Metamorphosis completely restructures a caterpillar’s wormlike body and, you would think, its brain. But caterpillars may not be so wasteful. New research shows that some of their brain cells remain intact through the supposedly annihilative process.
Scientists have long been interested in whether butterflies can remember experiences they had as caterpillars, but teasing memory from the simpler concept of familiarity is difficult, says Martha Weiss of Georgetown University in Washington, D.C.
Now Weiss and her colleagues Douglas Blackiston and Elena Casey have found a way. The team put tobacco hornworm caterpillars (Manduca sexta) in the stalk of a Y-shaped tube with one arm that contained a smelly, ephemeral gas. The researchers gave the caterpillars a mild electric shock when they went down the arm with the gas. Some of the caterpillars had undergone their third molt and others had completed their fifth and final molt.
When the critters emerged as adult sphinx moths, 77 percent of those that were shocked following the fifth molt remembered their aversion to the smelly gas, the team reports in the March PLoS ONE. Caterpillars shocked just after their third molt did not remember, suggesting that postmetamorphic memories are created in brain regions that develop later in metamorphosis.
Solitary insects like caterpillars may have more use for memory than social insects like wasps or ants because there’s no division of labor in the species, says Weiss. Unlike with bees, no scout is going to do a dance showing the butterfly where to find good food. A female butterfly needs to recognize a host plant for egg-laying, a nectar-bearing plant for feeding, and a safe spot for roosting. “I call it my single-working-mother hypothesis,” Weiss says.