A comparative study on sub-filament organization in primary myofilaments of basalar direct flight and tibial extensor muscles of the lepidopteran, Achalarus lyciades

Abstract

Primary myofilaments of direct flight muscle fibers are hexagonally arranged, are surrounded by six secondary myofilaments, and are composed of two sub-unit-fibrils, 90–120 A in diameter, at mid-sarcomere levels. Primary myofilaments are resolvable into electron-densities 15–25 A in diameter, which number 3–4 in each sub-fibril at mid-sarcomere levels, and number 6–8 elsewhere. Primary myofilaments of tibial extensor muscle are not found in hexagonal arrays, are surrounded by 10–12 secondary myofilaments, and are resolvable into electron densities which are 15–25 A in diameter similar to primary myofilaments of basalar flight muscle. However, a binary sub-fibril structure at mid-sarcomere levels is lacking in tibial extensor muscle fibers.