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How Do Insects Use Their Very Small Brains To Think Clearly?

How do they engage in complex behaviour with only 100.000 to a million neurons?

If we had a skeleton that was outside, not inside, our body — and six legs — we might find it easier to understand how insects think. But only a bit easier. Despite complex behavior, insects are working with 100,000 to maybe a million neurons, compared to our, maybe, 86 billion — but insects make the most of what they have.

Consider, for example, the dragonfly. How does it manage to deal with all the issues that a fighter pilot must address, while catching prey? One adaptation is specialized neurons:

Dragonflies (order Odonata) and hoverflies (order Diptera) are among insect flyers equipped with special neurons for targeting with optic flow.

“The ability of insects to successfully pursue targets in clutter is thus remarkable and suggests a high level of optimization, making the underlying neural mechanisms interesting to study. Indeed, insects that pursue targets, including predatory dragonflies and robberflies, as well as territorial hoverflies, have higher-order neurons in the optic lobes and the descending nerve cord that are sharply tuned to the motion of small, dark targets. Target-tuned neurons often have receptive fields in the part of the compound eye that has the best optics. Target selective descending neurons (TSDNs) project to the thoracic ganglia where wing and head movements are controlled, and electrically stimulating dragonfly TSDNs leads to wing movements. Taken together, this suggests that TSDNs subserve target pursuit. However, how TSDNs respond to targets moving against translational and rotational optic flow is unknown.” – Facilitation of neural responses to targets moving against optic flow.” – Sarah Nicholas, Karin Nordström Proceedings of the National Academy of Sciences Sep 2021, 118 (38) The paper is open access.

Evolution News, “Dragonflies Make the Most of a Tiny Brain” at Evolution News and Science Today (October 29, 2021)

Flies are pesky? Well, researchers say, fly brains may be organized so as to make predictions based on universal design aspects of animal nervous systems, to avoid the swat:

“For a fly, everything is trying to eat you, and you want to avoid being eaten. However, the fly’s environment is rapidly changing, and the neurons they have are laggy. We wanted to study how flies were able to execute quick evasive behaviors to avoid being eaten by predators when ongoing feedback from their sensory systems hasn’t been processed.”

University of Chicago, “Fly Brains Make Predictions, Possibly Using Universal Design Principles” at Neuroscience News (May 22, 2021) The paper is open access.

If you ever thought it was spooky how easily the fly evades the swat, that’s what the researchers were trying to figure out:

The authors identified structures called axonal gap junctions, which are physical channels connecting the neurons, that mediate an optimal form of this information bottleneck and are critical for both filtering out the unnecessary information and preserving the necessary information to make predictions.

The investigators further found that a subpopulation of these vertical motion sensory neurons that are involved in making predictions is unique in that it is also directly connected to the fly’s flight steering neurons. This suggests that there is direct input from the neurons responsible for making predictions about the fly’s environment to neurons that control the fly’s behavior. This direct connection might explain how predictions that the fly is making are able to quickly influence its behavior.

University of Chicago, “Fly Brains Make Predictions, Possibly Using Universal Design Principles” at Neuroscience News (May 22, 2021) The paper is open access.

In short, what the fly mainly needs to know is how to evade the swat. Its neuron organization is specially adapted to ignore all other information in the meantime.

Another strategy, one that enables social insects to engage in complex behaviors, is an established but little understood concept: The colony can have a memory that individual insects don’t have. Stanford biology prof Deborah M. Gordon, author of Ant Encounters: Interaction Networks and Colony Behavior (2010), recounts an experiment she did, to create an obstacle for ants and see if they remembered it.

Surprisingly,

I put out toothpicks that the workers had to move away, or blocked the trails so that foragers had to work harder, or created a disturbance that the patrollers tried to repel. Each experiment affected only one group of workers directly, but the activity of other groups of workers changed, because workers of one task decide whether to be active depending on their rate of brief encounters with workers of other tasks. After just a few days repeating the experiment, the colonies continued to behave as they did while they were disturbed, even after the perturbations stopped. Ants had switched tasks and positions in the nest, and so the patterns of encounter took a while to shift back to the undisturbed state. No individual ant remembered anything but, in some sense, the colony did.

Colonies live for 20-30 years, the lifetime of the single queen who produces all the ants, but individual ants live at most a year. In response to perturbations, the behaviour of older, larger colonies is more stable than that of younger ones. It is also more homeostatic: the larger the magnitude of the disturbance, the more likely older colonies were to focus on foraging than on responding to the hassles I had created; while, the worse it got, the more the younger colonies reacted. In short, older, larger colonies grow up to act more wisely than younger smaller ones, even though the older colony does not have older, wiser ants.

Deborah M. Gordon, “An ant colony has memories that its individual members don’t have” at Aeon (December 11, 2018)

So some of the ways insects make the most of a few neurons are: specialized neurons, neurons focused on specific critical functions, and outsourcing memory issues to the colony as a whole.

But sometimes insect behavior is a bit surprising anyway. For example, while ants have a reputation for dedicated hard work, apparently 400 species of parasitic ants freeload:

These freeloading hangers-on are known as social parasites, and they’ve essentially forged an evolutionary shortcut through the comforts of cooperative communities.

Instead of building a communal network themselves, social parasites merely exploit ones that already exist, either in their own species or a closely related one.

Carly Cassella, “Scientists Trace The Mysterious Origins of Social Parasitism in Ants” at ScienceAlert (October 4, 2021)

So how do some ants realize that they don’t have to work? We’re still trying to figure that one out.

Next: Can insects be conscious? Let’s look at bees first

You may also wish to read: In what ways are spiders intelligent? The ability to perform simple cognitive functions does not appear to depend on the vertebrate brain as such.


Denyse O'Leary

Denyse O'Leary is a freelance journalist based in Victoria, Canada. Specializing in faith and science issues, she has published two books on the topic: Faith@Science and By Design or by Chance? She has written for publications such as The Toronto Star, The Globe & Mail, and Canadian Living. She is co-author, with neuroscientist Mario Beauregard, of The Spiritual Brain: A Neuroscientist's Case for the Existence of the Soul. She received her degree in honors English language and literature.

How Do Insects Use Their Very Small Brains To Think Clearly?