How Silkworms Make Silk — and Why They're a Top Feeder

I keep silkworms for two reasons that turn out to be the same reason: they make one of the most remarkable natural fibers on earth, and that same biology makes them an outstanding soft feeder for reptiles. Understanding how a silkworm spins silk actually changes how you raise and feed them, so let's walk the whole process and then talk about using them at the bowl.

The animal is Bombyx mori, the domesticated silk moth — so thoroughly domesticated that it no longer exists in the wild and can't survive without us. In the feeder hobby we use the larvae (the "worms"); in sericulture, the larvae build the cocoons we harvest for thread.

The silkworm lifecycle, egg to moth

Silk starts with a pinhead-sized egg. In warm, humid conditions the egg hatches into a tiny larva that does essentially one thing: eat. Silkworm larvae feed almost exclusively on mulberry leaves, and they eat ravenously, growing through four or five molts (called instars) over several weeks until they're plump, pale caterpillars several thousand times their hatch weight.

When a larva is fully grown it abruptly stops eating and starts wandering, searching for a place to anchor a cocoon. That behavior change is the cue that the spinning stage has begun. After spinning, the larva pupates inside the cocoon and — left alone — emerges weeks later as an adult moth to mate and lay the next generation of eggs.

How the silk is actually made

The silk-making machinery lives in the larva's body, and it's elegant.

The anatomy behind the thread

A silkworm's body is divided into head, thorax, and abdomen. The abdomen houses the silk glands (sericteries), large paired glands that synthesize liquid silk protein. That liquid runs forward to the spinneret, a small opening near the mouthparts on the head. The thorax mostly provides the muscle for the figure-eight head motion used to lay the thread.

From liquid protein to solid fiber

Silk is two proteins working together. Fibroin is the strong structural core; sericin is a gummy protein that coats and binds the fibroin. Both are stored as liquid in the silk glands. As the larva forces the liquid through a narrowing duct toward the spinneret, water is drawn out and mechanical shear stress aligns the protein molecules. The instant the filament hits air it solidifies into a thread — a phase change driven by the physical act of pulling it out, not by drying alone.

Spinning the cocoon

Anchored to a surface, the larva swings its head in continuous figure-eight motions, laying the hardening thread in crisscrossing layers. Each cocoon is a single unbroken filament, often 1,000 to 1,500 meters long, spun over a few days. The result is a tough, lightweight, insulating shell that shields the pupa during metamorphosis. The whole sequence is pure instinct — the larva has never seen it done and gets it right the first time.

What makes silk unique

Fibroin's amino acids — mostly glycine, alanine, and serine — line up in a highly repetitive, crystalline arrangement. That ordered structure is why silk is simultaneously strong and elastic: it stretches without snapping. The triangular, prism-like cross-section of fibroin fibers refracts light at varying angles, producing silk's signature sheen. Sericin protects the fibroin during spinning and is usually washed off later to reveal the soft, lustrous finish.

These properties — strength, biocompatibility, lightness — are why silk has jumped beyond textiles into medical sutures, tissue scaffolds, and drug-delivery research. The same protein a feeder larva makes is being engineered for surgery.

Harvesting silk: cocoon to thread

In sericulture, harvested cocoons are steamed or boiled to dissolve the sericin "gum" that locks the filament together — a step called degumming. Softened, the single filament can be located and reeled off onto spools; several strands are often combined for strength before weaving or dyeing. Controlling temperature and pH through degumming is what preserves the fiber's fineness and shimmer. (It's worth being honest that conventional silk harvesting kills the pupa inside the cocoon; "peace silk" methods let the moth emerge first, at the cost of a broken, shorter fiber.) For a keeper raising silkworms as feeders, none of this applies — you're feeding the larvae out long before any cocoon matters.

Using silkworms as a feeder

Here's where the silk story pays off practically. Everything that makes silkworm larvae good at building cocoons — soft bodies, protein-rich glands, a clean mulberry diet — also makes them a premium feeder.

The honest nutrition picture: silkworms are high in protein and low in fat, soft-bodied with very little chitin, and they contain serrapeptase, an enzyme that can aid digestion. They're excellent for hatchlings, picky eaters, and animals recovering from illness that can't handle crunchy prey. But I'll correct the common myth here, same as I do for hornworms: silkworms are not calcium-favorable. Like nearly every feeder except black soldier fly larvae, they're phosphorus-heavy. If silkworms are a regular part of the diet, dust them with a plain calcium powder. Don't trust the "good Ca:P ratio" line you'll see repeated online.

You feed them what they're built to eat. In captivity that's a powdered mulberry-based silkworm chow mixed with water, or fresh mulberry leaves if you have a tree. Their entire diet is mulberry — and because what a feeder eats becomes what your reptile eats, that clean diet is part of their appeal. I source feeder silkworms and chow from the silkworm collection at All Angles Creatures.

Keeping feeder silkworms alive

Silkworms are more delicate than roaches or hornworms, and the killer is almost always moisture and dirt:

• ◆Warmth: keep them around 78–82°F for steady growth.
• ◆Dry and clean: never let water pool. Excess humidity breeds the mold and bacteria that wipe out a cup overnight. Feed chow that's moist enough to eat but not wet.
• ◆Fresh food, clean container: give fresh chow regularly and clear old food and frass. Handle gently — their soft bodies bruise.
• ◆Cull fast: any worm that darkens, goes limp, or smells off comes out immediately before it infects the rest.

Get those conditions right and silkworms reward you with a feeder almost nothing refuses — the soft, protein-dense counterpart to the hydrating, watery hornworm. I run both in rotation for exactly that contrast.

Pair these with the high-water, low-fat hornworm for hydration, and build the staple base of the diet on a discoid roach colony.