  We explore the properties of early-type galaxies (ETGs), including
ellipticals (E) and lenticulars (S0), in rich environments such as clusters of
galaxies (Virgo and Coma). The L_24/L_K distribution of ETGs in both Virgo and
Coma clusters shows that some S0s have a much larger L_24/L_K ratio (0.5 to ~2
dex) than the bulk of the ETG population. This could be interpreted as an
enhanced star formation rate in these lenticulars. We compare the optical
colors of galaxies in these two clusters and investigate the nature of these
sources with a large L24/L_K ratio by looking at their spatial distribution
within the cluster, by analyzing their optical spectra and by looking at their
optical colors compared to late-types. We obtain 10 Coma and 3 Virgo early-type
sources with larger L24/L_K ratios than the bulk of their population. We call
these sources Mid-Infrared Enhanced Galaxies (MIEGs). In Coma, they are mostly
located in the South-West part of the cluster where a substructure is falling
onto the main cluster. MIEGs present lower g-r color than the rest of the ETG
sample, because of a blue continuum. We interpret the excess L24/L_K ratio as
evidence for an enhanced star-formation induced as a consequence of their
infall into the main cluster.
