A new map of the adult male fruit fly brain and its nerve cord has been created, revealing more than 166,000 neurons and the millions of connections between them. The male fruit fly brain, about the size of a poppy seed, contains over 100,000 neurons, while the nerve cord adds additional neurons, bringing the total to more than 166,000. This new map joins a previously released map of the female fruit fly brain, published in 2024, which includes around 140,000 neurons. These detailed "connectomes," which are like wiring diagrams of the brain, allow scientists to compare male and female brains to explore how differences in brain structure might explain differences in behavior, such as mating rituals or aggressive actions. According to study co-author Gerry Rubin, who leads biology research at the Howard Hughes Medical Institute’s Janelia Research Campus, this is the first time researchers can compare the brains of both sexes in an animal with complex social behavior. “Male and female flies have many behavioral differences, and neuroscientists want to understand how the brain controls those behaviors,” Rubin said. “This now allows us to easily focus on the neurons responsible for these differences.” The map was initially shared as a preprint and later published in the journals Cell and Current Biology on September 3. Alongside the main study, three other papers used the new data to explore various aspects of fruit fly neurobiology. Carlos Ribeiro, a principal investigator at the Champalimaud Foundation in Lisbon, Portugal, and one of the researchers involved in the brain map, noted that the fruit fly nervous system performs highly complex tasks with relatively few neurons and minimal energy. He said the fly's brain structure could provide insights into creating more efficient artificial systems. Ribeiro also mentioned that this work provides a technical guide for future connectomics projects, such as mapping the brains of mice and eventually humans. In the near future, the team plans to map the brains of larval zebrafish and adult danionin fish. Long-term, the goal is to understand how vertebrate brains enable complex behaviors and use that knowledge to study human neurological and psychiatric disorders. One of the studies led by Ribeiro and his team focused on the circuits in the fly brain responsible for its sense of taste. Fruit flies have taste receptors in various parts of their body, including their legs, wings, mouthparts, and throat. Researchers identified these receptors and traced their connections to the brain. They then examined how these circuits interact with those governing behaviors like swallowing or walking. These circuits help the fly decide whether a food source is safe or harmful, ultimately determining whether to eat it. The other two studies published alongside the brain map explored visual processing in the fly brain and sex-specific differences in male and female brains. One study found that visual processing involves more than half of the 11,000 types of neurons identified in the map, indicating a deep integration of vision into the brain’s structure. The second study identified a network of cells unique to the male brain that coordinates male-specific behaviors, such as courtship and physical aggression. Female flies typically headbutt, while males tend to lunge. Although some networks were specific to each sex, the circuits responsible for sensation and movement were largely shared. Researchers are now investigating how specific switches in these circuits reroute signals differently in male and female brains, which could explain behavioral differences.