An instar is a developmental stage between molts. Most common insects go through 3-6 instars.
Monarch caterpillars, for example, go through several molts, or instars.
The first instar finds the monarch caterpillar pale yellow. At first, they donâ€™t even have the distinguishing stripes of the monarch caterpillar. Second instar caterpillars have distinct stripes. Further instars find caterpillars with legs closer to their heads, different eating patterns, and even different behaviors.
For a fantastic photo series on a monarch going through instars, then pupation, see this photo set.
The University of Minnesota has a great Field Guide to help you identify monarch instars, including identifying features and, best of all, a photo of all 5 instars + an egg together. Itâ€™s impressive.
To watch the instars of a painted lady caterpillar in person, try a Butterfly Student Kit.