"Two small and rather unappealing animals possess patterns of behaviour that have great relevance for the student of intelligent systems. These are the wood louse and the maggot of the common housefly, and it is the difference in their behaviour which is so illuminating. It has to do with the way in which they orient themselves to their environment. Wood lice like moist places and succeed in aggregating there by the simple device of slowing down their otherwise random movements as the humidity increases. The maggot, which, during a certain stage of its development needs to come out of the dark, finds light by a slightly more sophisticated system. It has a single, non-directional light-sensing organ at the forward end of its body and as it moves along it swings this end left and right, allowing the amount of light gathered during each swing to determine the extent of its forward motion. In this way it keeps altering its course until the amounts of light sensed are equal for both sides, by which time it must be heading straight for the light."