Abstract: Ultrastructure of the indirect flight muscles of a migratory moth, Oriental armyworm Mythimna separata (Walker) was examined with electron microscope. In cross section, the polygonal, close-packed elliptical myofibrils each has a diameter of 0.8-1.0#m and consists of 500-700 myosin filaments. Thick and thin filaments form a hexagonal lattice with the thin filaments midway between the adjacent thickfilaments with ratio actin and myosin in 3:1. In longitudinal section, myofibrils exhibit regular, well aligned sarcomeres (sarcomere length about 2.2-2.6μm, A-band about 2.0 -2.3μm) divided by homogeneous and continuous Z lines. Mitochondria are intercalated between myofibrils and completely filled the interstices between them. There are usually 3 mitochondria and 2 T-tubes along each sarcomere. Mitochondriaand T-system are estimated to be about 40% and 7% respectively of the fiber'stotal volume. T-tubes situate at midway between Z lines and middle of each sarcomere and make contacts with sarcoplasmic reticulum (SR) to form dyads and triads wherein. The SR is lesser in quantity as compared with the T-system and occupies only about 2.5% of the fiber's total volume. The characteristic feature of the SR is that there is a layer of SR cisternae transversely penetrating myofibril halfway between Z lines in every sarcomere. Ultrastructure of the flight muscle fiber of M.Separata moth is similar to those of fast acting insect muscle fiber in many aspects even though it may show some variations as the moth's age changes.

Key words: Mythimna separata, flight muscle, myofibril, sarcomere, sarcoplasmic reticulum, T-system.