The striking transformation of a caterpillar into a colourful, winged butterfly is one that has captivated scientists for years. The metamorphosis involves the breakdown of most of the caterpillar’s tissues before reassembling to form a butterfly. It therefore seems unlikely that butterflies or moths would remember experiences from their caterpillar days. However, scientists have now established that not only can a moth retain memories formed while it was a caterpillar, but that experiences gained during these early stages can have drastic impacts on adult life.
Memories live long
Moths and butterflies undergo drastic changes throughout their life-cycle, not just in their outward appearance, but also in their diet and overall lifestyle. Metamorphosis occurs within the pupal case. During this period, the larval brain stimulates the release of enzymes which dissolve most of its tissues into their constituent proteins through a process called histolysis. Then a group of specialised cells called histoblasts proceed to reconstruct the broken-down caterpillar body into that of a butterfly or a moth.
A few years ago, Martha Weiss and her group at Georgetown University discovered that aversive memories formed in the tobacco hornworm caterpillar (Manduca sexta) persisted throughout metamorphosis and were retained in adult moths.
Weiss trained caterpillars to avoid the odour of ethyl acetate, a chemical commonly used in nail polish removers. Caterpillars and moths are usually indifferent to the smell of ethyl acetate, but by pairing exposure to the odour with mild electric shocks, the scientists successfully taught these caterpillars to avoid the odour.
When given a choice of air or ethyl acetate, 78% of the caterpillars carefully avoided the odour in favour of air. Then, when adult moths developed from the pupae of trained caterpillars a month later, they continued to show a strong aversion to the smell, with 77% of the moths choosing air over ethyl acetate. Notably, the majority of moths choosing air as adults had also made the same choice as caterpillars, suggesting that individual preferences survived metamorphosis.