A fruit fly's brain is smaller than a sesame seed. But it has about 140,000 nerve cells. In 2024, scientists finished a map of every nerve cell and every connection in one fly's brain. A computer program made the first draft.

Then researchers and volunteers around the world checked it online. With this map, scientists could even guess which cells make a fly start eating.

A map of all the connections between nerve cells in a brain is called a connectome. The first one, published in 1986, was for a tiny worm with only 302 neurons. In 2024, an international team called FlyWire completed the connectome of an adult fruit fly's brain. They sliced one fly's brain extremely thin, photographed it with an electron microscope, and traced 139,255 neurons and about 50 million synapses.

Because this was too much work for people alone, AI first outlined the cells, and then researchers and citizen scientists corrected its mistakes online. Using the map, another team built a computer model that correctly predicted which neurons are needed for a fly to start feeding.

A connectome is a complete wiring diagram showing how every neuron in a nervous system connects to every other. The first, published in 1986, described a nematode worm with 302 neurons. In 2024, the international FlyWire consortium reported the connectome of an entire adult fruit fly brain, smaller than a sesame seed yet containing 139,255 neurons linked by roughly 50 million synapses.

The team sliced the brain of a single female fly into ultra-thin sections, imaged them with an electron microscope, and used machine learning to outline each neuron automatically; researchers and citizen scientists then corrected the errors online. A companion study classified the neurons into 8,453 cell types, of which 4,581 were new. A third study built a computational model of the whole brain from the wiring diagram and correctly predicted which neurons are needed to initiate feeding, as confirmed in living flies.

Even so, there are limits. The map comes from one individual, and electron microscopy reveals structure but not the strength or timing of signals. Mapping a human brain, with about 86 billion neurons, would be vastly harder.