Air box cover - Second filter in cover??

I was checking the status of my air filter this weekend and when I popped the airbox cover off I noticed a second filter that seems to be integrated into the cover of the box. I have the normal filter with the weird gasket sitting in the lower airbox, but there seems to be a second filter in the cover.

1. What is this thing?
2. Do I need to change it?
3. I can't seem to find a part number or replacement schedule.

I will try to get a quick picture at lunch today.

2007 Toyota Prius base w/ option 3. 130,000 miles.