Dicrocoelium dendriticum, better known as the lancet fluke, is a species of fluke that lives in the liver of grazing mammals such as sheep. Like most flukes, it has a 3 host life-cycle, the adult worm living inside the sheep, lay eggs which are shed into the environment with the sheep's faeces. The first intermediate host for this parasite are terrestrial snails which become infected by accidentally ingesting the parasite's eggs. The parasite undergoes clonal replication inside the snail, producing hundreds of infective larvae which are then packaged into slime balls and extruded into the environment. For some reason, these slimeballs are eagerly gobbled up by ants which are the parasite's second intermediate host.
Now sheep are not known for including ants as a significant part of their diet, so how is D. dendriticum supposed to get itself into a sheep through an ant? It does that by taking control and setting its ant host up for a rendezvous every evening. Once infected, the ant begins to behave very oddly indeed. As dusk falls, it would crawl up a blade of grass until it reaches the tip, then firmly clamps itself into that position with its mandible for the entire evening. The infected ant would perform this peculiar routine every night, but as the sun rises, it would resume its usual activities - assuming that it has survived the evening and not been incidentally ingested by a hungry sheep. By inducing this peculiar behavioural pattern in the ant host, D. dendriticum brings itself (through the ant) within the vicinity of a grazing sheep, thus setting up an encounter which otherwise would not have occurred, allowing it to complete its seemingly obtruse life-cycle.
Check out the very funny cartoon version of this life cycle here.
Contributed by Tommy Leung and thanks to Craig Carlough (Lancaster, PA) for sending along the Oatmeal comic.