MONARCH CATERPILLARS MOLTING

Since Monarch caterpillars go through five growth stages (called ‘instars’), they then have to molt four times. After molting, they eat the shed molt which gives them a source of nutrients. However they don’t eat the molt head, as can be seen below.

When freshly molted, the caterpillar head, legs and prolegs have a yellowish color, a bit translucent. It is in what is called a 'teneral' state, where the exoskeleton is still soft. This makes the freshly molted insect highly vulnerable. However the Monarch caterpillar has a good defense, thanks to the toxic cardenolides it ingests from Milkweed.

This page shows photos of Monarch caterpillars at different steps in the molting process.

(More general information on the Monarch butterfly can be found here.)

Monarch caterpillar eating its molt

The same caterpillar has now
finished eating its molt

Another Monarch caterpillar
eating its molt

Monarch caterpillar and its molt,
top view

Monarch caterpillar in 'teneral' state

The Monarch caterpillar shed
head is not eaten

The molting process can sometimes be slightly different, as seen from these photos below. This freshly molted Monarch caterpillar had its tentacles bent on its body. They remained this way for longer than other caterpillars.

Monarch caterpillar tentacles bent
and sticking to body

The tentacles are bent and
remain close to the body

Monarch caterpillar bent
head tentacles

Close up of the bent head tentacles

One head tentacle is now freed up

It took longer, but the
4 tentacles turned out OK

Here is now a series of 18 photos of a Monarch caterpillar molting, where we have a more detailed view of how the new caterpillar slips out of its old skin, head first. There is an interval of 18 minutes before the first and last photos.

The old head remains on top of the new head for a while before getting discarded and falling to the ground. The new head will have its typical black and yellow stripes after the hardening of the exoskeleton. The same goes for the legs and prolegs.

It is only after the exoskeleton has hardened that the Monarch caterpillar will turn around and eat its shed molt.

Monarch caterpillar molting,
new head now visible

Body darker and in straight position

New head out, dark line along body

Upper body larger, caterpillar slips
out from lower end

1st pair of legs out, but still folded

1st pair of legs now visible

The 3 pairs of legs are now out

Holding onto the leaf with the new legs

Body slips further from old skin end

Mid-body out, head tentacle visible

1st pair of prolegs visible,
old skin shrinking

2nd pair of prolegs visible

3rd pair of prolegs out, old skin
shrunk and darker

3 pairs of prolegs now holding
onto the leaf

4th pair of prolegs coming out

Monarch caterpillar just slipped
out of old skin

The old head finally came off

Waiting for exoskeleton to harden