Re: how does a cocoon becomes a utterfly

Hi Ninee,

Most of the work in becoming a butterfly actually occurs when the caterpillar turns into a pupa (the cocoon stage). Caterpillars are specialised eating machines and some increase their bodyweight by about 10,000 times. The muscles used by the caterpillar take up too much space to allow proper development of the wings. Therefore, when the caterpillar turns into a pupa, these muscles are broken down to allow the immature wing and the flight mechanism to grow. The immature wings, which have been growing as internal folds inside the insect, are turned outwards and the number of cells in the wings may increase 100 times at this point. Now there is space for the wing muscles, the powerhouse of an insect, to grow. The pupal stage is therefore a period in which enormous internal changes occur.

When the pupa is ready to hatch, there is little work to be done to change into an adult as most of the developmental changes have already taken place. The butterfly hauls itself out of the pupal case and starts pumping blood into the wings and other parts of the body to make them extend (rather like inflating an air mattress). When the wings are fully pumped, the insect withdraws the blood, which causes the wing tissue to dry up and die. This is because living tissue is full of water and too heavy to produce an efficient aerofoil. The result is a light wing strengthened by the hardened veins, which form the support matrix. Now the butterfly is ready to fly.

This is an absolutely fascinating process to watch, although it takes place over several hours. You can see it on several videos about insects or you can watch it occur in nature. Alternatively, you can find a mature caterpillar and watch it change at home which is great thing to do with children. Hope this answers your question.

Richard Kingsley

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