Left panel: three-colour composite image with Spitzer MIPS 70 μm in blue, Herschel PACS 160 μm in green, and Herschel SPIRE 350 μm in red; the density contours are the same as those in Fig. 9. Middle panels: dust emission distribution for image pixels in areas covered by the UMS+PMS and PMS-only classified elements, and by the same number of randomly selected unclassified elements. A Gaussian fit is plotted and the mean of the fit is indicated. Right panels: dust emission vs. young stars number density for the PMS and UMS+PMS classified elements. The error bars show the typical standard deviations within the dust emission bins.