[Imaginos]

There are quite a few parasites that control the behaviour of their hosts. One makes its ant host crawl to the top of grass stems so that it can be more easliy eaten by (I think) cows-the host for the next stage of its life cycle. My personal favorite is a species of barnacle that parasitises crabs (noticable by a fleshy growth on the underside of the crab) that essentially 'drives' the crab around to areas where the barnacle can feed.