Silkworm Size by Stage: A Feeding Size Guide

How Big Do Silkworms Get?

A silkworm hatches at two to three millimetres and reaches roughly seven centimetres — about two and a half inches — fully grown. It does that in about 28 days, through five growth stages called instars.

That is a bigger range than most feeders, and it is why "silkworm" on its own is not a size. A first-instar silkworm suits a dart frog; a fifth-instar one is a meal for an adult bearded dragon.

The Five Instars

A silkworm cannot grow inside its own skin, so it moults five times. Between moults it eats constantly — a silkworm can get through up to fifty times its own weight in mulberry across the larval stage.

• 1st instar — 2–3 mm at hatch. Needs finely chopped tender leaf; too small to handle.
• 2nd–3rd instar — still small and still on tender leaf, warm and humid.
• 4th instar — visibly a worm now, and able to take tougher leaves.
• 5th instar — full size, roughly 5–7 cm stretched out. This is the stage most feeders are sold at.

Matching Size to Your Animal

The general rule across reptiles is that prey should be no wider than the space between the animal's eyes, and for most keepers that is the useful test rather than a length in millimetres.

• Small geckos, anoles, small frogs → early-instar silkworms only.
• Crested geckos, chameleons, juvenile dragons → mid-size.
• Adult bearded dragons, monitors, tegus → full fifth-instar worms.

They Keep Growing After You Buy Them

This is the part that catches people out. A cup of medium silkworms does not stay medium — fed properly, they will be large within days. Buy a size below what you need if you are feeding over a week, and plan feedings around the fact that the cup is a moving target.

More in the complete silkworm raising and care guide.