Instar

An instar is a developmental stage of arthropods, such as insects and crustaceans, between molts. During an instar the animal grows and often changes form or coloration after shedding its previous exoskeleton. The term is used to describe the successive stages an organism passes through as it increases in size.

In insects, instars are most clearly defined in animals with complete metamorphosis (holometabolans) where the life cycle includes egg, larval instars, pupation, and adult. In species with incomplete metamorphosis (hemimetabolans), immature stages are nymphs that molt through multiple instars before reaching the adult form. The exact number of instars varies by species and can even differ within related groups.

instars are commonly studied to understand growth rates, development timing, and life-history strategies. Research often records measurements such as body length or mass at each instar, as molts mark the boundaries between stages. While size and morphology typically change across instars, some instars may look quite similar, depending on the species and environmental conditions.

Examples of organisms displaying distinct instars include caterpillars of butterflies and moths, beetle larvae, and many crustaceans. In Lepidoptera, for instance, the larval stage may pass through four or five instars before pupation. Across arthropods, the concept of instars provides a framework for describing growth, development, and timing without requiring a single definitive appearance for each juvenile stage.