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Memories Survive Metamorphosis

Sun, 9th Mar 2008

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Despite dramatic changes in every aspect of their biology, it seems that moths and butterflies can remember things they learned as caterpillars.

Tobacco Hornworm CaterpillarCaterpillars can be thought of as eating machines - as they do very little but eat to store up they energy they will need when they form a chrysalis.  During metamorphosis, as they become a butterfly or moth, they undergo a radical change in body plan as well as lifestyle and diet.  The change is so dramatic that it had been assumed that memories or associations learned during the larval, or caterpillar period would be lost to the adult stage.

Writing in the March issue of PLoS One, Doug Blackiston and colleagues at Georgetown university trained tobacco hornworm caterpillars to avoid certain odours by associating the smell with an electric shock.  After metamorphosis, the adult moths also avoided the odours, showing they had retained the association despite huge changes in physiology.

Tobacco Hornworm MothHowever, the ability to retain these memories depended on when they were learned - Caterpillars younger that three weeks old could learn to avoid the odour, but did not retain the memory into adulthood.  In those caterpillars conditioned shortly before pupation, the memories survived the pupal stage and were demonstrated in the adult.  

Retaining memories from before metamorphosis could allow female butterflies to lay her eggs on the same type of host plant that she fed on during her larval stage.  This 'habitat selection' could lead to the development of new sub species of butterfly.

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