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." This animation shows neurons that are specific to the male connectome, as well as neurons that are present in both males and females but differ between them.

The male connectome will allow researchers to compare male and female fly brains and understand complex social behaviors — like mating and aggression — that can vary by sex. (Image credit: Data acquired and analyzed by the FlyEM Project Team at HHMI’s Janelia Research Campus, the Cambridge Connectomics Group, and Google Research.

Video by Philip Hubbard/HHMI Janelia Research Campus) 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 animation shows all the types of neurons in the central nervous system of the male fruit fly.

The newly completed connectome is the first map of an entire male fruit fly central nervous system, comprising all the neurons in the brain, both optic lobes and the ventral nerve cord.