Number of channels that contain emission above the 3σ noise level in the observed spectrum (Table 4) as a function of the masses of the methyl formate sources. The different symbols represent the different evolutionary stages of the protoclusters: young, intermediate and evolved, as indicated on top of the figure, while the stars shows the methyl formate sources that are not associated with any compact dust continuum core. The horizontal dashed blue line shows the mass threshold of 8 M_⊙.