Dellie
The Lab
Episode 2 of 10 · 13–14 Sep 2026 · 4 min read
Giving the brain a body
The test said “passed”. The tongue was curling the wrong way.
Simulation onlyNo hardware in this episode.
A body in a physics game
A brain on its own only makes numbers. It needs something to move.
So the fly got a body: NeuroMechFly, an open, anatomically accurate model of a fruit fly with 42 moving joints. It lives inside MuJoCo, a physics simulator. Think of a very accurate video game, with gravity, friction and legs that can slip.
Brain and body take turns every 15 thousandths of a second. The body reports what it feels, the brain answers, the body moves, and round again. That back-and-forth is called a closed loop.
The bug that passed its test
The first closed loop ran on 13 September at 14:38. A foot touched sugar. About 50 thousandths of a second later, the brain’s tongue neuron started firing, all on its own, from the wiring. The tongue moved.
Up. It folded upwards instead of reaching down to the food: the first clip above.
And the automated test said everything was fine. It checked the numbers for each joint, not where the tip of the tongue actually ended up. Thirteen minutes later, version 2 reached the ground.
Look at the picture, not just the numbers.
Following a smell
Next came smell. The first try used the brain’s own turning cells to steer, and the fly veered away from the food.
What worked was one small, hand-written last step: turn toward the side that smells stronger. By that night he was a full fly, with smell, feeding and his nerve cord moving the legs.
With one hand-written rule, he finds the sugar every time
Runs in which he reached the sugar
Why is my gaming PC slower than my laptop?
On 14 September my gaming PC joined in. It has an RTX 5090, one of the most powerful graphics cards you can buy. And it ran the fly more slowly than my Mac.
“How come my PC with an RTX 5090 is slower than my Mac?”
The brain simulator wasn’t using the graphics card at all. So the team wrote a new brain engine for it.
Now one second of fly life takes about a quarter of a second to compute, and it gives exactly the same answer as the original, spike for spike: 53,385 of them, compared one by one.
He lives in a web page
The same day, the fly moved into a web page. You click to drop sugar, and watch him walk over and eat, with his brain lighting up in a little window beside him.