic wasps, because the caterpillar will lash out at anything that gets close with violent swings of its head. Even as swarms of predatory insects descend on what should be an easy meal, the caterpillar stands its ground, swinging to bat away the invaders. One study found that bodyguard caterpillars can fend off 58 percent of predators, whereas nonbrainwashed caterpillars fend off only 15 percent of predators. Not a bad rate of success for a squishy tube of a creature that normally ambles around eating leaves.

  Its moment of victory is a short one, though. As the wasps complete their life cycle, emerging from their cocoons and flying off, the caterpillar dies of starvation. It was used, through and through, and it dies an unfortunate death.

  I often find myself struggling to find words to describe how amazing this sort of thing is, but since I’m a writer and all, I should probably give it a shot. The majesty of a one-hundred-foot blue whale is one thing, and the beauty of a brilliantly iridescent blue morpho butterfly is another, but how astounding is it that a creature has evolved to mind-control another into doing its bidding? And ant-decapitating flies and Glyptapanteles wasps aren’t anywhere near the only animals to do this—all manner of critters have independently arrived at the mind-control solution to their problems.

  But such complexity is, weirdly enough, the product of the simple process that is natural selection. Wasps didn’t suddenly arrive at the idea of assuming control over caterpillars. The same way the mustache toad developed its weaponized mustache, over generation after generation, wasps with certain variations were more successful at reproducing. They may have begun, for instance, by injecting caterpillars in the first place and hollowing them out. That certainly helped. (Indeed, there are any number of parasitic wasps that stop there, having for whatever reason not evolved further complexity.) Then other variations resulted in the larvae avoiding major organs, guaranteeing a fresh meal for longer. That, too, helped the young better survive, and therefore pass down the genes responsible for such variations. Small modifications accumulate over time to add up to an awesome event like the mind controlling of a caterpillar.

  Evolution, though, is a two-way street. Creatures can evolve this kind of onslaught, but their victims can evolve a defense. And there’s one caterpillar out there that isn’t going to sit by and suffer the indignity of predation. No, that won’t do.

  IF THE BODY OF ANOTHER CREATURE FITS, WEAR IT

  Different species arriving at similar adaptations like mind control is known as convergent evolution. A good example is flight. Birds evolved from dinosaurs into critters capable of flying, but bats—which are mammals—also evolved this ability, only they use a membrane of skin as a wing instead of feathers. These are different lineages, but both bats and birds found it advantageous to take to the air. The same goes for the many creatures that have stumbled upon mind control. As the saying goes: If the body of another creature fits, wear it.

  Asp Caterpillar

  PROBLEM: Slow, plump caterpillars are easy meals.

  SOLUTION: The asp caterpillar grows a groovy, irritating hairdo with secret weapons underneath: spines that sting with such ferocity that they can trigger breathing difficulties (and panic attacks) in humans.

  Having already dumped her husband some years earlier, naturalist Maria Sibylla Merian at last gave in to an overriding obsession with insects, taking her youngest daughter and traveling from Amsterdam across the Pacific Ocean to Suriname. Here she spent two years traipsing through jungles collecting bugs, taking them home, and raising them—studying them, drawing them.

  Few caterpillars she found were as bizarre as the flannel moth, with its shock of hair (an entomologist friend of mine rightly points out its resemblance to Donald Trump). “The skin of these caterpillars, under the hair, looks very like human skin,” Merian wrote. “They are very poisonous; if one touches them with the hand, it swells up immediately and is very painful, as I discovered myself.” And poor Merian didn’t have the luxury of modern medical treatment. You see, the year was 1699, more than 130 years before Darwin stepped foot in the Galápagos. Merian was literally a trailblazer in a man’s world, a woman whom science has largely forgotten. And she was probably the first nat
ural historian, regardless of sex, to undertake a scientific expedition to a foreign country.

  Merian was in Suriname to expose the mysteries of insect metamorphosis, which in the seventeenth century was almost totally unstudied by modern European science, itself in its infancy. The big questions: Why should butterflies and moths bother with such extreme transformations? Why not hatch into a butterfly and forgo all the caterpillar business?

  Well, it turns out it has a lot to do with food. By taking on such a radically different form, the caterpillar can exploit a niche different from the adult’s, so it doesn’t need to compete. But this life cycle leaves the caterpillar—a piece of meat just asking to get assaulted, as the Glyptapanteles wasps have noted—vulnerable. Unless, that is, it evolves a defense.

  Among the many species whose caterpillars have adopted weaponized hairdos, none is more powerful than the one Merian tangled with: the asp caterpillar, aka the puss caterpillar, of the Americas. Yeah, there’s a bit of a disconnect between the names “asp caterpillar” and “puss caterpillar.” But then again, there’s a bit of a disconnect between how the caterpillar looks and what it can do to the human body. Reports of envenomations vary, likely due to the dose of venom received, but a sampling of reactions include: “It immediately felt like a hammer hit me” and “it felt as though my arm had been broken” and “I have had kidney stones before, but I believe the pain I am experiencing from the asp sting is worse.” Symptoms can range from faintness to a burning sensation, fever, and, rather vaguely and intriguingly, “abdominal distress.” The paper that reported these cases noted that victims, not suspecting that a caterpillar could cause such suffering, are often “launched into panic attacks by the unexpected onset of the pain.”