THE FINE STRUCTURE OF THE VENTRAL INTERSEGMENTAL ABDOMINAL MUSCLES OF THE INSECT RHODNIUS PROLIXUS DURING THE MOLTING CYCLE : II. Muscle Changes in Preparation for Molting

The development of the ventral intersegmental abdominal muscles of Rhodnius prolixus is triggered by feeding. The early muscle (1 day after feeding) contains essentially nonstriated fibrils. However, in cross-sections, areas indicating early I bands, Z lines, and A bands can be recognized. Interdigitating thick and thin myofilaments do not assemble into a precise lattice until sometime between 4 and 5 days after feeding. As development continues, the number of fibrils increases, the region corresponding to the Z line increases in density, and the fibrils contain more recognizable striations. The newly formed fibrils broaden as myofilaments are added peripherally. At all stages throughout development, the ratio of thin to thick myofilaments is always 6:1. The formation of fibrils in the abdominal muscles of Rhodnius is different from that in chick embryo skeletal muscle. The major differences are that at all stages in Rhodnius there are (1) a constant ratio of thin to thick myofilaments, and (2) detectable Z-line material. Other findings in Rhodnius suggest (1) that fusion of mononucleated cells with the multinucleated muscle cell occurs, (2) that microtubules develop in the tendon cell concomitantly with development of myofibrils in the associated muscle cell, and (3) that filaments 55A in diameter aggregate into microtubules.