Mass distribution of the methyl formate sources as a function of their parent cloud mass. 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 error bars show the range of masses obtained considering the uncertainties on T_d (300 ± 100K for the most extreme objects and 100 ± 50K for the rest; see Sect. 5.4). The horizontal dotted line shows the 8 M_⊙ mass threshold.