Moths Remember Lessons Learned While Caterpillars

John Roach for National Geographic News

March 5, 2008

Adult moths can remember their "childhoods" as caterpillars, a new study has found.  Recently scientists trained tobacco hornworm caterpillars in the lab to avoid a nail polish-like odor delivered in association with a mild shock.  These bugs then entered the pupal stage and metamorphosed into moths. As adults, they also avoided the nail-polish odor—showing that they had retained their larval memory.  "We concluded that indeed the association does persist and is accessible to the adults in this artificial scenario," said study senior author Martha Weiss, a biologist at Georgetown University in Washington, D.C.  The finding also supports the idea that a piece of the caterpillar brain persists through metamorphosis, she added.  (Related: "Scientists Rethinking Nature of Animal Memory" [August 22, 2003].)  Weiss and colleagues report their research today in the Public Library of Science journal PLoS ONE.  Memory and Metamorphosis  Scientists have long wondered whether memory could survive the dramatic reorganization of the moth brain during metamorphosis, Weiss noted.  "The transition from a caterpillar to a moth or butterfly is really very dramatic," she said.  (See a picture of caterpillar larvae that look like bird poop.)  For example, caterpillars and moths move, eat, and sense the world differently—not to mention appear nothing alike.  The study also showed that memory retention depends on the maturity of the developing caterpillars' brains.  Caterpillars younger than three weeks old learned to avoid the nail-polish odor but could not recall the information as adults.  However caterpillars conditioned to avoid the odor in the final larval stage before pupation, called the fifth instar, retained the lessons.  Larvae trained during the third-instar stage also demonstrated an aversion to odor as fifth instars, but they did not avoid the odor as adults, the authors said.  The findings jibe with the idea that memories are retained in one lobe of a mushroom-shaped part of a caterpillar's brain that forms during the late stages of larval development and survives metamorphosis.  "Basically, the brain is not completely taken apart and rebuilt from scratch," Weiss said.  Reinhard Stocker, a biologist at the University of Fribourg in Switzerland, studies the nervous system of fruit flies.  In an email, he noted that Weiss team's data appear strong, but he said that the moths' memory retention only after the fifth instar was perplexing. "Metamorphic rewiring of the brain is generally thought to be completed only in later stages of metamorphosis, which for the moth would be well after the fifth instar," said Stocker, who was not involved in the new research. "Thus it should essentially not make any difference if one trains third- or fourth-instar larvae. I don't have any simple clue to interpret such an observation." Shock and Odor  The research may help explain how adult female moths that can eat a variety of food choose to lay their eggs on the same type of plant they fed on as larvae.  Study author Weiss describes it as an "if it was good enough for me, it's good enough for my kids" type of selection.  If the moths retain some memories from their larval stage, as this research shows, then they could remember what they ate as "kids."  Other researchers have theorized that moths have what's called a chemical legacy from their larval stage that could cue them what to eat.  "That could look like the larva is actually remembering something," Weiss noted.  Her research team was careful to show that larval memory was based on the formation of an association between a shock and an odor—not a chemical legacy, she said.  "For me, it is exciting to think that a learned association really can be transferred from one phase to another through this very radical transition."  Story