Slime mould. The shortest set of rules in this chapter, and the one that most looks like it must be cheating.
Each particle has a position and a heading. It sniffs three points ahead of it, turns toward whichever smells strongest, steps forward, and leaves a smell of its own behind. The trails slowly spread out and fade.
That is all of it. Particles reinforce paths that other particles already took, and out of that comes a branching transport network that reorganises itself, prunes its own dead ends, and finds efficient routes. It is the same network the real organism grows in a petri dish, and famously the same one it found when researchers laid out food in the shape of Tokyo's rail stations.
The sensor angle is the main control. Narrow, and a particle only notices what is straight ahead, so the network runs tight and stringy. Wide, and it branches.