It’s like something out of a horror movie. To procreate, the jewel wasp turns an unfortunate cockroach into a “zombie slave” by poisoning its brain. The wasp then leads the mindless cockroach into a lair and uses its living body to feed and grow a baby wasp, which ultimately emerges from the cockroach’s abdomen. That ends the cockroach's story, leaving only one terrifying mystery: how exactly the wasp “zombifies” its victim.
Having labored to answer this question for years, researchers at Israel’s Ben-Gurion University of the Negev recently made a breakthrough: identifying sensors on the wasp’s stinger that guide it into the cockroach’s tiny brain.
The findings, published in the science journal PL0S ONE in February, help explain the wasp’s unique mind-control ability and may hint at the neurobiology of free will.
“The brain sting is the most important step in a highly specialized process,” said Dr. Ram Gal, a neurobiologist who led the study in Prof. Frederic Libersat’s laboratory at Ben-Gurion’s Department of Life Sciences. “It leaves the cockroach’s sensory and motor systems working fine, but something else has changed. In humans, we would call this thing free will.”
Look and feel in the insect world
A pregnant female jewel wasp is loaded with fertilized eggs, each of which needs its own unfortunate cockroach to develop on. When the wasp finds a suitable host, she swoops in and stings it in the thorax, briefly paralyzing the front legs.
She then inserts her long stinger through the cockroach’s neck and into its brain – delivering a cocktail of mind-altering venom. The sting, which takes a full minute to deliver, has two effects. For the first half hour, it causes the cockroach to obsessively groom itself, giving the wasp time to fly off and find a nest (and possibly removing potentially harmful infections). And for up to a week, it makes the cockroach completely passive.
To understand how the wasp finds the cockroach’s miniscule brain – one of two such structures in its relatively spacious head – the researchers first looked at the wasp’s stinger under a powerful electron microscope. They identified two types of sensors that have been found on other parasitic wasps: one for feeling and one for tasting.
The researchers next ran a series of experiments to see how the wasp uses the sensors. They found that the wasp stung the cockroach normally when the cockroach’s brain was intact or replaced with a rubber-like pellet of similar density. The wasp did not deliver a sting when its stinger sensors were destroyed or when the cockroach’s brain was removed, mashed up or replaced with a less-dense pellet.
Electrodes inserted in the wasp’s nervous system showed that the wasp was getting different brain signals in these cases too. The results prove that the wasp uses a sense of feel to locate the cockroach’s brain; it’s still unclear what the taste sensors do.
It wants its kids to eat the brain
The sensors on the wasp’s stinger are an unusually specific adaptation. While other parasitic wasps use sensors to root out hidden prey, the jewel wasp uses them exclusively to hunt cockroach brain.
Such an adaptation requires a huge amount of evolutionary “energy,” and insects, in particular, must be highly energy efficient. So the researchers know precisely locating the cockroach’s brain must be essential to the wasp.
With this fact in mind, they are now carefully looking at how its venom interacts with the cockroach’s brain. Maayan Kaiser, a doctoral student who works in the lab, is analyzing the various peptides and proteins that make up the venom. Ultimately, the researchers hope to be able to explain how the wasp commandeers the cockroach’s free will.
They think the answer will have applications for other animals, even people.
“Insects aren’t, in fact, that different from humans," said Gal. “The main difference is in complexity, but the building blocks are the same. We think if we understand insects’ internal representation of the external world, it may give us some clues about what happens in ‘higher’ organisms – like us.”
By the way, this story doesn't end prettily. After the wasp has found a nest – a hole in a tree or pile of rocks – it returns to the zombie cockroach and bites off its antennae. Hungry from all the work, the wasp sucks some of the cockroach’s sugary, protein-rich blood through the antennae stumps. Then, walking backwards, it guides the cockroach by the stumps to the nest. There, it lays a single egg on the cockroach and covers the entrance of the nest with rocks, mostly to keep out predators. When the egg hatches, the wasp larva spends a few days feeding on the cockroach’s blood, before building a cocoon inside its abdomen. The cockroach feels all this. A month later, an adult wasp climbs out of the cocoon, and the circle of life continues. It could almost make you cry.
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