Scientists have mapped an insect brain in greater detail than ever before

Charting a larval fruit fly’s nerve cells and their connections took 12 years

This diagram shows every nerve cell (multicolor) in a larval fruit fly brain, with the spheres representing the cell body and the elongated tails as branches that send and receive information. The connections, known as synapses, aren’t visible here.

Michael Winding

By McKenzie Prillaman

March 9, 2023 at 2:00 pm

The wiring of one insect’s brain no longer contains much uncharted territory.

All of the nerve cells — and virtually every connection between them — in a larval fruit fly brain have now been mapped, researchers report in the March 10 Science. It’s the most complex whole brain wiring diagram yet created.

Previously, just three organisms — a sea squirt and two types of worm — had their brain circuitry fully diagrammed to this resolution. But the brains of those creatures have only a few hundred neurons. The scientists who conducted the new study wanted to understand much more complicated brains.

Fruit flies (Drosophila melanogaster) share a wide range of behaviors with humans, including integrating sensory information and learning. Larvae perform nearly all the same actions as adult flies — except for some, like flying and mating — but have smaller brains, making data collection much faster (SN: 7/19/18).

The idea for this project came 12 years ago, says neuroscientist Marta Zlatic of the MRC Laboratory of Molecular Biology in Cambridge, England. At that time, she and her colleagues captured electron microscope images of the entire larval fruit fly brain. They then stitched those images together in a computer and manually traced each neuron to create a 3-D rendering of the cells. Finally, the team found the connections where information gets passed between the cells, and even determined the sending and receiving ends.

The researchers identified more than 3,000 neurons and about 550,000 connections, known as synapses.

This 3-D rendering of the larval fruit fly’s neurons was constructed from serial images captured with an electron microscope. Spheres represent each nerve cell body and the elongated tails are branches that send and receive information.

Neurons transmit information to one another in circuits. Exploring the neurons’ connectivity patterns — not just directly linked partners, but also the links of linked cells and so on — revealed 93 different types of neurons. The classes were consistent with preexisting groupings characterized by shape and function. And nearly 75 percent of the most well-connected neurons were tied to the brain’s learning center, indicating the importance of learning in animals.

The researchers hope that this work serves as a blueprint for fellow scientists studying brain circuitry. “Now we have a reference map,” Zlatic says.