Saturday, September 11, 2010

Mind-controlling parasites

Whether just intestinal worms or gut-tearing aliens, parasites are already a creepy thought.  The idea that something will use your body as a host to live and breed in is something that most people don’t like to think about.  And the fact that so many of these organisms are specifically targeting their host species and so are searching them out gives them an air of malice.  It turns out that matters could be far worse – there are many examples of parasites that will seize control of their host, attacking its brain and causing it to do things that it wouldn’t ordinarily do, in the interests of its parasitic master.
For example, there are many species of wasp that lay parasitic eggs, but one variety of Hymenoepimecis wasp works a little differently.  The wasp egg is attached to the back of an orb spider which carries on being a spider – for now.  At some point, the larva decides that it wants a cocoon, and it starts injecting chemicals into its host.  Now the spider builds a very different web and sits still in the middle.  The larva emerges, devours the spider and uses the web to finish building its cocoon.
There are many other examples of mind-controlling insects (another favourite is the flatworm Leucochloridium paradoxum), but I was surprised to learn about fungi that also take over their hosts brains.
David Attenborough’s voice is so calming as he talks about a fungus that controls a species of ant.
The Cordyceps genus of fungus are all parasites, but there are some species that go beyond just infecting their hosts and will modify their behaviour.  One attacks the bullet ant, infecting its brain and causing it to seek out a spot that suits the fungus.  The ant grabs hold of a twig to stabilise itself, and the fungus grows out of the ants head to spread spores over more ants in the nest.
It turns out that this adaptation is very old.  There is evidence from as far back as 48 million years ago of ants exhibiting this characteristic behaviour.
For some more mind-controlling parasites, take a look at these lists of examples.
Hmm, an “Invasion of the body snatchers” scenario immediately suggests itself.  Or perhaps a colony of insects commanded by a fungus that took control of their queen?

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