Google Research and Janelia map the male fruit fly nervous system with 166,000 neurons

What the researchers published

Google Research and HHMI's Janelia Research Campus published a full map of the male fruit fly's central nervous system on 3 September 2026. The connectome covers the brain and the ventral nerve cord, which the authors compare to a spinal cord. It contains more than 166,000 neurons and 125 million synaptic connections, which the post calls the largest brain map by number of neurons to date.

The main paper, "Sexual dimorphism in the complete connectome of the Drosophila male central nervous system", is in Cell, with three companion papers on vision, taste and social behaviour released the same day. Janelia led the project with the MRC Laboratory of Molecular Biology, the University of Cambridge and other groups. Research scientists Michał Januszewski and Viren Jain wrote the Google post. The data can be explored and downloaded through Neuroglancer, Google's open-source viewer, and from Janelia.

Where the AI comes in

Mapping starts by cutting a brain into very thin slices and imaging each one with an electron microscope. Google's flood-filling networks, convolutional neural networks that trace every pixel belonging to one neuron from a starting point, reconstruct the cells across slices. For this map Google improved its PATHFINDER reconstruction system by adding synthetic neurons to the training data, and used newer tools to label specific neuron types. Human experts at Janelia then proofread and verified the result.

The post puts the work in sequence: an automated reconstruction of the female fly brain in 2019 and a human-verified half-brain map with 25,000 neurons and 21 million connections in 2020. The new map complements the female brain map from FlyWire and a female brain and nerve cord map published in Nature.

Why it matters

With complete male and female maps, researchers can compare wiring between the sexes neuron by neuron, which is the subject of the main paper. The authors are clear on limits: the map is a static wiring diagram, verification still took years of human effort, and modelling the human brain's 86 billion neurons is out of reach. Other science results from Google's labs this autumn include AlphaGenome Atlas.

What to watch next

The next step the authors describe is cutting the human proofreading time with better AI tools, and applying the methods to larger brains; the post points to zebrafish work at Harvard and an elephantnose fish study at Columbia University. The post gives no imaging resolution, data volume or count of proofreading hours, so those details will have to come from the papers. More research coverage is under AI Research.

Questions people ask

How many neurons are in the male fruit fly connectome?

More than 166,000 neurons with 125 million synaptic connections, according to Google Research and HHMI Janelia.

Where was the fruit fly connectome published?

The main paper is in Cell, titled "Sexual dimorphism in the complete connectome of the Drosophila male central nervous system", with three companion papers.

Can I download the data?

Yes. The post says the map can be explored and downloaded through Neuroglancer and from HHMI Janelia's project page.

Sources

1. A connectomics milestone: Mapping the complete male fruit fly brain (Google Research, 3 Sep 2026)
2. Sexual dimorphism in the complete connectome of the Drosophila male central nervous system (Cell, 3 Sep 2026)