Control of growth and ultrastructural maturation of a cricket flight muscle.

The metathoracic dorsal longitudinal muscle (DLM) in the cricket Teleogryllus oceanicus increased in mass and rapidly acquired interfibrillar tracheoles and an increased proportion of mitochondria around the time of the adult molt. Both neural input and juvenile hormone levels were investigated as possible factors controlling this rapid maturation. Motor axons to the muscle were cut early in the last nymphal instar, and muscle growth slowed but ultrastructural maturation continued; the percentage of muscle volume occupied by mitochondria tripled and tracheoblasts invaded the fibers in both the denervated and contralateral innervated muscles. Newly molted last instar nymphs were treated with methoprene, a juvenile hormone analog, and examined four days following the next molt. Muscle growth was slowed but not stopped. Both mitochondrial proliferation and tracheoblast formation were completely blocked by hormone treatment. This study shows that both neural input and low levels of juvenile hormone are required for muscle growth. However, ultrastructural maturation seems to depend exclusively on low levels of juvenile hormone.