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Both the proliferation and differentiation of ventral diaphragm myoblasts are controlled by ecdysteroid during metamorphosis of the moth, Manduca sexta, but the responses have different hormonal requirements. Tonic exposure to moderate levels of ecdysteroid are required to stimulate myoblast proliferation. This is due to the presence of an ecdysteroid-dependent control point in the G(2) phase of the cell cycle. As a result, proliferation can be repeatedly turned on or off simply by adjusting the concentration of ecdysteroid to be above or below a critical threshold concentration. In contrast, high levels of ecdysteroid trigger irreversible proliferative arrest and differentiation of myofibers. Myoblast proliferation and differentiation also differ in their response to the juvenile hormone mimic, methoprene. Ecdysteroid-dependent proliferative arrest and differentiation are blocked by coculture with methoprene but methoprene has no effect on ecdysteroid-dependent proliferation. In the animal, premature exposure to high levels of ecdysteroid in the absence of juvenile hormone triggers precocious differentiation of the myoblasts, resulting in the formation of several thin bands of muscle rather than a complete diaphragm. Thus, ecdysteroid and juvenile hormone collaborate to determine the size and shape of the adult musculature.

The motor system of the moth Manduca sexta is completely remodeled during the pupal-adult transformation (PAT). It is stable until the formation of the pupal stage (0% PAT), but larval motor end plates become disrupted by 5% PAT and are lost by 10% PAT, at the time that the muscle has begun to degenerate. Most of the axonal arbor is retracted by 15% with the first signs of adult sprouts appearing by 20% PAT, coinciding with proliferative activity in the remains of the larval muscle. Extensive growth of the axonal arbor begins after 30% PAT, with an initial phase of rapid longitudinal growth (35–50% PAT) and then the production of short transverse branches that then form sprays of end plates (50–70% PAT). Growth and maturation of the end plates occupies the remainder of metamorphosis. Neuromuscular metamorphosis was interfered with by systemic or local treatment with a mimic of the insect juvenile hormone. The results of these treatments suggest that some aspects of the removal of larval axonal branches requires cues from the target. For the sprouting response, the rapid longitudinal growth over the muscle appears to be due to ecdysteroids acting directly on the cell body of the motoneuron. By contrast, the subsequent production and maintenance of transverse sprouts and the corresponding end plates may be an indirect response to stimulation of muscle growth and differentiation by ecdysteroids.

The relationship between the ecdysteroid titre and eclosion hormone was explored for the pupal and adult ecdyses of Manduca sexta . Ecdysteroid treatment late during either moult caused a dosedependant delay in the time of ecdysis. Sensitivity to exogenous steroid treatment dropped off as the respective moults neared completion and in both cases coincided with the time of the low point in the endogenous ecdysteroid titre. It was concluded that an ecdysteroid decline is a normal prerequisite for the ecdyses of both stages. The steroid drop is important for two aspects of the eclosion hormone system: it causes target tissues to become sensitive to the peptide and it is a prerequisite for the subsequent release of eclosion hormone itself. Thus, the dual action of the declining ecdysteroid titre insures that when eclosion hormone is released, the tissues will be competent to respond to it.

In the tobacco hornworm, many larval motoneurons become respecified and supply new muscles in the adult. Changes in the morphology of one such neuron were examined through metamorphosis. The dendritic pattern of the adult comes about both by outgrowth from the primary and secondary branches of the larval neuron and by the development of new branches that are unique to the adult.