INCREDIBLE INSECTS: A Celebration of Insect Biology
The growth of larval insects, their molting cycle, and their metamorphosis to pupa and adult are controlled by hormones. The brain (yes, insects have brains) secretes a hormone called PTTH via a pair of glands called the corpora cardiaca. This hormone then stimulates the prothoracic glands to secrete the molting hormone, ecdysone. Each time ecdysone is secreted, the larva undergoes a molt.
During larval life, another hormone, called the juvenile hormone, is secreted by the corpora allata. As long as juvenile hormone is present, a larva molts to a larger larva. When the larva is fully grown, juvenile hormone secretion stops, and in its absence the larva now molts to a pupa or an adult.
The life cycle of insects is divided into two phases: the juvenile larval phase, during which they feed and grow, and the adult phase, during which they disperse, mate and reproduce.
Insects can be divided into two major groups, depending on the type of change they undergo. Insects with complete metamorphosis first molt from a larva to a pupa and then to the adult form, whereas insects with incomplete metamorphosis molt directly from larva to adult.
Contrary to popular opinion, during metamorphosis the inside of a caterpillar does not liquefy. Instead there is a lot of remodeling of various internal organs, and the growth of structures that will give rise to the wings and legs of the adult. Tiny wings, called imaginal disks, are actually already present under the skin of the caterpillar.