instar

thumb|300px|Imperial moth (Eacles imperialis) development from egg to pupa, showing all the different instars

An instar (; ) is a developmental stage of arthropods, such as insects, which occurs between each moult (ecdysis) until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, changes in the number of body segments or head width.