Dear Anabelle,

When I was a kit, I looked a lot like the adult cat I would become—even though I was smaller and fluffier. But wiggly caterpillars don’t look like butterflies at all.

I talked about this 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 “cocoon” isn’t really a cocoon at all. It’s called a chrysalis.

A brown silk cocoon on a green leaf
A moth’s cocoon, credit: entomart

Both butterflies and moths belong to a big group of insects that go through complete metamorphosis. They have four life stages: egg, larva, pupa and adult. They go through a massive change to become an adult.

That big change happens when the insect is a pupa. That’s like their teenager stage. A moth pupa usually changes inside a silk cocoon. Sometimes people harvest that silk to make fabric.

A butterfly pupa might look like a cocoon, but it’s different.

“Many butterflies have what we call a naked pupa or chrysalis,” Felsot said. “The wings, mouthparts and antennae are glued to the body, and it’s compressed. But if you poke it, you’ll see it wiggles around.”

A butterfly pupa is covered with the same tough skin that you see on any insect. It’s just a temporary, baggy version of that skin called a chrysalis. The chrysalis is often tethered with silk, so it stays put.

A mostly green chrysalis hangs from a silk thread. The chrysalis is partly transparent, and the butterfly's wing is visible inside.
A butterfly’s chrysalis

Insect silk generally comes from the same organs that make saliva.

“There are lots of things that salivary glands do,” Felsot said. “One thing is produce silk proteins. These are in the form of a gel. So, it’s very viscous, and it’s forced out as a drop. But then the insect pulls away from it—maybe they wiggle their head or move their body a little bit—and that spins it into a fiber.”

As the gel hits the air and the insect pulls away from it, the silk crystallizes. The particles in the silk line up in an orderly way. That makes the silk strong. The silks made by different kinds of insects are all a little bit different.

Insects use silk for all kinds of things. Some insects like moths wrap silk around their bodies to make a silk cocoon. Some insects use silk like glue to make cases out of stuff they find.

A caddisfly and its case made of bits of sand and gravel
A caddisfly’s case, credit: NSF

One of my favorites is the caddisfly. They’re related to butterflies and moths, but their larvae live underwater.

Some caddisflies use silk to glue together tiny bits of sand and debris. It forms a little house a larva can live in and carry around. When it’s time to change into an adult, the insect usually seals up the entrance to the case with more silk.

Caddisfly silk is so special—sticky, stretchy and waterproof—that scientists want to copy it so they can make better bandages and stitches. Scientists study insect and spider silks to learn how to make all kinds of things.

It’s just one more way insects make our lives smooth as silk.


Dr. Universe