About

Somebody sliced a fruit fly’s brain into pieces thin enough to photograph in an electron microscope, traced every cell through the stack, and counted the connections between them. That map is called a connectome. This one is public and free to use, and it is the largest of its kind.

About five hundred of those cells are wired up and running on this page. They are the fly’s compass — the part that keeps track of which way it is facing — plus the part of its eye that tells colours apart, and a handful of cells that set how fast it walks. The connections between them are the real counts, and they are never edited.

This is not a fly and it is not thinking. It is a few hundred numbers going round in a loop.

Not everything here is measured. The wiring is. But a connection count is not a strength, so each kind of connection is given a number that turns one into the other, and those numbers were chosen rather than found: eleven for the compass, eight for the colour circuit. They were set by searching for the values that best matched results published from living flies, not by taste. Fifty more chosen numbers sit in the rest of the model and three in the paint. The four pigments are a choice too. All of them are listed, with where each came from, on the proof page, and the list is generated from the code rather than typed, so it cannot go stale.

The light is the real sky over London, clouds included: the sun’s bearing and height set the key light and every shadow, and cloud decides whether there is a shadow at all. What the weather does not decide is how bright the room looks. The studio is exposed to a fixed level, the way a gallery lights the work and leaves it, because a room lit by a real overcast London morning and rendered honestly is a black rectangle. The fly keeps its own hours: it works through the night under a lamp, and the sky reaches the work rather than switching it off. Nothing here decides what is good, and there is no language model anywhere in it.

The obvious suspicion is that any wiring would do this — that it is a random number generator in a costume. So the circuits were re-run on wiring that had been deliberately broken, three ways, a hundred times each. The compass forms and holds a heading in 100 of 100 runs on the real wiring, and in 0 of 300 on the broken ones. The colour circuit survives one of the three, and the proof page says exactly which and by how much. You can run it yourself: Shuffled under the studio rebuilds the same hour on rewired connections and names which break it used.

Where it’s shaky

Neurons, names, synapse counts, skeletons: Read from the connectome.
The width of the compass reading: Fitted, not discovered. Twelve numbers were tuned until the compass’s active patch came out about 90° wide, and it then measures 92.6°. What was not tuned for, and is therefore the finding: that one patch forms at all from the real connection counts, that it has a single peak, and that it stays put for the whole run.
Colour tuning: Partly reproduced. Of the four colour cells, one matches what was measured in living flies, two are close, and one is wrong — it prefers red where the real cell prefers blue. That one cannot work here: the cells that would push it the other way are not in the patch this model contains. Its selectivity comes out at 0.39 against a published 0.55.
The compass has favourite directions: Push the compass to each of sixteen headings and let go, and it slides back to one of only two. The real connection counts are not symmetric, and evening them out would mean editing them. So the compass cannot be turned by the fly’s own steering signal: a strong turn command moves it less than a degree in twenty-five seconds. What moves it is the sun.
One unit of the eye: Seven. A fly’s eye comes in two flavours of unit and neither carries all the cell types this needs — none of the 892 in the dataset does. So the model uses one unit and its six neighbours.
Which connections excite and which inhibit: Read from the dataset. Measured for most cells, predicted by the dataset’s own model for the rest. None of it is guessed from the literature.
Breaking the wiring on purpose: The two circuits break differently, and only one of them breaks. The compass forms a heading in 100 of 100 runs on the real wiring and in 0 of 300 on the broken ones. The most telling break keeps every connection exactly where it is and only flattens the counts to 1 each: the compass is gone. Turning the strength back up until the network is as excitable as the real one does not bring it back. So the counts matter, not just the shape of the graph. The colour circuit is tougher: shuffle the connections while keeping each cell’s type and its number of partners, and three of the four colour cells keep almost all of their tuning. What carries it is which types connect to which, not which individual cells do. Two limits travel with these numbers and are printed on the proof page: on one broken rung a heading forms in 44 of 100 runs, and the shuffle that keeps types cannot rewire more than 39% of the compass because it runs out of legal swaps.
Sky, weather, light, eye pigments: Physics and published measurements, not the connectome. One number in the eye’s sensitivity curves was chosen rather than measured.
The paint: The numbers are not the ones the model needs. Mixing paint properly needs two measurements per pigment — how much light it absorbs, and how much it scatters — and nobody has published both for these four. What is used instead is the colour of each pigment at full strength, turned into one number where two are needed. The five caveats that come with that are printed in full on the proof page.
When the fly walks, stops and flies: Which way it turns is the connectome’s: the steering command is banked and spent whole, so the total turn over a piece is exactly what the circuit asked for and nothing is invented. When it stops and for how long is not. Every signal in these circuits settles to one level and stays there — there is no slow rhythm in them to read bouts out of — so the walking and stopping is a published pattern applied as a rule, with its own seeded clock. It is listed as one on the proof page.
Which four pigments: The one purely aesthetic choice. Named on the proof page.
Ultraviolet: The fly sees it and you do not, so one of the four pigments stands in for it. That is a translation, not a measurement. Worse: the pigment data stops short of the ultraviolet for two of the four, and below that point the value is held flat — which is exactly where two of the fly’s five light sensors do most of their work.
The colour barely changes during a piece: Paint, look at the paint, paint again: the loop settles almost at once and a piece comes out close to one colour. It used to settle completely, and the reason was a modelling mistake rather than a parameter. The formula the fly’s eye used for the colour of wet paint is one that describes an infinitely thick layer, and it gives the same answer no matter how much paint there is — so the amount under the brush was invisible and only the ratio of the four pigments got through. The renderer had always used the finite-thickness version. The fly uses it now too, and the fly walks far enough to cross its own paint, so the colour does move. Over thirteen measured pieces across the day and the night it moves 1.26 to 1.37 in the units colour difference is measured in. A difference you could see is about 2.3. That is the measurement, printed rather than tuned away, and what is holding it down is which cells are in the model rather than how far the fly walks — two of the four colour outputs barely move at all, and the proof page names them.
Dopamine setting the walking speed: The direction of the effect is documented. How strong it is here was chosen.
The fly’s body and gait: Not a mesh. Nothing is downloaded and no third-party geometry is distributed: the fly is built from primitives, and what is borrowed is the flybody model’s proportions — flybody’s proportions, our triangles. The gait is procedural. Cosmetic. Not driven by motor neurons. Provenance and Apache-2.0 attribution in the body model notice.
The dealer’s eye: Receptive field is arbor position. The weakest link. Not yet built.
Familiarity rule on visual compartments: Documented for olfactory compartments, applied by extension. Not yet built.
A sale is sugar: True as a description of what the model receives. Not yet built.
Dealer to artist wire: A real path if one exists at length two or less; otherwise the one invented connection. Not yet built.
It’s a fly painting: It is a compass and a hue detector in two loops, wearing a body. This must stay the joke.

Credit

The connectome is the MaleCNS v1.0 dataset, a collaboration between FlyEM at HHMI Janelia, the University of Cambridge, the MRC Laboratory of Molecular Biology and Google Research, released under CC-BY 4.0.

Weather data by Open-Meteo.com.

The fly’s proportions are measured from flybody (Google DeepMind / HHMI Janelia Research Campus), used under the Apache License 2.0. No mesh of theirs is distributed by this site; the full attribution required by §4 of that licence, and the list of which proportions are measurements and which are free parameters, is in the body model notice.