ranges: $5\degr < |\ell| \le 65\degr$ and $|b| \le 1\degr$; $2\degr < |\ell| \le 5\degr$ and $|b| \le 1.5\degr$; $|\ell| \le 2\degr$ and $|b| \le 2\degr$, comprising a total of 274 square degrees. The IRAC camera provides images at four filters centered at wavelengths 3.6, 4.5, 5.6, and 8.0 , with an angular resolution of $\sim 2''$.

The GLIMPSE surveys have revealed very peculiar structures in star-forming regions [a summary is provided in Section 2 of @Churchwell2009]. The 8.0  filter is particularly useful to detect the presence of bright fluorescent emission from polycyclic aromatic hydrocarbons (PAHs), which are excited by the stellar far ultraviolet (UV) field, but are destroyed by the harder UV radiation present within ionized gas regions. Thus, PAH emission is often observed from *IR bubbles*, which appear projected as ring-like structures and in many cases are tracing molecular material swept up by the expansion of regions created by the ionizing radiation from massive stars [@Deharveng2010]. On the other hand, *infrared dark clouds* (IRDCs), already found in previous MIR surveys, are seen as extinction features against the bright and diffuse mid-infrared Galactic background. They represent the densest and coldest condensations within giant molecular clouds and are the most 