What Actually Happens When a Silkworm Spins

The Last Thing a Larva Does

At the end of the fifth instar a silkworm stops eating, empties its gut, and begins to spin. Everything from that point is one continuous behaviour ending in a sealed cocoon, and it is the most-watched thing this animal does.

Stage One: Anchoring

Before any cocoon exists, the larva lays down loose scaffolding strands, attaching itself to whatever surface it can reach. This is why a pupating container needs paper, mesh or crumpled card: a worm with nothing to anchor to will wander, and wandering worms produce poor cocoons or none.

Those first strands are heavy on sericin — the gluey protein — doing structurally what it does industrially: sticking silk to things.

Stage Two: the Figure of Eight

Once anchored, the animal begins the movement that defines the process. The spinneret is in the head, so drawing thread means moving the head, and the silkworm sweeps it in a repeated figure-eight, laying fibre in overlapping arcs.

The cocoon is built from the outside inward. The worm disappears from view long before it stops working.

Stage Three: Sealed In

When the cocoon is complete the larva moults one final time inside it, into a pupa. From there the animal reorganises entirely, and a moth emerges roughly 10 to 14 days later.

If You Are Rearing Rather Than Feeding

Two practical points follow from the biology. A worm that has stopped eating and started wandering is not ill — it is looking for somewhere to spin, and it needs moving to a pupating container rather than more food. And once spinning starts, leave it alone: a cocoon interrupted partway is usually a cocoon abandoned.

For the rest of the sequence, see how to breed silkworms.

Related: the silkworm genome · what silk is made of