The Memories of Moths

Moths can remember things that happened to them as caterpillars. Seriously. Butterflies, too.

In a 2008 paper, researchers from Georgetown University demonstrated that when a caterpillar is trained to avoid a particular smell, the moth who was once that caterpillar will retain the memory of the training, and continue to avoid the scent. Caterpillars were treated with mild electric shocks while simultaneously being exposed to particular odors, conditioning them to avoid that smell. The caterpillars were then placed in tubes with a “fork” in their “road,” with the target smell emanating from one path but not the other. The caterpillars strongly preferred to head away from the smell, down the other direction. But here’s the crazy part: when these caterpillars became moths, the vast majority of those moths also avoided the smell, and took the other option. The researchers tried the same setup with caterpillars and moths who had not been exposed to the shock-training, and found that the avoidance behavior was not exhibited by these untrained individuals. In other words, caterpillars and moths don’t just avoid this smell on their own; that comes only with training. And the only training occurred during the caterpillar stage of the moths’ life cycle. Since the caterpillars avoided the smell, that means it worked. And since the moths did too, that means they remembered their caterpillar training. Which means that memories are preserved across the metamorphosis stage.

I found this research fascinating. It’s my kind of discovery: upending conventional wisdom by actually testing it against the real world, and finding it wanting. Like most folks out there, I grew up with the idea that butterfly and moth caterpillars are essentially liquefied during their metamorphosis. The pupal crucible breaks down everything the caterpillar is, and rebuilds it as a moth, or as a butterfly. When the caterpillar’s pupa breaks open, the moth who emerges is something quite new and different. This idea is the root of numerous metaphorical comparisons: When we feel that we are losing who we are, that our lives are being entirely upended and destabilized, we are reminded that “What the caterpillar calls the end of the world, the Master calls the butterfly.” Except it turns out that even the caterpillars aren’t quite so prone to exaggeration. Perhaps the caterpillars are secretly Zen Masters themselves.

However, to me, the thing that jumped out at me wasn’t that caterpillars’ memories are retained by moths and butterflies, but rather that caterpillars have been proven to form memories in the first place. Of course, it comes as no surprise to me, but I imagine that most folks today think of caterpillars as mindless “eating machines,” with no psychological life at all. It would be “anthropomorphizing” to attribute mentation to caterpillars; how very unscientific! And yet the entire premise of this research is the tacit understanding that caterpillars form memories. There is something it is like to be a particular caterpillar, something different from what it’s like to be a different caterpillar. (Or a bat.) Caterpillars feel pain, just as we do, when zapped with an electric shock. (If you prick us, do we not bleed?) And it traumatizes them. During metamorphosis, caterpillars essentially takes their own bodies apart and start over from scratch, and even that is not enough for them to get over it. To put it in the words of John Locke (not the Lost character; I mean the O.G.), caterpillars have psychological continuity over time. In case you missed it, that’s Locke’s criterion for personhood, and the persistence of personal identity. When we speak of caterpillars, we should not say “the caterpillar that became a moth,” but rather “the caterpillar who became a moth.” It isn’t just something, it’s someone.

Remember that next time you are considering a silk scarf. Sericulture, the 5000-year-old practice of breeding and raising silkworm caterpillars in captivity, then tearing apart their cocoons when they enter into the metamorphosis phase, is lethal to the silkworm caterpillars involved. And although a single caterpillar produces as much as one thousand yards of raw silk, because the harvesting process is very inefficient, producing a single kilogram of silk fabric requires the murder of more than five thousand caterpillars. There’s a reason that Gandhi (yes, that Gandhi) was a vocal opponent of sericulture. If you believe that every living thing has a conscious self (a soul, if you will), then every single filament of silk ever worn is steeped in a five thousand year history of genocide.

And if you accept, as shown by this among other experiments, that a caterpillar forms associative memories of subjective experiences like pains and smells, memories written so deeply and indelibly into the caterpillar’s being that they survive even the near-death and winged resurrection of the caterpillar itself, then on what grounds can you deny that each of these caterpillars has a conscious self?