For a sample of Herbig stars, the sub-mm flux at 1.3 mm (normalized at a distance of 150 pc) is compared to the MIR spectral index given by the flux ratio at 30.0 and 13.5 μm. Filled symbols indicate sources for which gaps have been reported in the literature. The sub-mm flux can be used as a proxy for the disk mass while the MIR spectral slope may trace gaps in the temperature range T ~ 200–400 K. Because gaps are found in most group I objects, the evolutionary link from group I to group II is no longer obvious.