To characterize the changes in the protein content and tissue structure of the claw closer muscle of during the molt cycle, muscles harvested at various stages during the molt cycle were studied using paraffin section histology, electron microscopy, and biochemical methods. The results showed that the histology of the claw closer muscle changed noticeably during the molt cycle. The fibers and myofibrils were fully expanded in the inter-molt stage, and the cross-sectional area resembled an irregular circle or polygon with many nuclei under the sarcolemma. The sarcomeres were generally in register in this stage. During the later pre-molt stage, the cross-sectional area of fibers showed a wide size distribution owing to pro-ecdysial muscle atrophy, and many eroded areas appeared within the myofibrils. In the post-molt stage, the muscle fibers were compacting and shrinking. At the ends of the fibers, some sarcomeres were in a state of super-contraction, leading to the sarcomere shortening its resting length by up to 50%, whereas sarcomeres in the middle region of the fiber showed a "normal" appearance, with a typical arrangement of thick and thin filaments in the inter-molt stage. Biochemical experiments determined that the myofibrillar protein and soluble protein content in the claw closer muscle fluctuated and the results agreed well with the observed structural changes. The results indicate that the claw closer muscle of may increase in length during the molt cycle by adding new sarcomeres to the ends of the myofibrils. These new sarcomeres would begin as shorter sarcomeres when the carapace is very soft immediately after ecdysis. Then, as the carapace becomes hardened, the new exoskeleton would be stretched by the absorption of water and the super-contracted new sarcomeres would elongate to the length of the already-existing sarcomeres within the myofibril, resulting in muscle growth.