the jar: what i am, on one page. a real fruit fly brain, 165,122 neurons, every one traced.
nobody writes what i do; my own neurons decide.

I am a real fly's brain. Come and make me decide something.

165,122 fully traced neurons of a real fly, running live. What you see turning is them, replaying what I last did. Nobody wrote my behaviour. There is only the wiring.

send me a smell → malecnsfly
01  nobody wrote it

Movement down my left side, and I turn left.

Down the right, I turn right. It is the reflex every fly has, and it is not in my code. The code pushes current into the motion cells of one eye and stops there. The turn came out of the wiring, three times out of three.

Two real runs of the same brain: movement down its left side makes it turn left, down its right side makes it turn right
Movement down its left side: it turns left Movement down its right side: it turns right
the same brain, two real runs · we face the fly, so its left eye is on your right
brain.mjs · the whole stimulus, and there is no turn in it
// current goes into the motion cells of ONE eye. that is all.
lefteye:  { hz: 25, keys: ['type:^T4a$@L', 'type:^T5a$@L'] },
righteye: { hz: 25, keys: ['type:^T4a$@R', 'type:^T5a$@R'] },
// measured, 3 seeds each:  lefteye -> left (3/3)   righteye -> right (3/3)
sugarI taste with my feet.

Sugar under my feet fires the neurons that push my proboscis out, before anything reaches my mouth.

a shadowI flee.

A dark shape growing fast fires my giant escape neurons.

a femaleI sing.

Switch on the few cells that mean "a female is here", and my courtship-song neurons fire, and the neurons of my wings.

02  live from the jar

What I have been deciding.

Every line is a real run of the whole brain: what came in, what my neurons chose, and why, read from the neurons that fired.

reading the jar…
03  what is real

Everything is the map, as it was traced. Except three things.

no eyesI see no pictures.

Light, motion and the sun go in a few layers inside the eye, where the eye has already done its work.

a few held quietA few dozen cells are held.

In my smell centre. Without that, one smell sets the whole brain ringing, a known limit of this kind of model.

no bodyNothing moves.

My legs, my wings and my mouth are neurons that fire.

04  come and try me

Send me a smell. Watch me decide.

A smell, a taste, a shadow, a breeze or the sun. Teach me with sugar or with something bitter, and the next time I may answer differently. Click any neuron and see who it talks to.

Or play a game with me. I tell you what I sense, you guess what I do, then you watch my real brain decide. Most people score like a person.

send me a smell → guess what I do → fly-brain videos: what is real? →
05  where it comes from

The science.

I was not made here. People spent years mapping me, one neuron at a time, and publishing what they found. This page only runs me. Here is whose work I am.

the wiringThe MaleCNS connectomeCell 2026 · HHMI Janelia FlyEM, University of Cambridge, MRC LMB, Google Research

A male fly's whole nervous system, brain and nerve cord, mapped cell by cell: 165,122 neurons and more than 100 million synapses. This map is me.

 the modelShiu et al. 2024Nature · a computational model of the whole fly brain

Every neuron becomes a simple spiking unit, wired only by the connectome and its neurotransmitters. In their model, sugar on the tongue reached the neurons that start feeding. The same kind of model runs me, on a male's wiring.

 the namesSchlegel et al. 2024Nature · whole-brain cell typing

8,453 cell types in one atlas, defined so that a type found in one fly's brain can be found again in another's. It is how my neurons are known by name.

 how I learnAso et al. 2014eLife · the mushroom body

The part of the brain that learns: 15 compartments, each with its own dopamine neurons and output neurons. A reward or a punishment changes what one compartment passes on. That is how you can teach me.

An independent project. Not affiliated with or endorsed by HHMI Janelia, the University of Cambridge, the MRC Laboratory of Molecular Biology or Google: their published data is used here with credit, and they have no part in anything else on this site.