Study: Moths Can Remember Caterpillar Days

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A new study finds that moths can remember things they learned when they were caterpillars — even though the process of metamorphosis essentially turns their brains and bodies to soup. The finding suggests moths and butterflies may be more intelligent than scientists believed.

RENEE MONTAGNE, host:

Those moths that spend all night crashing into your porch lights can seem like really stupid insects. But wait. A new study says your basic moth brain can do something amazing. It can remember lessons learned when the moths were caterpillars, before the trauma of metamorphosis.

NPR's John Nielsen has more.

JOHN NIELSEN: Caterpillars don't just change their clothes when they turn into moths or butterflies, according to biologist Martha Weiss. Instead, they go through a biological meltdown that reduces them to soup.

Professor MARTHA WEISS (Biologist): I mean, that's what I learned, is that the caterpillar turned to minestrone and that those ingredients that made up the caterpillar were completely reorganized into a butterfly that threw away the leftovers that it didn't need from the soup and was off.

NIELSEN: Weiss studies butterflies and moths inside a tiny laboratory on the campus of Georgetown University in Washington, D.C. When I met her there last week, the only living insects on the shelves in this place were some hungry-looking praying mantids and crickets that some of the mantids would soon be eating.

But last spring, this lab held a lot of big, green caterpillars that eventually turned into tobacco hornworm moths. It also held a research team armed with canisters full of foul-smelling gas called ethyl acetate and boxes rigged to zap the caterpillars with electric shocks.

Weiss says it was all part of a study of moth memory.

Prof. WEISS: The question that we asked is can a moth or a butterfly remember something that it learned as a caterpillar?

NIELSEN: To find out, Weiss and her colleague Doug Blackiston put a lot of big, green tobacco hornworm caterpillars into the electric boxes and then gave them whiffs of stinky gas. Then Blackiston zapped them.

Prof. WEISS: So that the caterpillars would get a little bit of smell, and then they'd get a shock, and you could tell that they noticed the shock. And I think he did it once an hour for eight hours.

NIELSEN: Weiss says the caterpillars quickly learned that the stench would be followed by the jolt. As a result, the caterpillars wouldn't go near anything that smelled of ethyl acetate.

Next, the researchers let the caterpillars start the process that would turn them into moths. One by one, these caterpillars disappeared into brown, urn-shaped pupal chambers that dissolve their bodies and their brains. Five weeks later, the moths hatched out. At that point, the researchers gave the moth a choice of fresh air or air that stank of ethyl acetate.

Prof. WEISS: And wouldn't you know it, the moths that had learned to avoid ethyl acetate as larvae still avoided it as adults.

NIELSEN: In other words, somehow, the caterpillar memories had survived the biological meltdown. Weiss and her co-authors report on their results in the journal PLoS ONE, which is published by the Public Library of Science. And by some accounts, it's the kind of work that might eventually help experts on the human brain learn more about how damaged neurons sometimes fix themselves.

But for now, Martha Weiss of Georgetown says she is much more interested in what this paper seems to say about the learning skills of moths and butterflies. Basically, it looks like they are smarter than we think.

Prof. WEISS: They're not good at crossword puzzles. They're not good at calculus, but they can make connections that are relevant to their lives.

NIELSEN: For example, which red flowers are the poisonous ones and where is the safest place to lay your eggs. But one big question did go unanswered in this project. Weiss says she's still not sure why moths can't figure out why it's futile to keep crashing into those porch lights.

John Nielsen, NPR News, Washington.

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