What really happens when a caterpillar turns into a butterfly?

Eric Carle’s well-loved story The Very Hungry Caterpillar is often children’s first introduction to this amazing transformation.

We all know the life story of a butterfly; the egg hatches into a caterpillar, which then encases itself in a chrysalis until it emerges as a butterfly. But do you know what happens to the caterpillar during its transformation? It’s fascinating but also a bit gruesome!

When the caterpillar hatches from its egg it truly is a very hungry caterpillar. Many species eat the egg shell because it contains valuable nutrients, and then go on to eat lots of young green leaves. Unfortunately the caterpillar’s skin doesn’t grow bigger as its body does, so it has to grow a new and larger skin beneath the old one before it sheds it. This is called moulting and it happens several times as the caterpillar eats more and more and grows larger and larger. The last time is a little different, however, as the caterpillar anchors itself to the stem or leaf of a plant with silk before it moults; the new skin is hard and forms the chrysalis.

(Do you know the difference between a chrysalis and a cocoon? A chrysalis is a hard case made from proteins, whereas a cocoon is spun from silk. Butterflies emerge from a chrysalis, while moths and some other insects emerge from cocoons).

Once the caterpillar is safely inside its chrysalis it is known as a pupa, and this is where things start to get a bit grisly. The pupa’s body releases enzymes, which are a kind of protein, and these begin to digest the body until it’s just a nutrient-rich soup. The only parts that aren’t digested are the imaginal discs, which are like a blueprint for the butterfly’s body; they form while the caterpillar is still inside its egg but aren’t active until the pupa is dissolved. Once that’s happened they use the caterpillar soup to fuel the formation of the butterfly’s body. After 10-14 days (the length of time varies according to the species) the butterfly emerges. Its wings are small and wet, but it enlarges them by pumping fluids into them from its abdomen. Once the wings are fully expanded and dry, the metamorphosis is complete and the butterfly is able to fly.

A paper kite butterfly. (Image: Jim McSweeney).
A paper kite butterfly. (Image: Jim McSweeney).

As if all this wasn’t gruesome enough, researchers at the University of Georgetown in the USA have discovered that moths and butterflies have some of the same memories they had when they were caterpillars. They proved this by teaching caterpillars to associate certain smells with bad things, and then testing the adult moth or butterfly’s reaction to those same smells. This makes me wonder whether the caterpillar is in a sleep-like state as it dissolves and is remade, or whether the adult butterfly can remember being caterpillar soup…?!

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