Disk luminosity at 1.3 mm (L₁₃₀₀/L_∗) compared with the MIR spectral index F_13.5/F₃₀ ratio. Filled symbols indicate the objects for which the 69 μm feature has been detected. The 69 μm feature is often detected for disks with low L₁₃₀₀/L_∗. The MIR spectral index can be used as a tracer of large disk gaps. The symbols show flaring/transitional disks without silicate features (group Ib, orange circles, ), with silicate features (group Ia, dark green squares, 2.1 <F₃₀/F_13.5< 5.1), and self-shadowed disks with silicate features (group IIa, purple triangles, ). The numbers refer to the objects and are given in Table 2. The grey symbols show the objects without forsterite detections at Spitzer/IRS and Herschel/PACS wavelengths.