Care guide

A Silkworm Grows 10,000 Times Its Weight

By Matt Goren2026-09-05 14:20:00 -04002 min read

The Numbers Are Absurd

A silkworm increases its weight roughly ten thousand times between hatching and the end of the fifth instar — in about 20 to 25 days.

It starts at two or three millimetres and finishes near seven centimetres. Nothing about the animal's life is slow.

Almost All the Eating Happens at the End

A single caterpillar gets through roughly 30 grams of mulberry leaf across its entire larval life — and around 80 to 85% of that is consumed during the fifth instar alone.

The first four instars are, in feeding terms, a warm-up. The final stage is where the animal does almost all of its eating and almost all of its growing, and it is why a fifth-instar cup empties so much faster than a cup of smaller worms.

A Quarter of the Animal Is Silk Gland

The silk glands make up about 25% of a mature silkworm's body weight. Not a quarter of its length — a quarter of its mass is the organ that makes thread.

And during the fifth instar, which lasts about a week, that gland synthesises silk protein equivalent to 25–35% of the larva's entire body weight. In seven days the animal manufactures roughly a third of itself, in protein, for a structure it will use once.

What That Means Practically

Three things follow directly, and every one is something keepers run into:

Sizes change fast. A cup of mediums is not a cup of mediums next week. The animal is on a schedule you cannot slow, because refrigeration kills them.
Fifth instars eat alarmingly. If food is disappearing far quicker than it was, nothing is wrong — you have reached the stage where 80% of the eating happens.
Then it stops entirely. When the gut empties and feeding ceases, that is the wandering stage, not illness.

Why Domestication Went This Way

Every one of these numbers is a farmer's number. More silk gland per animal, more protein per gland, more growth per day. The 40-genome study found selection concentrated in exactly these tissues — silk gland, midgut, reproductive tract.

This is what five thousand years of choosing the best cocoons produces: an animal that is a quarter thread-factory and spends its last week building it.