How do butterflies evolve from the cocoon?
This is a good question, because it is so hard to imagine how a butterfly that is all cramped and folded in its chrysalis manages to emerge. First of all, though, let’s make sure we’re using the right words since vocabulary is very important in science. When insects change from their larval form (for example, a caterpillar) to the adult form (for example, a butterfly) they form a “pupa”. In butterflies, we use a special word: “chrysalis”. For moths, it’s just “pupa”. Many moths go an extra step and cover their pupa with a cocoon. The cocoon can be made of just silk, produced by glands in the caterpillar’s mouth, or the silk can be used to tie leaves or chunks of soil together. Butterflies do not make cocoons for this extra layer of protection.
There are two words we use for describing an adult insect coming out of the pupa: “emergence” and “eclosion” (French for “hatching”). If you use the word “eclosion”, everyone will know you are a real “entomologist” (a scientist who studies insects). The term “evolution” is best left for things like birds evolving from a type of dinosaur.
Getting back to the chrysalis, when the butterfly is ready to eclose, the outer skin starts to soften and the butterfly flexes its muscles to make the skin split, so that it can get its legs out. Then it pulls the rest of the body out. At this point, the wings are all crumpled, and the body is short and plump. The butterfly pumps fluid into the wing veins, which helps them to expand. Once the wings are dried and hardened, excess fluid, called “meconium” is excreted. This “butterfly poop” is usually a very bright yellow, orange, pink or red. The butterfly has one more task before it flies: it needs to assemble its proboscis, the long, coiled straw it uses to feed on nectar. When a butterfly first emerges, its proboscis is split into two coils. After repeatedly coiling and uncoiling the proboscis, the two halves eventually zip together. Once it has working mouthparts and dry wings, and has pooped out the meconium, it is ready to fly. – Dr. Naomi Cappuccino