Because of their rigid exoskeleton, insects can grow only by periodically shedding their exoskeleton - called molting . Molting occurs repeatedly during larval development. At the final molt, the adult emerges. In several insect orders, the adult looks entirely different from the larva that preceded it. This marked transformation is called metamorphosis .
Metamorphosis takes place during a dormant stage called the pupa .
Molting and pupation require the hormone, PTTH, secreted by a cluster of cells in the brain of the larva. If these cells are cut out of the brain of a full-grown larva, pupation does not occur. PTTH does not drive pupation directly but, as its name suggests, acts on the prothoracic glands . There are two prothoracic glands located in the thorax. Under the influence of PTTH, they secrete the steroid hormone ecdysone .