Pupation is the initial stage of pupa development. It starts shortly after the larva stopped feeding. When the larva reached a certain size, the levels of juvenile insect hormone start to drop, the larva stops feeding and moving and its skin starts to change in color and texture. Moth caterpillars produce a silky cocoon using special glands. Butterfly caterpillars fasten their body to a leaf using the same silk glands.
When the larva is fully enclosed in the cocoon, chrysalis or other type of pupal case, its body starts to suffer drastic transformation through a process called histolysis. Larvae have specialized cells called transformative, which remained inert throughout their lives. However, once the pupation process started, these cells are activated initiating several biochemical processes that breaks down the internal structure of the caterpillar.
Histogenesis is the formation of organs present in the adult animal, such as wings, legs and compound eyes. After the breaking down of the larval structure, cells called histoblasts start to reshape the new body. These histoblasts can differentiate into various types of cells, which will form distinct organs. The histogenesis period varies according to species. In the moth of the genus Manduca, the eyes start to form three days after pupation while the wing pigmentation occurs from 15 to 18 days after pupation.
Once the adult body is formed inside the pupa and the metamorphosis is completed, the insect breaks the case and eclodes. Among the factors that help to trigger eclosion are changes in temperature or light and liberation of new hormones inside the insect's body. The total length of the pupal stage depends on quantity of light exposure in most butterfly and moth species.