The earliest insects, which strongly resembled today's silverfish, lacked metamorphosis and their juveniles looked very much like adults except that they didn't have functioning genitalia. After the evolution of flight, more advanced species, such as cockroaches and grasshoppers, developed incomplete metamorphosis. Their immature stages, called nymphs, still resembled the adults except that they lacked genitalia and bore wing buds that only transformed into functional wings during the molt to the adult stage. In both cases, the insects molt, or shed their external skeletons, several times as they grow to adults. The higher insects, species with complete metamorphosis, spend their juvenile life as larvae that bear no resemblance to the adults.
Juvenile hormones suppress the development of adult structures.
In insects with partial or no metamorphosis, the absence of JH during embryo formation and development allows the embryo to become a miniature version of the adult. In embryos of insects with complete metamorphosis, Truman and Riddiford said, there is an early appearance of JH that suppresses some of the adult-directed growth and promotes formation of the larval stage. Juvenile hormones remain as the larva grows, then disappear to allow growth of imaginal discs, which will give rise to specific adult structures. A complex interplay between JH and ecdysteroids then allows the larva to progress to a pupa, and finally ecdysteroids alone drive the transformation to adult. Juvenile hormones play such an important role in the embryonic and larval development of metamorphosing insects that they have been used as the basis for insecticides.