In simple metamorphosis, what is each developmental stage referred to as?

In simple metamorphosis, the developmental stages of an insect are referred to as "instars." An instar represents a specific period of growth between two molts, where the insect sheds its exoskeleton to accommodate its increasing size. During this time, the insect does not undergo significant changes in body structure – maintaining its overall form as it grows. Each instar can vary in size and characteristics, but the basic body structure remains consistent throughout these stages.

The term "instar" is essential because it distinguishes the life cycle stages of insects undergoing simple metamorphosis from those with complex metamorphosis, where distinct forms like larva, pupa, and adult are observed. The other choices, such as phase, stage, and form, while they might be used in broader biological contexts, do not specifically capture the concept of developmental growth as accurately as "instar" does in this context.