SiO emission (in units of T_mb) of the six MDCs studied. For each MDC we show, on the left, a greyscale showing the total integrated SiO emission and, on the right, three overlaid spectra extracted from the datacubes. The grey, blue, and red circles in the left-hand panels show the positions of the grey, blue, and red spectra displayed on the right. The grey spectra exemplify the narrow profile of the SiO emission, and the blue and red spectra are taken at positions where there is significant wing emission. Sources and outflow directions are as labelled in Figs. 2 and 3.