Slime

The plasmodial slime mold Physarum polycephalum builds remarkably efficient transport networks with no nervous system at all. The classic agent-based abstraction gives each cell three rules: drop pheromone where you stand, sniff the patches ahead, and turn toward the strongest scent.

Here every mold deposits chemical on its patch, the field diffuses to neighbors and evaporates each tick, and the gradient-following built-in samples ahead-left, ahead, and ahead-right to steer each crawler uphill. From a uniform random scatter, positive feedback fuses wandering paths into branching trails that thicken, compete, and slowly rewire.