The Sound of Science - "Caterpillar Soup"

May 10, 2019

Sam: Welcome to the Sound of Science on WNIJ. I’m Sam from NIU STEM Outreach.

Kate: And I’m Kate! With spring in the air, Jack wants to know why do caterpillars form cocoons. 

NIU STEM Outreach

Sam: Most insects go through complete metamorphosis for a straightforward evolutionary reason: separating the adult population from the larva population means there’s more food for each group. If the caterpillars are busy getting fat from leaves, then there’s more sweet nectar for the adult butterflies. This reduces the competition and makes it more likely for their species to thrive and continue.

Kate: What’s really fascinating about caterpillars and food, though, is that they basically digest themselves! Gross right? But transforming from a caterpillar into a butterfly takes a lot of time and energy. Caterpillars are biologically programmed to get big and fat in order to build up a large supply of nutrients.

Sam: It IS gross – but kind of cool. When the time is right, they form a cocoon and enzymes activate within their bodies. These enzymes break almost everything down. Legs, eyes, skin, just about all of it turns into a high energy soup that will fuel the metamorphosis.

Kate: However, some parts, called imaginal disks, don’t break down. Those are the foundations of the butterfly. Around 20 or so little chunks of organic matter float around in the caterpillar goop and begin to form the new eyes, antennae, wings, legs, and a few other parts. Interestingly, these chunks are so self-contained, they can be transplanted into other species of insects. The transplanted parts from the original bug will form on the body of the new one during development. 

Sam: There are some other parts that remain intact, such as some digestive organs and their brain. We know this through an interesting experiment where researchers trained caterpillars to avoid the smell of ethyl acetate by shocking them. After the caterpillars emerged as moths, they ran the experiment again. Turns out, the moths that were part of the experiment were still afraid of ethyl acetate.

Kate: Research into imaginal discs and caterpillar memories will one day open doors into research on genetics, memory storage, tumors, and cloning! 

Sam: Keep bugging us with your questions by emailing us at This has been the sound of Science on WNIJ.

Kate: Where you learn something new every day.