Butterflies Remember Caterpillar Experiences

March 6, 2008 — -- Don't be cruel to caterpillars -- they won't forget it. Moths and butterflies can remember what they learned as caterpillars, a study reveals.

The findings challenge the accepted wisdom that the insects – brains and all – are completely rewired during metamorphosis, and may provide clues about neural development.

"Practically everything about the two phases of the organism are so different – morphology, diet, how they move, and what they sense," says Martha Weiss of Georgetown University in Washington, DC, in the US.

"We were curious to see if we could train a caterpillar to do something it could remember as an adult," she says

Weiss and colleagues exposed tobacco hornworm caterpillars, Manduca sexta, to ethyl acetate – a chemical often used in nail polish remover -- and a series of mild electric shocks.

'Caterpillar soup'Seventy-eight percent of the caterpillars that were shocked directly after exposure avoided the compound in subsequent tests while still in the larval stage.

The tests were conducted inside a Y-shaped pipe that allowed the animals to choose an area smelling of ethyl acetate or of unadulterated air.

About a month later, after the caterpillars had metamorphosed, the adult moths were given the same choice test. Seventy-seven percent of them avoided the ethyl acetate pipe, suggesting that the lesson learned as a caterpillar is remembered as an adult.

"People always thought that during metamorphosis the caterpillar turns to soup and all the ingredients are rearranged into the butterfly or moth," says Weiss. "That clearly isn't what happens. Parts of the brain are retained that allow memories to persist through this very dramatic transition."

Plant preferenceWeiss says the findings could have implications for how neurons and synapses can reconnect after the upheaval following a stroke or other kind of brain damage. The study could also explain how females know which plants to lay their eggs on, since they may remember the type of plant they fed on as a caterpillar.

Jeremy Davis of the University of Arizona in Tucson says the findings lend strong support to the theory that larval memory can persist to adulthood, a phenomenon previously only seen in fruit flies.

However, he says he is not sure if memories spanning the two life phases are strong enough to influence where females lay their eggs.

"Whether or not larval experience can cause individuals to prefer a certain host plant is still an open question," Davis says. "Positive association may not be strong enough to be maintained over metamorphosis."