Molting caterpillars

When feeding or tending to caterpillars, it is safest not to move a caterpillar unless it is moving its rear legs itself. If it isn’t moving, you can gently tickle the rear of the caterpillar. If it isn’t molting, it will normally move.

Although some species do fine if they are moved when their legs are locked in the silk, moving some species at this time is a death sentence. Monarch caterpillars normally do well when moved while they are molting.

Although caterpillars grow, their skin/cuticle does not grow. This would present a dilemma if it was not for the fact that caterpillars molt.

Monarch caterpillar molting

Monarch caterpillar molting

At a certain point in growth, they lay a mat of silk on an object and lightly lock their legs into it. They release enzymes that dissolve the inner layer of their cuticle. After about a day, the caterpillar’s cuticle splits above the thorax and the caterpillar literally crawls out. Underneath is a new cuticle, a bit baggier and will fill out to a larger size.

It is fairly obvious when a caterpillar is preparing to molt. They withdraw their heads from their head capsules, causing a larger bulge than normal behind the head capsule.

Monarch butterfly caterpillar preparing to molt

Monarch caterpillar preparing to molt

The old head capsule sits on the face of the caterpillar, much like a doctor’s mask.

Monarch butterfly caterpillar molting

The scrunched up old cuticle is at the rear
The old head capsule is still on the front

Monarch butterfly caterpillar preparing to molt

Monarch head capsule is tiny and black
New head capsule is larger and yellow
When the new head capsule dries, it will be black

The caterpillar literally crawls out of its cuticle, raising its hind prolegs high to completely leave the old cuticle behind.

Monarch releasing its anal prolegs from its old skin

Monarch releasing its anal prolegs
from its old skin

Monarch caterpillar removing its head capsule

Monarch caterpillar removing
its head capsule with its legs

Some species will eat their old cuticle. Others will leave it behind.

Gulf Fritillary butterfly caterpillars eats its old cuticle after molting

Gulf Fritillary butterfly caterpillar
eats its old cuticle after molting

Sleepy Orange butterfly caterpillars normally do not eat their old cuticle after molting

Sleepy Orange butterfly caterpillars
normally do not eat their old cuticle

A few species leave their old cuticles behind in this manner (below). The old cuticles look as if the caterpillars died and are just dried up, hanging from the plant.

Mourning Cloak butterfly caterpillars leave their old cuticles on the limb after molting

Mourning Cloak caterpillar’s old cuticles
Left behind after molting appear to be
dead caterpillars hanging from the limb

Many species leave their host plants to molt. This can cause confusion to enthusiasts who find a new-to-them species of caterpillar and aren’t sure what to feed them. Gulf Fritillary caterpillars are notorious for molting off their plants.

Gulf Fritillary butterfly caterpillar molting on grass seed head

Gulf Fritillary caterpillar molting
on a seed head of grass.
Passion vine is their host plant.

Imperial Moth caterpillar molting

Imperial Moth caterpillar molting
The head capsule is on the tip of its head

Luna moth incomplete moth

A Luna Moth caterpillar failed to molt properly
A band of old cuticle is constricting the caterpillar
The band was removed to save its life

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