Inside the Silk Gland: How a Silkworm Makes Thread

Three Sections, Three Jobs

The silk gland is a paired organ running most of the length of the larva, and it is divided into three parts that each do something different.

• Posterior silk gland (PSG) — synthesises fibroin, the structural protein that becomes the fibre.
• Middle silk gland (MSG) — synthesises sericin, the coating that glues the fibre together.
• Anterior silk gland (ASG) — processes the liquid proteins and carries them to the spinneret.

The order matters. Fibroin is made furthest back and travels forward, picking up its sericin coat on the way, arriving at the spinneret as a coated liquid ready to be turned into thread.

Liquid Going In, Solid Coming Out

This is the part that makes silk remarkable as a material. Inside the gland the protein is a liquid. It becomes a solid fibre during extrusion itself — not by drying, not by cooling, but through the physical forces of being squeezed and drawn through a narrow duct.

The current understanding is that shearing and extensional flow in the spinneret trigger the transition. The animal is not extruding a fibre that hardens afterwards; the act of extrusion is what makes it a fibre.

The Figure of Eight

The spinneret is in the head, which is why a spinning silkworm moves its head the way it does. The characteristic figure-eight head movement is the animal drawing its own thread — pulling the fibre out and laying it down in a pattern at the same time.

If you have watched a silkworm start a cocoon, you have watched a spinning machine operating its own draw mechanism.

Why Industry Cares About the Detail

Spinning silk artificially — taking dissolved fibroin and forcing it through a nozzle — has never fully matched what the animal does. The gap between a laboratory-spun fibre and one that came out of a caterpillar's head is why the spinneret is still actively studied.

Related: the silkworm genome · what silk is made of