The body of an insect is composed of a hardened outer shell or exoskelton that protects and supports soft vital tissues inside.
Insects must shed their exoskelton in order to grow larger and assume a new form. Each time an insect sheds its exoskelton it passes into a new life stage or instar. Differences between instars can often be seen in altered body proportions, colors, patterns, or changes in the number of body segments.
In the photo above, a colony of second instar walnut caterpillars are feeding on pecan leaves. The first instar of walnut caterpillars are olive green, while the 2nd, 3rd and 4th instars have a dark red color. The fifth and final walnut caterpillar instar will be entirely black.
Knowledge of instars is important important for two reasons. First, insects in the early stages of development (early instars) are much easier to control with insecticides than insects near maturity. Secondly, insects often are most damaging when fully sized during their their final instar.