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We study the relationship between the mid-IR (5-18 $\mu$m) emission oflate-type galaxies and various other star formation tracers in order toinvestigate the nature of the dust heating sources in this spectral domain. Theanalysis is carried out using a sample of 123 normal, late-type, nearbygalaxies with available data at several frequencies. The mid-IR luminosity(normalized to the H-band luminosity) correlates better with the far-IRluminosity than with more direct tracers of the young stellar population suchas the H$\alpha$ and the UV luminosity. The comparison of resolved imagesreveals a remarkable similarity in the H$\alpha$ and mid-IR morphologies, withprominent HII regions at both frequencies. The mid-IR images, however, show inaddition a diffuse emission not associated with HII regions nor with thediffuse H$\alpha$ emission. This evidence indicates that the stellar populationresponsible for the heating of dust emitting in the mid-IR is similar to thatheating big grains emitting in the far-IR, including relatively evolved starsresponsible for the non-ionizing radiation. The scatter in the mid-IR vs.H$\alpha$, UV and far-IR luminosity relation is mostly due to metallicityeffects, with metal-poor objects having a lower mid-IR emission per unit starformation rate than metal-rich galaxies. Our analysis indicates that the mid-IRluminosity is not an optimal star formation tracer in normal, late-typegalaxies.Comment: accepted for publication on A&

Mid-IR emission of galaxies in the Virgo cluster and in the Coma supercluster