Which of the following is not a stage of complete metamorphosis?

In the context of insect development, complete metamorphosis, also known as holometabolism, consists of four distinct stages: egg, larva, pupa, and adult. The larva and pupa stages are critical components of this process, where larvae often have different feeding habits and physiology compared to the adult form, and pupae represent a transformation phase where the organism undergoes significant reorganization.

The term "naiad" refers to the immature aquatic stage of certain insects, particularly in groups like dragonflies and mayflies, which undergo incomplete metamorphosis, also known as hemimetabolism. In this type of development, the nymphs (naiads) resemble smaller versions of adults and undergo a series of molts without a distinct pupal stage.

Thus, naiads are associated with incomplete metamorphosis and do not belong to the stages of complete metamorphosis, making this option the correct choice. The other stages—larva, pupa, and adult—are integral to the cycle of insects that undergo complete metamorphosis.