The Loom

Imagining My Homicidal Liver

i-23266f87a5162e780e510c909418bc21-copidosoma.jpgParasitoid wasps (or rather, one group of them called the Ichneumonidae) are the subject of one of Charles Darwin’s most famous quotations: “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.”

Scientists have learned a lot more about parasitoid wasps since Darwin wrote about them in 1860, and their elegant viciousness is now even more staggering to behold. Not only do they devour their hosts alive from the inside out, but they also manipulate the behavior of their hosts to serve their own needs (see my post on zombie cockroaches for one particularly startling example).

To be fair, though, parasitoid wasps are not just vicious to their hosts. They can be just as nasty to other parasitoid wasps. Some wasp larvae can only mature inside other parasitoids, turning their host into a grotesque Russian doll. And, as I write in tomorrow’s New York Times, some wasps turn their caterpillar host into a battlefield, waging all-out war with other wasps. They kill other species of wasps, and will even kill their own siblings by the thousands. (Be sure to see the diagram of the sci-fi life cycle of the wasp Copidosoma floridanum. By the end of it, the caterpillar is a mummified mass of pupae.)

These creatures are certainly bizarre, but bizarre in an scientifically interesting way. Scientists have found that the evolutionary forces that shape other animals can also explain these wasps. As I explain in the article, the warfare among the wasps probably arises thanks to the peculiar way they develop. A single egg (like the one being laid inside a host egg in the picture) gives rise to thousands of genetically identical siblings. Up to a quarter of them become vicious soldiers, while the rest become passive feeders. The soldiers are sterile, lacking any sex cells. In a way, they’re not even really individuals. In a genetic sense, they’re like disembodied organs. Imagine you could send your liver off to kill your enemies.

The soldiers benefit their siblings by killing–either killing competitors or even killing other siblings. Evolutionary trade-offs between conflict and cooperation are also at work in the families of other species, from cooperative honeybee hives to cub-killing packs of lions to humans. What would Darwin make of these wasps? I’ll leave that for more theologically-minded folks to speculate.

[For those interested in all things Web 2.0-ish, I’m pleased to say that this is the first article I’ve written for the Times that includes a couple links to relevant scientific papers. It’s downright bloggy.]

[For more fun with parasitoid wasps, you can also check out my book, Parasite Rex.]


  1. #1 RPM
    August 14, 2007

    [For those interested in all things Web 2.0-ish, I’m pleased to say that this is the first article I’ve written for the Times that includes a couple links to relevant scientific papers. It’s downright bloggy.]

    It’s too bad one of those links is not formatted correctly. The link to the article from the Journal of Evolutionary Biology should be this. Otherwise, cool stuff. It would also be helpful if the cited papers were listed at the end of the article, for easy perusing.

  2. #2 George
    August 14, 2007

    Not specific to this post, but the page does not display properly. It is quite messed up with the posts far down the page at the bottom?

  3. #3 lylebot
    August 14, 2007

    Nice article. But… a web page that contains links to other web pages is now considered “Web 2.0-ish”? Isn’t that what the Web was about from the beginning? Seems to me the NY Times is finally catching up to Web 1.0…

  4. #4 Jim Lemire
    August 15, 2007

    what a coincidence – I recently posted something about a personal experience with parasitoids in my own vegetable garden. Even when gruesome, the wonders of biology are incredibly cool.

  5. #5 Marco
    August 16, 2007

    Carl, will you please, PLEASE, put all the binomial terms in italic, as should always be done. 😎
    Just kidding. I really enjoy your pieces, and I deeply envy you. In my magazine, I can write just of furry and cuddly animals. When I got the chance of writing something on inverts, they MUST be beautiful (caterpillar and the like). All the other ones are simply ugly and not worth bothering with.

  6. #6 Andrew Derksen
    August 17, 2007

    For the truly curious, and for those who happen to be in town, Dr. Strand is currently scheduled to give a talk entitled “Polydnaviruses: symbionts and potent immunosuppressive pathogens of insects.” at the University of Florida‘s department of Entomology and Nematology in Gainesville on October 18th at 3pm.

  7. #7 bob
    August 30, 2007

    Sorry, but I can’t help myself:

    Parasite Flies should be Parasitoid Hymenoptera, flies are a different order.

    The wasps you refer to are not in Ichneumonidae. Ichneumonidae have other cool evolutionary stories, like the viruses that inject their host with to surpress immune responses.

    The picture and Copidosoma floridanum are Encyrtidae, which are Chalcidoidea wasps.

    The cockroach parasitoids are Ampulicidae, which a more closely related to thread-waisted wasps and bees than to the group normal considered to be the parasitoid hymenoptera (i.e. mircohymenoptera)

  8. #8 Carl Zimmer
    August 30, 2007

    Bob [7]: What are you referring to when you say “Parasite Flies”? If you’re referring to the category header for this post, please look again: it’s Files, not Flies.

    I’m aware that Ichneumonidae are only one group of parasitoid wasps, which is why I used the term “one group of them.”

    Thank you for the other taxonomic information.

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