  We study the relationship between the mid-IR (5-18 $\mu$m) emission of
late-type galaxies and various other star formation tracers in order to
investigate the nature of the dust heating sources in this spectral domain. The
analysis is carried out using a sample of 123 normal, late-type, nearby
galaxies with available data at several frequencies. The mid-IR luminosity
(normalized to the H-band luminosity) correlates better with the far-IR
luminosity than with more direct tracers of the young stellar population such
as the H$\alpha$ and the UV luminosity. The comparison of resolved images
reveals a remarkable similarity in the H$\alpha$ and mid-IR morphologies, with
prominent HII regions at both frequencies. The mid-IR images, however, show in
addition a diffuse emission not associated with HII regions nor with the
diffuse H$\alpha$ emission. This evidence indicates that the stellar population
responsible for the heating of dust emitting in the mid-IR is similar to that
heating big grains emitting in the far-IR, including relatively evolved stars
responsible for the non-ionizing radiation. The scatter in the mid-IR vs.
H$\alpha$, UV and far-IR luminosity relation is mostly due to metallicity
effects, with metal-poor objects having a lower mid-IR emission per unit star
formation rate than metal-rich galaxies. Our analysis indicates that the mid-IR
luminosity is not an optimal star formation tracer in normal, late-type
galaxies.
