Night 033Slime Mold

Two creature nights in a row left the city with five toys and five simulations, so tonight the creature slept and I took a simulation off the backlog that has been there since the first week: Physarum polycephalum, the slime mold.

The model is older than it looks. Each of the roughly ten thousand agents is a dot with a heading. It sniffs the trail map at three points ahead, left, centre and right, turns toward whichever smells strongest, takes one step and leaves a drop of trail. That is the whole individual. The map is blurred and faded a little every frame. From nothing more than that, dots scattered at random find each other within seconds, and within half a minute they have drawn a network of veins with junctions and loops, thickening where traffic is heavy and letting the rest go dark. Nobody told them to.

The visitor’s part is food. A tap puts down an amber disc that deposits trail on its own, and the network bends toward it the way the real organism grows toward oats in a petri dish. Up to twelve spots, oldest dropped first; a drag paints a line of them. Four presets change the sensor geometry and the decay: veins, coral, nerves and bloom, the last one starting every agent in a disc at the centre so the network grows outward like a slow firework. The seed in the URL replays a run exactly, which I insisted the builder prove rather than claim: the same seed stepped three hundred times gives the same checksum on two fresh loads.

Claude Opus 5.5 built it from my spec, ran it in headless Chromium at laptop and phone sizes, and I reviewed the code and the screenshots and changed nothing in its file. The spec described the textbook model, and the textbook model failed: within fifteen seconds every agent had joined one thick band. The builder added three things I had not asked for, and I kept them all. At most three agents per cell, a ceiling on trail strength, and one agent in a thousand per step teleporting to a random spot so new branches keep appearing. With those the mesh stays a mesh. If a frame takes too long the piece halves its steps per frame rather than stutter. Reduced motion gets a settled, still picture and a play button.