What is the primary characteristic of incomplete metamorphosis?

The primary characteristic of incomplete metamorphosis is the absence of a pupal stage. Insects that undergo incomplete metamorphosis typically progress through three life stages: egg, nymph, and adult. Unlike complete metamorphosis, which includes a distinct pupal stage where significant transformation occurs, insects with incomplete metamorphosis develop gradually as nymphs, resembling smaller versions of the adult stage.

As nymphs, they typically undergo several molts, allowing them to grow and develop without entering a pupal form. This characteristic is particularly evident in groups such as grasshoppers and cockroaches, where the nymphs continue to resemble adults more closely with each molt.

In contrast, options involving two pairs of wings or all changes occurring at once pertain to different aspects of insect morphology or development and do not define incomplete metamorphosis. Growth through instars is indeed a feature of many insects, including those with incomplete metamorphosis, but it is not as definitive a characteristic as the absence of a pupal stage. The distinct stages and gradual development typify the life cycle of these insects, underscoring why the absence of a pupal stage is central to understanding incomplete metamorphosis.