Which stage of development do immature forms undergo in gradual metamorphosis?

In insects that undergo gradual metamorphosis, the term "nymph" refers to the immature stages that follow the egg phase. During this phase, nymphs typically resemble smaller versions of the adult insects but lack fully developed wings and reproductive organs. They go through several molts as they grow, gradually developing into adult forms. This process is distinct from complete metamorphosis, where there is a distinct pupal stage.

The nymph stage is crucial for development, as it allows for growth and maturation, facilitating the transition into the adult life stage. This is why nymphs are central to discussions of insects with gradual metamorphosis, sometimes termed hemimetabolism.

In contrast, naiads refer specifically to the aquatic immature forms of certain insects like dragonflies and mayflies, and are not a broader term for all insects undergoing gradual metamorphosis. The egg stage is the initial phase of development and does not involve any characteristics of the mature insect. The pupa is pertinent to insects undergoing complete metamorphosis and does not apply to those that develop through gradual metamorphosis. Thus, the nymph stage is the most accurate descriptor of the immature forms in gradual metamorphosis.