Insects show a remarkable ability to learn and retain memories. Honeybees not only remember directions from fellow workers to find suitable foraging grounds, but can also sniff out landmines when trained using classical conditioning (like Pavlov’s dogs). Wasps and other hymenopterans (bees, wasps, and ants) have shown the ability to learn how to detect drugs, fungal infections, and even cancer. But these are all examples of insects learning in adulthood. What happens to the memories an insect makes in its larval form? Do insects that go through complete metamorphosis – like flies, bees, ants, wasps, butterflies, and beetles – retain memories of their so-called childhood?
It’s widely known that holometabolous insects (those that go through complete metamorphosis with four distinct life stages – egg, larva, pupa, and adult) liquefy during their pupal stage, only to emerge later as (solid) adults. However, the amorphous blob within the pupal chamber does not spontaneously reform into an adult; the goo is the remnants of digested larval tissue cells, and once the larval cells break down, they cannot be reconstructed. The adult structures form from minuscule sacks of cells (roughly 10-50 cells in size) called imaginal discs.
Imaginal discs contain the building blocks of the adult insect form: wings, legs, antennae, etc. They form during embryonic development and remain dormant during the larval stage. During pupal development, they survive larval tissue degeneration and begin to rapidly divide and form the adult structures. Each imaginal disc is responsible for forming a different adult structure, and within a few days or weeks, the adult emerges, wings and all.
By all accounts, memories and associative behaviors should not persist after metamorphosis. In 2023, an extensive study following the brain activity of fruit flies during metamorphosis showed that, while some neurons were retained, all neural pathways were broken and reformed, essentially giving the flies brand-new brains. Even simple associative memories (those formed through classical conditioning) could not survive a total rewiring. However, previous studies indicate that insects with more complex nervous systems, like bees and butterflies, do retain associative memories after metamorphosis. In a 2008 study, tobacco hornworm moths avoided negative stimuli they were exposed to as larvae.
Additionally, butterflies and other insects will lay eggs on plants they matured on as larvae despite no longer utilizing those plants as food sources. “[O]ne wonders how these two types of observations relate,” says James Truman, professor emeritus at the University of Washington and principal investigator in the 2023 study. He suggests that these neural shuffles could look different in other insect species and other parts of the insect brain. “The challenge is in trying to find out the nature and extent of these effects.”
There is still plenty of work to be done in regards to the insect brain, how it forms memories, and how those memories persist despite goo-ification. But as scientists try to untangle the complex web of neurons and how they communicate and reorganize, just remember: If you insult a caterpillar, it might remember the slight when it’s a butterfly.