An instar is a stage in the growth of an insect larva. As an example, there are three instars in the growth of a housefly. During each instar it eats and grows. Then it sheds (moults) its outer covering (exoskeleton) and grows some more. After the third instar it changes into a pupa, the next stage in its development. The adult (imago) which comes out of the pupa is the familiar housefly. With many insects, most of the eating and growing is done by the larvae. The function of the adults is to reproduce.