Optical spectroscopic diagnostic diagrams for the mETGs with available SDSS spectra. We adopted a conservative threshold of S/N > 5 for the lines detection. The solid lines are from Kewley et al. (2006) and separate S.F. galaxies, LINER, and Seyfert; in the first panel, the region between the two curves is populated by the composite galaxies. Triangles are used to mark upper or lower limits. The blue symbols represent mETGs located within the region populated by S.F. galaxies in the [S II]/Hα vs. [O III]/Hβ diagram.