An international research team has published the first complete wiring diagram of an adult male fruit fly's brain and central nervous system, linking the insect's sensory organs and brain to the nerve cord that controls its body.
The connectome contains 166,700 neurons, more than 125 million synaptic connections and 11,710 cell types. It covers the central brain, optic lobes and ventral nerve cord through an intact neck, giving researchers a continuous map from visual, auditory and olfactory inputs to motor outputs.
AI traced the cells, then people checked them
Researchers sliced the fly's central nervous system into extremely thin sections and captured millions of electron-microscope images. Computational tools and AI turned those flat images into three-dimensional neuron shapes, but the reconstruction still required extensive manual verification and annotation.
The MRC Laboratory of Molecular Biology says the proofreading represented the equivalent of 44 years of human labor. The work was led across HHMI Janelia, Google Research, the University of Cambridge, MRC LMB and other collaborating institutions.
A matched map for studying sex differences
Scientists already had complete maps of female fruit fly circuitry. Adding the male central nervous system allows synaptic-resolution comparisons across the sexes in an animal with well-studied courtship, aggression and other social behaviors.
The researchers found that most neurons are shared, while sex-specific and structurally different cells are concentrated in higher-order regions involved in behavioral control. That does not make the connectome a recording of thoughts or a complete model of behavior. It gives scientists a structural reference for experiments that test how particular circuits transform sensory information into action.
The dataset is publicly available to view and download through Neuroglancer. Its scale also marks how far connectomics has moved beyond the roughly 25,000-neuron fruit fly “hemibrain” released in 2020, while underscoring the distance between mapping a fly and reconstructing the roughly 86 billion neurons in a human brain.
