Abstract: The development of thoracic dorsolongitudinal muscle in the female adult Oriental armyworm, Mythimna separata (Walker) reared at 23℃ , aged from emergence to sixteenth day of the adult life was studied through quantitative and qualitative analysis of elecron micrographs. Myofibrillar diameter, mitochondria and T-system volume fractions of the muscle fiber increased from 0.7μm, 35% and 2.5% at emergence or 1-day-old to 1μm, 45% and 8.5%, respectively by day 7 of the adult life, while sarcomere length short- ened from 3.0μm to 2.2μm during this period, but the hexagonaI organization of myoffiaments and the number of thick filaments per myofibril remained unchanged.Thereafter, the myofibrillar diameter, mitochondria and T-system volume fractions become decreased and the sarcomere length increased gradually. Thin filaments and the hexagonal arrangement of myofilaments disappeared and the number of thick filaments per myofibril reduced from about 600 to 450 by day 10 of the moth life; they are indicative of muscle degeneration. Developmental changes of the dyads and triads in volume fraction of T-system of the muscle varied: the ratio of dyads was high when the volume fraction ofT-system was low while the ratio of triads was high when that of T-system was high. These results indicate that ultrastructural changes do occur in flight muscle development of the moth, even though variation in volume fractions of myofibrillar and sarcoplasmicreticulum of the muscle fiber shows no obvious trend. Flight muscle development seems to be related to flight performance and ovarian development in female moth of M. separata when their relationship is investigated in light of published findings. The moth's flight muscle grows and flight capacity increases as its ovary develops, but its flight muscle degenerates and flight capacity decreases after the moth begins laying eggs.

Key words: Mythimna separata, flight muscle, myofibril, sarcomere, mitochondria, T-system