Simple repetitive behavior is common, as in circadian and brain rhythms, as well as peristalsis and walking. (In a millipede there are, for example, typically just two modes of locomotion, both simple, involving opposite legs moving either together or oppositely.) Many structures built by animals have repetitive forms, as in beehives and spider webs; the more complex structures made for example by termites can perhaps be understood in terms of generalized aggregation systems (see page 978). (Typical models involve the notion of stigmergy: that elements are added at a particular point based only on features immediately around them; see also page 1184.) Nested patterns may occur in flocks of birds such as geese. Fairly regular nested space-filling curves are sometimes seen in the eating paths of caterpillars. Apparent randomness is common in physiological processes such as twitchings of muscles and microscopic eye motions, as well as in random walks executed during foraging. My suspicion is that just as there appear to be small collections of cells—so-called central pattern generators—that generate repetitive behavior, so also there will turn out to be small collections of cells that generate intrinsically random behavior.