The zombies with six legs
As the popularity of vampires wanes, zombies seem to be coming into their moment. “The Walking Dead” has become a hit show on AMC. Atlanta is trying to claim the title of Zombie Capital of the World. Even the federal government is getting in on the action: This year the Centers for Disease Control and Prevention posted instructions about emergency preparedness by warning about a “Zombie Apocalypse.”
But when it comes to wandering around mindlessly, feasting on brains and generally creeping out the populace, the champion zombies haven’t been drawn from human ranks. They have six legs and have been perfecting their skills for millions of years. Insects routinely do things too grotesque for even the most gruesome horror film, and along the way make us question some of our assumptions about what it means to be alive. Or, for that matter, undead.
The best examples come from wasps engaged in the nurturing task of providing food for their babies. Wasp larvae eat meat, but the mother wasp often doesn’t live long enough after she lays her eggs to ferry prey back and forth to her growing brood. What is more, the prey, usually other insects or spiders, need to be kept fresh while the wasp larvae feed, since no one, not even young wasps, wants to eat rotting insect flesh.
To solve this problem, the tiny emerald jewel wasp does what many apartment dwellers might like to do: direct the movements of a cockroach. The wasp provisions her young by paralyzing the roach and bringing it back to the nest. The paralysis, as opposed to out-and-out killing, helps the prey stay fresh while the young wasp larvae feast on it. Of course, paralyzed insects can’t put themselves into the nest, so wasps usually have to do all the heavy lifting, staggering under the weight of the groceries as they fly back to the young.
Except, that is, in the case of the jewel wasp, so named for the glittery green of her exoskeleton. The female wasp doesn’t send the roach into an immobile stupor; instead, she makes it into a zombie via a judicious sting inside the roach’s head that leaves the insect passive but able to walk on its own. Then, as science writer Carl Zimmer describes, “the wasp takes hold of one of the roach’s antennae and leads it, like a dog on a leash, to its doom.”
For years scientists were mystified about the precision of this sinister manipulation of the nervous system. How could a single injection of venom manage to produce what neuroscientists Ram Gal and Frederic Libersat called “a living yet docile” victim? Finally, last year, through a series of meticulous manipulations of the cockroach nervous system, including a kind of sting-mimicking injection at various nerve cells in the head, the researchers demonstrated that the drive to walk in response to most stimuli is seated in a tiny cluster of cells called the sub-esophageal ganglia. By poisoning just this minuscule part of the nervous system, the wasp is able, in Gal and Libersat’s words, to “hijack the cockroach’s free will.”
Zimmer refers to the discovery as finding “the seat of the cockroach soul.” I am not sure I buy the idea that roaches have souls to be found, nor that free will is residing in all those cockroaches lucky enough to miss an encounter with a jewel wasp, but then I am not sure about either of those things in humans, either.
Still, the finding is fascinating. If we can so completely alter a roach’s motivation to walk by throwing a monkey wrench into a couple of cells, what does that tell us about the nature of motivation?
Another case of arthropod subjugation was discovered by a group of scientists in Brazil. The prey in this case is a caterpillar, upon which the wasp lays her eggs; once the wasp larvae hatch, they feed on its flesh while the caterpillar continues to munch leaves on its own. But the parasitized caterpillar makes an unwitting sacrifice even after the wasp larvae have emerged from their host. The ravaged caterpillar stands guard over the developing wasps and defends them against intruders with vigorous swings of its body, a most un-caterpillar like behavior. Apparently the wasps exert a kind of mind control over their caterpillar host that persists even after they leave it, doomed to die before it will ever become a moth.
The zombie insects illustrate one of my favorite principles about the six-legged: Anything we can do, they can do better, or at least with less equipment. They require no laboratory with bubbling beakers, no deals with the devil. And they also provide fodder for creativity. This year, if you want to dress like a zombie for Halloween, consider dressing up as a cockroach.
Marlene Zuk is a professor of biology at UC Riverside and author of “Sex on Six Legs: Lessons on Life, Love and Language From the Insect World.”
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