PTTH: The Brain Hormone That Starts a Moult

Moulting Starts in the Brain

An insect does not moult because its skin is tight. It moults because its brain releases a hormone telling a gland to release another hormone. That first signal is PTTH — prothoracicotropic hormone — and it is the reason moulting can be scheduled at all.

The Cascade

PTTH is a brain secretory peptide. Its target is the prothoracic glands, a pair of endocrine organs in the thorax. On receiving it, they synthesise and release ecdysone, which travels in the haemolymph and is converted at the tissues into 20E, the active moulting hormone.

Brain → gland → bloodstream → tissue. Each step amplifies the one before, which is how a small peptide signal becomes a whole-body reorganisation.

The Gland Is Not Always Listening

This is the detail that makes the system a clock rather than a switch. The prothoracic glands of a freshly-moulted fifth instar larva do not respond to PTTH at all. The signal can arrive and nothing happens.

What silences them is juvenile hormone. Remove its source surgically and the same glands become active and responsive again. So the animal is not simply waiting for a command — it is waiting to become capable of hearing one.

Why This Was Worked Out in Silkworms

Insect endocrinology leaned heavily on Bombyx mori for the same reasons every other field did: the animal is large enough to dissect, available in enormous numbers, genetically well-characterised, and runs a fixed developmental schedule. When your subject moults on a predictable timetable, you can ask what happens if you interrupt it.

Much of what is now taught as general insect physiology was established in a caterpillar sold today as reptile food.

Related: silkworm size by stage · the silkworm genome