Stink Bugs

I'd like to know the life expectancy of the Stink Bug. I would also like to know what these things feed on, AND what their purpose on this planet is. It's the middle of December and my house is still littered with the things! I pulled out Christmas boxes today to put gifts in and there's a load of them living in my tissue paper.... if that's not gross, I find them floating in my water cup, in every corner of almost every room, and hopping around the rugs of the house (making a lovely crunching sound when you step on them). I have 3 dogs that adore catching flies but they won't kill a Stink Bug if their lives depended on it. The fact that these bugs are still alive this late in the year is amazing, especially because I have no idea what they're eating in my closet and on my rugs... they better not be laying any Stinky babies, that's all I have to say!