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Moths remember life as caterpillar

During the dramatic transformation from caterpillar to moth, memories and minds remain at least somewhat intact, according to a new study.

The study, published in the journal PloS ONE, found that moths and perhaps butterflies often recall what they learn as ground-dwelling insects, revealing how resilient memory may be.

"Caterpillars are built for crawling and chewing plant material, while moths and butterflies are built for flying and drinking nectar," says Dr Douglas Blackiston from Georgetown University, who led the recent study.

"As such, the adult butterfly must rearrange its digestive system and develop distinct muscles for flight and walking on its long legs."

The morphing caterpillar's brain also grows in size and complexity to reflect the insect's drastic lifestyle transition.

Whether its memories endure during those changes was a mystery, until now.

Inside the mind of an insect

To test more specifically what goes on in the insect's mind during metamorphosis, Blackiston and his colleagues borrowed tobacco hornworm caterpillar larvae from the insect laboratory at North Carolina State University.

The researchers conditioned the caterpillars to avoid the chemical ethyl acetate, which smells like pear juice.

Since caterpillars don't naturally avoid ethyl acetate, the researchers gave some of the insects a mild electrical shock whenever they encountered it. The insects quickly learned to avoid the telltale odour.

After the caterpillars morphed into moths, the scientists exposed them to the chemical again.

Just as babies do not appear to remember anything until reaching a certain stage of brain development, the moths only recalled what they learned if the experience occurred after they were three weeks old.

Insects that had been conditioned before that age did not avoid the chemical, while those conditioned after that point appeared to retain the memory.

"We now know that the [insect's] brain is extensively rewired, although not built from scratch by any means," says Blackiston.

He says that many memory-storing neurones may remain intact during the transformation and make new connections as the insect ages.

Pest control

In the future, the findings could improve pest control techniques, since the research may help explain why insects often lay eggs on plants they apparently remember eating when they were larvae.

The research could also help to determine how brains cope with large-scale changes, and how memory survives even after a nervous system is largely rewired, Blackiston says.

Fred Gagnon is the curator of butterflies at the Magic Wings butterfly conservatory in South Deerfield, Massachusetts, and doubts caterpillars could form many memories beyond odour-based ones.

He says this is because they have very poor vision and "just eat all of the time without moving much".

But the conditioning aspect of the study doesn't surprise him as butterflies, like dogs and cats, can be socialised and trained.

"If you raise them in captivity, they are much less afraid of human contact than those taken from the wild," Gagnon says. "You can even train them to land on your finger."