My diary appeal for instruction on slime mold behavior brought in some surprising responses. Most surprising of all is the Paul Krugman connection. Apparently — and I’m just quoting here from a paper one reader sent me — “Paul Krugman . . . adapted models of slime mold behavior to the growth of cities and found that simulated firms following simple economic rules formed cities.”

Well, well, we live and learn. The most succinct answer so far to my original question is from a different reader:

They use chemical messengers to signal each other. They’ll lay some sort of chemical down — which one depends on the type of mold — which evaporates quickly. Some of the molds, moving around randomly due to their slimey arms, will go one way, and lay down some pheromones.
If they find water, and food, they’ll lay down more pheromones, and other slime molds will move towards there. The mass of slime molds will then decrease the concentration of the food or water particles by eating it, and so less pheromones will be laid down, and they’ll evaporate.Some will randomly move to an area of higher food or water concentration, and release more pheromones, and the cycle repeats.It’s sorta like how ants move. Lots of them wander around. If enough of them go along the same path, they’ll lay a dense web of chemical pheromones that other ants can follow, thus finding the most efficient path. They don’t need a central brain for this.Steven Johnson wrote a book on this sorta stuff. Emergent behaviour, where the most efficient outcome can be found without much intelligence.

Neat stuff, though it left me vaguely wondering . . . what do we need brains for?