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How Similar Are Insect Brains to Human Brains?

A fly's brain can tell us a lot about insect intelligence and maybe a thing or two about our own.

By Avery Hurt
Apr 17, 2023 1:00 PM
Fruit Flies on a Lime


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During the early days of the COVID-19 pandemic, while the rest of us were binge-watching TV shows and baking banana bread, a couple of researchers at Johns Hopkins University took their minds off the healthcare crisis by counting the brain cells of fruit flies and mosquitoes. 

The researchers, Joshua Raji and Christopher Potter, discovered that the two animals had around 200,000 brain cells on average, mostly neurons, with about 10 to 15 percent non-neuronal cells, such as glial cells. (Glial cells are the brain's connective tissue, providing a supportive network for neurons, though they have some important metabolic functions as well).

Compared to larger animals, that's not a lot of neurons. Humans have approximately 86 billion neurons, and mice have about 12 billion. But don't be too quick to write off these tiny critters as dunderheads. Fruit flies feel pain, have working memory, make predictions about the future and even pause to think before acting (a skill not all humans have managed to master). 

Read More: Do Insects Have Feelings and Consciousness?

The Fly Brain

In 2023, an international team published research that mapped out the complete "connectome" (a brain-wide synaptic wiring diagram, or what we might call a connectivity map) of a fruit fly larva. 

According to the study, they chose this particular animal because it's small, but has a "relatively complex brain" and a "rich behavioral repertoire." They found that the way the fruit fly larva's neurons connected with each other was quite complex. Rather than just sending messages from the axon of one neuron to the dendrite of another neuron — which is the typical model of neuronal activity — nearly half the fly's neuronal connectivity involved sending messages axon to axon, dendrite to dendrite, or dendrite to axon.

The researchers also found that the fly's neurons did not specialize, as had been previously assumed. In fact, most neurons were multitaskers, responding to different sensory cues. When the researchers looked not just at how many neurons there were but at how these neurons interacted, the insect's brain looked far more complex than its size would indicate. Fruit fly brains may be tiny, but they're efficient. The researchers hope that this understanding of the connectome will help shed light on the wiring of the brains of humans, which share many genes with fruit flies.

Evolutionary Connection

Frank Hirth is an expert in evolutionary neuroscience at King's College London. For decades, he and his colleagues have been studying how neural circuits form and how they function in vertebrates and in insects. 

"Flies, crabs, mice, humans: all experience hunger, need sleep and have a preference for a comfortable temperature, so we speculated there must be a similar mechanism regulating these behaviors," said Hirth in a 2013 article about his work. "We were amazed to find just how deep the similarities go, despite the differences in size and appearance of these species and their brains."

Hirth's ongoing research has indeed supported the view that though the brains of insects and humans are structurally very different, they share many of the genetic mechanisms involved in the brain areas that control behavior. These surprisingly deep similarities are, he says, likely due to a common ancestor, one that may have lived as long as half a billion years ago.

Are Insects Conscious?

Does this mean that insects are conscious? That's another question, and one that's difficult to answer, even for animals we know far more about than we know about insects. But it's a question that scientists and philosophers are already grappling with. What we're learning about the brains of fruit flies suggests that insects are a lot more like we are than we once thought, and that could well include consciousness.

Read More: How Intelligence Is Measured In The Animal Kingdom

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