sSFR of the cluster members. The dot-dashed line represents the expected sSFR vs. M_∗ relation for a 24 μm source of 0.2 mJy flux density (i.e. at the completeness limit of our Spitzer observations), obtained in Biviano et al. (2011) using the relations of Lee et al. (2010) and Kennicutt (1998). The dashed line corresponds to the relation of Oliver et al. (2010) for galaxies from the Spitzer Wide-area InfraRed Extragalactic Legacy Survey in the redshift range 0.2 <z< 0.3. The galaxies in A983 and 1731 are plotted in the top and bottom panels, respectively.