New map of a male fly central nervous system includes all 166,000 neurons — and enables direct comparisons to female fly brain

A fruit fly’s brain is roughly the size of a poppy seed — and yet that tiny package contains over 100,000 neurons. A new map charts every single neuron in the male fruit fly brain, as well as the insect’s equivalent of a spinal cord, totaling more than 166,000 neurons.

This new map joins a map of a female fruit fly brain that was unveiled in 2024 and covers about 140,000 neurons. These two wiring diagrams, also called “connectomes,” can now be compared to see if there are differences between the sexes’ brains that help to explain behavioral differences reflected during mating or in aggressive actions, including sex-specific fighting moves.

“It is the first time we can compare both sexes of an animal with complex social behavior,” study co-author Gerry Rubin, head of biology and a senior group leader of the Howard Hughes Medical Institute’s Janelia Research Campus, said in a statement. “Male and female flies have a lot of differences in their behavior, and neuroscientists want to understand how the brain controls those behaviors. This now allows us to easily home in on the neurons that are causing those differences.”

Initially released as a preprint, the new fly brain map was published in the journal Cell and Current Biology Thursday (Sept. 3). The study describing the map was published alongside three other papers, each of which uses the new data to explore a specific aspect of fruit fly neurobiology.

“The fly nervous system performs remarkably sophisticated computations with relatively few neurons and little energy, and its architecture could suggest principles for designing more efficient artificial systems,” said Carlos Ribeiro, a principal investigator at the Champalimaud Foundation in Lisbon, Portugal, whose team contributed to the brain map and led one of the related studies.

“This work also provides a technical roadmap for more ambitious connectomics projects in the future, such as those for mice and humans,” Ribeiro said in a separate statement from the Champalimaud Foundation.

In the near term, the scientists aim to map the brains of larval zebrafish (Danio rerio) and adult danionin fish (Danionella). In the long run, the goal of this research is to understand how vertebrates’ brains enable complex behaviors, and then use that knowledge to help unravel the basis of neurological and psychiatric disorders in humans.

The study led by Ribeiro and other Champalimaud Foundation scientists aimed to map the circuits in the fly brain dedicated to the insect’s sense of taste. Fruit flies have taste receptors in many body parts, including their legs, wings, mouthparts and inside of the throat. The researchers identified these taste receptors and traced their connections back to the fly brain. From there, they examined how those circuits interacted with those that govern behaviors such as swallowing or walking. When it comes to eating behaviors, this circuitry helps the fly determine whether a given morsel is safe or harmful and ultimately choose whether to eat it, the researchers concluded.