Activation of nonmuscle myosin is essential for correct morphology of the ARSs. (A–I) Nerve cords were dissected from third instar larvae carrying MLC Sqh (Sqh-GFP) under its own promoter (A–C) and myosin heavy chain Zipper (Zip-GFP; D–F) or a dominant-negative form of Zipper fused to GFP (DNZip-GFP; G–I) both driven by the Moody-Gal4 driver. A, D, and G show GFP labeling, B, E, and H show phalloidin labeling, and C, F, and I are the corresponding merged images. The arrows in A–F show the ARSs and indicate overlap staining between phalloidin and myosin labeling. The arrows in G show dissociation of Zip-GFP from the ARSs and in H show abnormal morphology of the ARSs. J–L show a cross section of third instar larvae expressing moesin-GFP in SPG cells labeled with anti–P-MLC (red) and GFP. Their merged image is shown in L. An overlap between the ARSs and the P-MLC staining is observed (arrowheads). M and N show the border between two SPG cells of nerve cord dissected from larvae expressing the Ca²⁺ indicator GCaMP3 in SPG cells (M) and labeled with phalloidin (red; N). An overlap between the phalloidin staining representing the ARSs and the fluorescence of the Ca²⁺ indicator is observed (arrowhead).