Those precious silk garments in your closet were made by the caterpillar of a fuzzy, white moth — thousands of them. Silkworms spin a cocoon with a single strand of silk up to 10 city blocks long.

I talked about your question with my friend Allan Felsot. He’s an insect scientist at Washington State University. He told me cocoons are mostly silk. But they’re usually made by moths. A butterfly ...