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Silkworms Spin Cocoons That Spell Their Own Doom

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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. Humans have bred these insects into weaving machines that can no longer survive in the wild.


This fuzzy moth has wings, but it will never fly.

Why? Because we humans made it that way.It’s a domestic silk moth. It looks almost nothing like its wild cousin.

For 5,000 years humans have bred silk moths, changing the course of their evolution to be their own unique species. All because we covet their shimmering threads. 

The silkworm — this moth’s caterpillar — spits out an extraordinary fiber. It’s ideal for making everything from satiny sheets to glamorous gowns to terrible ties. 


But we don’t just love it for its looks. 

Silk is one of nature’s strongest fibers. The thread is thinner than a human hair, but for its weight, it’s five times stronger than steel. 

The silkworm begins its life snuggled in a tiny, translucent egg. The hungry caterpillar nibbles its way out. It’s raised on a farm where it can chomp and chomp and chomp. Here they’re feasting on ground-up mulberry leaves.

Each larva starts out smaller than the date on a penny and in just a few weeks it’s as long as your finger. Now this plump caterpillar can get to work on its next phase of life: making its cocoon. It secretes a liquid from a spinneret on its head. 

As soon as the solution touches the air, it becomes a pliable fiber.  

What we know as “silk” is a long, unbroken strand constructed primarily from two proteins. 

 The inside is fibroin — it’s the structural center. The outside is sericin — the glue binding it all together. 

The larva spins raw silk for two to three days straight. The end result is a single string of silk, up to 10 city blocks long. This tiny shelter maintains just the right humidity and temperature for the caterpillar to transform into a moth. 

Once the cushy casing is complete, farmers harvest the raw silk. For most silkworms in captivity, this is where their journey ends.

They die when manufacturers boil, steam, or dry them out in the sun.  

Silk harvesters unspool the single thread of silk that makes up each cocoon. If they were to let the moth hatch, it would break the valuable thread.

It takes up to two thousand unbroken cocoons to make one silk dress.

However, a few lucky pupae are allowed to continue developing, and become moths, so they can spawn the next generation.

After centuries under our watch these industrious weavers can’t fly, and their camouflage is gone, so they wouldn’t last long in the wild. They can hardly move. 

Once outside the cocoon, a flightless male must seek out a female quickly because they only have a few days to live. So they now rely on us to make sure they find a mate and reproduce.

Silk production is a multibillion-dollar global industry. And it’s not just for fashion and luxury.

Silk fabric is a natural insulator, yet it also allows air flow. 

Silk is used in modern medicine to stitch together wounds, stabilize bones, and even replace tendons inside our bodies.

And so, our relationship with the humble silkworm continues — as it has for millennia — at least until we figure out how to make silk as well as it can.

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