identification of hole cover

My slider windows have holes through them at the bottom. The plastic piece on the bottom snaps in and covers the the bottom hole. The upper plastic piece is door-like in that it is two pieces and swings open from the bottom. Crazy! Maybe a drain? Whatever it is, I am missing some and insects can get in through the holes. The size is about an inch long and 1/4th inch high. I await your reply. Ann