How do they engage in complex behavior with only 100,000 to a million neurons?:
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.
News, “How do insects use their very small brains to think clearly?” at Mind Matters News
Takehome: Researchers are finding that insects have a number of strategies for making the most of comparatively few neurons to enable complex behavior.
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.