Every other Friday on Morning Edition, the Outside/In team answers a question from a listener about the natural world.
“What happens inside a chrysalis during metamorphosis?” asked Chris, writing from Massachusetts.
Note: This piece originally aired in September 2023 and was published Sept. 29, 2023.
Submit your question about the natural world to the Outside/In team. You can record it as a voice memo on your smartphone and send it to outsidein@nhpr.org or leave a voicemail on our hotline, 1-844-GO-OTTER. We also accept questions sent to Twitter or Instagram. We’re @OutsideInRadio in both places.
It’s all hormones
“Basically, the definition of [metamorphosis] is just changing,” said Karen Oberhauser, entomologist and director of the UW Madison Arboretum. She’s been studying monarch butterflies from Wisconsin to Mexico since 1984.
Metamorphosis is not a term used to describe mammals. That’s because mammals, like humans, essentially have the same body as a juvenile as as adults. We just develop.
But flies, ants, moths, bees, and butterflies (including monarchs) go through complete metamorphosis, a four-stage process: egg, larva, pupa, adult. Other insects, like dragonflies, go through incomplete metamorphosis, which means they skip the pupa stage.
“All of the changes are driven by hormones,” said Oberhauser.
When a caterpillar creates a chrysalis, it’s actually shedding its skin.
“But instead of another larva skin underneath, it is the pupa or the chrysalis skin,” said Oberhauser.
A chrysalis, by the way, is a term used only for butterflies. If it’s another insect, it’s called a pupa.
Inside the chrysalis, some parts of the caterpillar literally dissolve. The caterpillar has the same six legs on the front that it will have as a butterfly, but it won’t need its big caterpillar muscles anymore. Same goes for the digestive system.
“The digestive system really changes a lot from being able to handle, you know, all that milkweed that lets it grow so much to just handling nectar,” said Oberhauser.
“All of that material is then used to construct new things.”
Beyond the chrysalis
Many parts of the monarch stay the same from caterpillar to butterfly, including the reproductive organs.
“The testes are paired in a male monarch… it’s just this beautiful red ball inside the adult, but it's also there inside the caterpillar,” said Oberhauser. “So, that’s an example of an organ that starts forming early on.”
The caterpillar doesn’t just turn to total mush, which is a common misunderstanding. If you were cut into a chrysalis halfway through the process, “you would pretty much see a butterfly, but it would be white,” Oberhauser said. That’s because the pigment is still forming, and the scales are one of the last things to develop.
However, because this would kill the butterfly and monarchs are endangered, cutting open a developing chrysalis is inadvisable.
Finally, metamorphosis is a term that describes the entire process of transformation, not just the transition from caterpillar to butterfly.
“The process of metamorphosis isn't something that just gets turned on when the caterpillar sheds its skin and becomes a pupa,” said Oberhauser. “It's something that's happened for a long time.”
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