There's an evolutionary arms race going on between a rare species of butterfly and ants:
Maculinea alcon butterflies infect the nests of Myrmica ants by hatching caterpillars nearby, hoping that the caterpillars will be ‘adopted’ and cared for by ants that mistake them for their own young. The caterpillars achieve this by mimicking the surface chemistry of the ants. Getting this chemistry right is important: if an ant doesn’t recognize a caterpillar as one of its own it will eat it, says David Nash, a zoologist at the University of Copenhagen in Denmark.
Successfully adopted caterpillars are bad for the ant colonies, as ants may neglect their own young in favour of the intruders. But the ants are fighting back. “The ant larvae seem to be evolving as a result of being parasitized,” says Nash. “It’s an ongoing evolutionary arms race.”
http://www.nature.com/news/2008/080103/full/news.2007.405.html (Photo: David Nash) - via 3quarksdaily