Long-time readers know how much I love parasitic wasps--those nifty insects that lay eggs on some other creature. The egg hatches, then eats its host alive. I wrote about some cockroach-killing parasitic wasps in this article. If memory serves, I also wrote about the spider-killing kinds in The Red Hourglass.
Another kind of parasitic wasp specializes in aphids. Scientists have recently made fascinating new discoveries about these. The aphids can develop an immunity to the wasps with the help of a virus. The virus doesn't parasitize the aphid itself, but the bacteria that live in its gut. The scientists haven't figured out all the mechanics of this situation yet, but it's a startling development. It means an animal can develop an advantageous trait simply by carrying the right virus.