First Complete Map of a Male Fruit Fly's Central Nervous System Details 124 Million Connections

In 2024, researchers unveiled the complete wiring diagram of a female fruit fly’s brain, mapping roughly 140,000 neurons in what was then the largest connectome ever produced. Now, a massive international consortium has mapped the male equivalent—and crucially, they did not stop at the brain. By extending the map down through the insect’s ventral nerve cord, the equivalent of a human spinal cord, scientists have produced the first complete, synaptic-resolution diagram of an entire central nervous system, capturing 166,691 neurons and more than 124 million connections that link sensory input directly to motor action.[1][3][4]

To build the map, researchers at the Howard Hughes Medical Institute’s Janelia Research Campus and Google Research sliced a single male Drosophila melanogaster into thousands of microscopic sections. They imaged each slice using electron microscopy, generating millions of two-dimensional pictures. Artificial intelligence models, specifically flood-filling networks, then traced the boundaries of individual cells across the slices, stitching the flat images into a cohesive three-dimensional network.[5][6]

The resulting dataset details 11,691 distinct cell types across the insect's body. Because the map includes the ventral nerve cord, researchers can now trace a continuous path from the fly’s optic lobes—where visual information enters—straight through the central brain and down to the motor neurons that control its wings and legs. This allows neuroscientists to observe exactly how a visual stimulus translates into a physical evasion or approach.[1][2][3][5]

The inclusion of the male brain also allows the first direct structural comparison between sexes in an animal with complex social behaviors. "It is the first time we can compare both sexes of an animal with complex social behavior," said Gerry Rubin, a senior group leader at the Howard Hughes Medical Institute's Janelia Research Campus. When researchers overlaid the new male map against the 2024 female connectome, they found that the vast majority of the central nervous system is isomorphic, meaning it shares the exact same wiring.[3][4][5]

However, the mapping identified 262 male-specific cell types, 69 female-specific types, and 114 dimorphic types that exist in both but wire differently. Together, these sexually dimorphic elements comprise roughly 4.8 percent of the male central brain. These sex-specific differences are not distributed evenly. The sensory and motor peripheries are nearly identical, but the dimorphic neurons are heavily concentrated in higher brain centers.[3]

In the male brain, specific structural hotspots act as circuit switches, rerouting sensory information to drive distinct behaviors. These dimorphic pathways help explain how the same sensory input can trigger different responses depending on the sex of the fly, governing complex actions like courtship rituals or sex-specific aggression.[2][3]

While the structural map is complete, a connectome is only a static snapshot. It shows where the wires go, but not necessarily how much signal travels down them at any given moment, or how neuromodulators like dopamine dynamically alter those circuits in a living, behaving animal. Researchers are now working to integrate this anatomical data with functional recordings of brain activity to see the circuits in motion.[2][6]

The computational pipeline used to process the fly’s 124 million synapses is already being pointed at larger targets. The consortium is currently mapping the brains of larval zebrafish and adult Danionella fish, which will provide the first full connectomes of vertebrates. The techniques refined on the poppy-seed-sized brain of the fruit fly are now laying the groundwork for the decades-long effort to map the 86 billion neurons of the human brain.[1][2][5]