The debate over sex differences picked up one small but inconvenient exhibit this month: a fruit fly with 166,700 mapped neurons.
Google Research, HHMI Janelia Research Campus, Cambridge researchers, and other collaborators finished the first complete map of an adult male fruit fly’s brain and central nervous system. The finished connectome includes 124.2 million proofread synaptic connections.
Google highlighted the work on X this week. Researchers used artificial intelligence to help combine millions of two-dimensional images into three-dimensional neural shapes.
For the first time, scientists have mapped the complete brain and central nervous system of an adult male fruit fly — a key model organism in science. 🪰
— News from Google (@NewsFromGoogle) September 3, 2026
Working alongside HHMI Janelia Research Campus and the scientific community, @GoogleResearch scientists and researchers used… pic.twitter.com/dpcXH4jmNS
Scientists already have female fruit fly connectomes. Now they have a male one to put next to them, connection by connection.
About 95 percent of the brain’s cells are shared between males and females. The remaining cells include neurons that are sex-specific or sexually dimorphic.
The Cell paper found 289 male-specific neuron types, 71 female-specific types, and 138 dimorphic types. Many of those differences were found in higher-order brain centers. The sensory and motor ends looked much more alike.
Then there is the behavior.
UK Research and Innovation (UKRI) points to male courtship and female egg laying among the behaviors tied to those differences in circuitry.
Cambridge researchers found the same sensory information can end up taking a different route in male and female flies. Their description is pretty plain: “sex-specific behavioural circuits” can produce different responses to the same stimulus.
Joint first author from UKRI, Dr. Isabella Beckett, said researchers can now start working backward from those differences.
“We now know from the connectome the extent to which sex-specific cells and different wiring distinguish the male from the female fly brain. These findings are a launch pad for future experiments to trace how genes ultimately shape the wiring differences that switch behaviour.”
The amount of work behind that sentence was substantial.
Researchers cut the male central nervous system into 66 pieces. Seven electron microscopes ran for more than a year. The Cell team estimates the proofreading work alone came to the equivalent of 44 person-years.
The final map contains 11,710 neuron types across the brain and nerve cord.
One male-specific pathway is already fairly easy to understand.
Researchers identified a neuron called LoVP92 in visual circuitry used during courtship. It helps process information when a male tracks another fly in front of him. Google Research called the pathway the fly’s “love spot.”
Nobody mapped a human brain here. This is a fruit fly.
But they did map an entire male nervous system, compare it with female brains, and find sex-linked differences in the neurons, the wiring, and what those circuits do.
For a fruit fly, biology got pretty detailed.