Metamorphosis (Caterpillars, Moths)

Abstract

Growth and metamorphosis in insects is punctuated by a series of molts that allow abrupt increases in body size and morphology. During a molt the epidermis detaches from the overlying cuticle, a new cuticle is secreted, and the old cuticle is finally shed at ecdysis. In insects that undergo complete metamorphosis, such as moths, the transition from larva to adult occurs during two sequential molts, first to the pupa and then to the adult (Fig. 1). The larval stages have reduced visual and olfactory organs and an elongate, soft body supplied with a complex array of muscles that maintain the tone of the hydraulic skeleton and are used for locomotion. The larvae possess internal imaginai discs, discrete collections of epidermal cells that later form some of the adult cuticular structures such as legs and wings. The first metamorphic molt results in the pupal stage, which is characterized by a rigid exoskeleton and immobile head and thoracic appendages. With the exception of the intersegmental muscles that move some of the abdominal segments of the pupa, the larval muscles degenerate during this transition. During the subsequent molt from the pupa to the adult, new adult muscles differentiate, and complex sensory structures such as the compound eyes and antennae form as do locomotor structures (legs, wings, etc.). After the moth ecdyses from the pupal cuticle, the intersegmental muscles, the last remnant of the larval musculature, degenerate.