Butterflies and moths hatch from an egg into a caterpillar and when it's time for that caterpillar to become an adult, it protects itself in a hard, rounded case before dissolving into goo and re-forming into a beautiful adult. But what are these protective cases called? Are they cocoons or chrysalises? Are those just different words for the same thing? Short answer: no. Only butterfly caterpillars have chrysalises and only moth caterpillars use cocoons. But what makes them different?
Cocoons are soft sacs made of silk that moth caterpillars spin around themselves before beginning the transformation into an adult. Often times they will also incorporate leaves and other material into the cocoon for protection and camouflage. Many species of flies, beetles, and bees also spin cocoons before changing into their adult form.
Butterfly caterpillars don't coat themselves in silk. Instead, when the caterpillar reaches the right size, it forms beneath its skin a tough, hard shell made of chitin, which is a bit similar to what hair and nails are made of. Once the chitin shell (this is the the chrysalis!) is ready, they attach themselves to a twig with a sticky substance and shed their skin one last time to reveal the shell beneath. They can then begin metamorphosing into adult butterflies.
It's important to try not to disturb a cocoon or chrysalis if you see it. The pupae inside cannot survive outside of its protective casing.
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