See the First Complete Map of an Insect’s Brain

Over 12 years, scientists charted more than 3,000 neurons and the nearly 550,000 connections between them in a larval fruit fly

A fruit fly sitting on a grape
A fruit fly sits on a grape. Adult fruit flies are typically only a few millimeters long. FREDRIK VON ERICHSEN / DPA / AFP via Getty Images

A fruit fly larva is only a fraction of an inch long, and its brain is the size of a grain of salt. But that didn’t stop scientists from trying to get a complete picture of what’s inside the tiny organ.

Now, researchers have constructed a detailed map of the neurons and the connections between them in the brain of a larval fruit fly. With 3,016 neurons and 548,000 connections, called synapses, the result is by far the most complex map of a whole brain ever made.

“It is a tour de force of how we understand the ways in which brains are connected,” Timothy Mosca, a neuroscientist who studies fruit fly synapses at Thomas Jefferson University and did not contribute to the research, tells Gizmodo’s Lauren Leffer.

colorful spheres in the shape of a brain's two hemispheres with several multicolored strings coming from the bottom
The map of neurons in a larval fruit fly's brain. Johns Hopkins University / University of Cambridge

The new diagram is “a reference map,” that could help researchers better understand how the brains of animals work, Marta Zlatic, a neuroscientist at the University of Cambridge in England and a co-author of the study, tells Science News McKenzie Prillaman.

Until now, scientists had only made this sort of map, called a connectome, for the brains of three organisms: two types of worm and a sea squirt larva. These animals’ brains only had a few hundred neurons each. Researchers have also mapped 25,000 neurons and 20 million synapses in the brain of an adult fruit fly, but this is still just a partial connectome, per Live Science’s Nicoletta Lanese.

The connectome of the larval fly, published Friday in the journal Science, took 12 years to complete. Imaging a single neuron required about a day, according to a statement from Johns Hopkins University, where some of the researchers work.

Still, the team managed to use an electron microscope to image thousands of slices of the insect’s tiny brain. Then, they identified the neurons in each image and traced their connections to other nerve cells, writes Live Science. They were also able to sort the neurons into 93 types.

Complete Set of Neurons in an Insect Brain

The resulting diagram yielded a number of interesting findings. Almost 75 percent of the most well-connected neurons were linked to the brain’s learning center, underscoring its importance. And while the brain had multiple layers with connections between them, it also had shortcuts that skipped layers, perhaps to make up for the low number of neurons, per Nature News Miryam Naddaf. The two hemispheres of the brain were heavily connected, and they appeared to be similar—a surprise for the researchers, since the left and right hemispheres in humans have some specialized functions.

Scientists typically think of neurons as sending signals via a long structure called an axon and receiving signals in their shorter dendrites. But the study revealed that a third of the connections in the fruit fly’s brain did not follow this pattern—they were between two axons, between two dendrites or from a dendrite to an axon. “Given the breadth of these connections, they must be important for brain computation,” Michael Winding, a co-author of the study and a neuroscientist at the University of Cambridge, tells Gizmodo.

Human brains have an estimated 86 billion neurons and hundreds of trillions of synapses—vastly more neural connections than the number of stars in the Milky Way. But fruit fly brains and human brains still have similarities. For example, both have regions that correspond to decision making, learning and navigation, Joshua Vogelstein, a co-author of the study and a biomedical engineer at Johns Hopkins University, tells NPR’s Jon Hamilton.

The researchers now hope to use the map to better understand the structures involved in decision making and learning, according to the BBC’s Jason Goodyer.

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