Most insects undergo complete metamorphosis. Each stage of the life cycle – egg, larva, pupa, and adult – looks different from the others. Entomologists call these insects holometabolous (holo = total).
The larvae of holometabolous insects bear no resemblance to their adult parents. Their habitats and food sources may be entirely different from the adults as well. Larvae grow and molt, usually multiple times. Some insect orders have a unique name for their larval forms: butterfly and moth larvae are caterpillars; fly larvae are maggots; and beetle larvae are grubs.
When the larva molts for the final time, it transforms into a pupa. The pupal stage is usually considered a resting stage, although much activity occurs internally, hidden from view. The larval tissues and organs break down entirely, then reorganize into the adult form. After the reorganization is complete, the pupa molts to reveal the mature adult with functional wings.
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.