With a few odd exceptions, all insect life begins as an egg. After leaving the egg, an insect must grow and transform until reaching adulthood. Only the adult insect can mate and reproduce. The physical transformation of an insect from one stage of its life cycle to another is called metamorphosis.
Insects may undergo gradual metamorphosis, where transformation is subtle, or complete metamorphosis, where each stage of the life cycle appears quite different from the others. In some insects, there may be no true metamorphosis at all. With regard to metamorphosis, entomologists divide insects into three groups – ametabolous, hemimetabolous, and holometabolous.
Illustration by Debbie Hadley, using drawings from Insects - Their Ways and Means of Living by Robert Evans Snodgrass, U.S. Bureau of Entomology. These drawings are in the public domain.