The butterfly has become a favorite cliché for a fresh start in life. Despite the drastic metamorphosis, however, scientists have disagreed about whether a butterfly makes a clean break with its caterpillar past. New research suggests that butterflies and moths come with mental baggage, painful memories left over from their lives as larvae. The tobacco hornworm (Manduca sexta) is a green caterpillar that eats the leaves of tobacco plants across the United States. After 3 weeks of feeding, the larva secretes a hard brown shell around itself--a pupa--and spends 18 to 24 hours as anatomical soup, breaking down and remodeling everything from its digestive tract to its central nervous system. The brown-and-yellow moth that finally emerges is seemingly brand-new inside and out, with wings, complex eyes, and a taste for flowers.
Studies in fruit flies--which also spend their youth as larvae--have shown that the memories and neural connections of some insects can survive such a dramatic physical overhaul. It's not so clear-cut in the hornworm. Efforts to demonstrate persistent memory in M. sexta have relied on showing that adult moths retain a preference for the food they ate as larvae. Skeptics suggest that the moths are influenced not by their memories but by traces of food that cling to the pupae and come into contact with the emerging adults.
Martha Weiss, an evolutionary ecologist at Georgetown University in Washington, D.C., tried a different way to assess moth memory and answer these criticisms. She placed 2-week-old larvae into a tube with two chambers; one contained fresh air, the other was scented with the neutral odor of the organic compound ethyl acetate. Normally, the creepy crawlers show no preference for either compartment. But when she gave some a mild electrical shock while exposing them to the chemical, they learned to avoid the scented chamber.
The shocked creatures kept behaving this way after transforming into moths, Weiss reports online today in PLoS One. "They avoided the odor up to 35, 45, even 50 days later," she says. To strengthen the case that this effect stems from enduring memories and neural structures, Weiss repeated the experiment using younger, week-old larvae with less developed brains. They forgot the connection between shock and scent as adult moths. To eliminate effects caused by traces of leftover chemical, she sprayed another group of pupae with a heavy dose of her ethyl acetate. The chemical by itself had no impact on their behavior as adults. Weiss also scrubbed the pupae of her shocked moths to remove any lingering scents, just in case.
"She's done a good job of ruling out alternative hypotheses," says Mark Stopfer, a neurologist at the National Institute of Child Health and Human Development in Rockville, Maryland. Follow-up studies will be needed to pinpoint the brain structures that survive metamorphosis, he says. Stopfer adds that the findings might also provide a new model to study memory formation and preservation.