Fig. 4. Electron micrographs of transverse (A,D,C,F) and longitudinal (B,E) sections obtained from dorsal opener muscle 271 of immature (A–C) and mature (D–F) females. In immature females, note the presence of numerous tracheoles (Tr), small irregular myofibrils and mitochondria (M). T-tubules (T) and dyads (open arrow) occur at the level of the A-band (Ab). Multiple microtubules (Mt) are present within the sarcoplasm. Elements of the sarcoplasmic reticulum (SR) are rare and not well developed. By contrast, myofibrils of mature females appear well organised and are divided by regular chains of sarcoplasmic reticulum profiles. Comparatively large mitochondria (M) are present at the level of the I-band (Ib), adjacent to the Z-line (Z). N, nucleus. Scale bars, 0.25 µm (C,F); 0.5 µm (A,B); 1 µm (C,D).