The 8.0/4.5 μm flux ratio versus the 5.8/3.6 μm flux ratio. The small black dots refer to the SWIRE sample analysed by Vaccari et al. (2010, and in prep.). The largest symbols represent the XMMOMCDFS sources with available IR data from the previous sample, with the same meaning as the symbols in Fig. 3. In addition, the crosses indicate X-ray variable sources according to Paolillo et al. (2004, and in prep.). Symbols are also reported in the legend.