Caterpillar Instars

"I hear the term 'instar' often when people talk about Monarch caterpillars. What does this term mean, 'first instar' or 'second instar'?"

A caterpillars' skin doesn't grow. When the caterpillar grows too large for its skin, it must crawl out of its old skin (actually a cuticle, not a living skin)to continue to grow. Underneath its old skin is a loose baggier new skin that will stretch only to a certain point before the caterpillar must crawl out of it.

Looking at the photo with a large caterpillar and a tiny one on the right, it is very evident that the tiny caterpillar on the right could never grow to become the larger caterpillar if its skin didn't grow OR if it didn't crawl out of its old skin and have a larger skin underneath. The smaller caterpillar is first instar. The larger caterpillar is fifth instar, about to become a chrysalis.

Caterpillars crawl out of thier skin four times. This process is called 'molt'.

The time period between molts is called an 'instar'. Because Monarch caterpillars molt four times, they have five instars.

1st instar; hatched out of its egg
2nd instar; after the first molt
3rd instar; after the second molt
4th instar; after the third molt
5th instar; after the fourth molt and before becoming a chrysalis.

The last time it sheds its skin is when it pupates into a chrysalis.

The instar of a Monarch caterpillar is measured by its head capsule, not the size of the caterpillar. If a caterpillar doesn't eat much, it will be smaller during an instar.