The great brain robbery:

EVER NOTICED A SLATER (pillbug, sowbug) wandering around during daylight hours? Chances are its behaviour has been altered by a parasite. Normally, slaters avoid light like children flee from injections.

Cleverly designed computer viruses mess around with the computer's 'brain', making it behave in abnormal yet predictable ways. Did you know that some small parasitic worms can do the same to their host? Their tinkering produces well-orchestrated, predictable conduct. How they do it remains largely a mystery to behavioural and molecular biologists.

The creatures that are responsible for such changes in behaviour are always the larval stage of various parasites that are parasitic throughout all their life stages. (As distinct from other parasites that are free-living as adults.) The vast majority of such parasites go through highly complex life cycles, with larval stages living in one kind of host, while the adult stages live in totally different creatures. Though the basic theme is similar, the variety of ways employed by the various stages of different parasites to find their way from one host to another stretches human imagination.

Such parasites change the behaviour of their intermediate host so that the probability of their gaining access to the final host is greatly increased. Thus, some species that live their final stage in starlings cause their intermediate host, such as pillbug, to wander around in broad daylight (instead of cowering in the dark, as self-respecting pillbugs all do) where they are much more likely to be seen and eaten by a starling.

The question of how one creature is able to change the normal behaviour of another creature and enslave it to a new mode of behaviour is fascinating. As with everything God does, there is variety in the method. In some instances, the behaviour is changed not by affecting the victim's instinct, but by, for instance, encysting in its muscles in such a way that the animal is no longer able to move about as normal. For example, the larvae of the nematode Tetrameres americana settle into the muscles of certain grasshoppers, making their victim less active. The grasshoppers are then far more likely to be eaten by their final hosts-chickens. Others change the behaviour of their intermediate host by invading its central nervous system.

Such amazing behaviour, involving extremely complex and complicated molecular interactions simply cannot be explained by standard evolutionary mechanisms.

Long version