What is an "Instar"?
The Five Stages of Caterpillar Growth

Caterpillars go through 5 stages of growth. Each stage is called an "instar."

As a caterpillar grows, it "molts" 5 times before it becomes a chrysalis. Each time it molts the caterpillar progresses to the next instar (1st instar, 2nd instar, 3rd instar, 4th instar and 5th instar).

Its skeleton is on the outside of its body, like clothes. So, as it grows, it can no longer fits in its skin.

Look! >>
Can you see the shedded skin behind the caterpillar?

But as Dr. Lincoln Brower explains, the analogy of growing out of clothes doesn't fit exactly:

"The caterpillar doesn't just shed that skin, it digests and reabsorbs most of it. Before the skin starts shedding it does get tight. But it doesn't just slip off. What happens is that the cells beneath the skin start releasing enormous amounts of enzymes and actually absorb most of the skin. Before it's shed it becomes a thin sheen over the body. So what is shed is just a thin outer part of the cuticle. You can see how it looks when shedded (see picture to the right). Sort of like a snake's skin. So a snake skin analogy is really much better."

Some definitions:

  • Cuticle--the proper name for the skin, or exoskeleton, of the caterpillar.
  • Epicuticle--a waxy layer on the outside of the cuticle that protects the caterpillar from water loss.
  • Chitin (pronounced "kite in")--found in the cuticle, this is the material that makes the exoskeleton of an insect strong and lightweight, and protects its soft insides. Chitin can be thin and pliable, as it is in the monarch caterpillar stage. Or, if thick, chitin protects the insect with a hard, rigid exoskeleton.