Care guide
Why Silkworms Moult Five Times: The Hormones Behind the Instars
Two Hormones, One Argument
A silkworm passes through five larval stages before it spins. That number is not arbitrary and it is not counted by the animal. It is the outcome of two hormones running against each other.
- Ecdysteroids — principally 20-hydroxyecdysone, usually written 20E — trigger a moult.
- Juvenile hormone (JH) decides what the animal moults into.
Ecdysone says change now. Juvenile hormone says change into another larva. As long as JH is high, every moult produces a bigger caterpillar. When JH falls away at the end of larval life, the same ecdysone signal produces metamorphosis instead.
Where the Signal Comes From
The chain runs from the brain outward:
- The brain releases PTTH — prothoracicotropic hormone.
- PTTH stimulates the prothoracic glands to make and release ecdysone.
- Ecdysone enters the haemolymph — insect blood — and is converted at the tissues into the active form, 20E.
- 20E drives the moult.
Juvenile hormone acts on this system as a brake as well as a switch: it inhibits ecdysone secretion and makes the prothoracic glands less responsive to PTTH.
The Experiment That Proves It
The cleanest evidence is a silkworm mutant that cannot make normal juvenile hormone. It does not go through the usual sequence of larval moults. It metamorphoses early — precociously — producing a miniature moth from a larva that should still have been growing.
Remove the brake and the animal skips ahead. That single result ties the whole model together.
Why a Keeper Might Care
It explains the thing that catches everyone out: a cup of silkworms does not hold its size. The animal is on a hormonal schedule, and nothing in a plastic tub pauses it. It also explains why cooling is so dangerous — you are interfering with a developmental clock in an animal bred over five thousand years to run it on time.
Related: silkworm size by stage · the silkworm genome