Given to Fly

I have no idea what the deuce is going on, but our house has been invaded by dozens of house flies in the last week.  The first night, I killed 6-ish.  Then the second night, I lost count but at least 8 were slaughtered.  The next morning I killed another one.  L came home Thursday night and saw at least 3 and I killed 3 more.  We don't know where they are coming from or why, but it is so gross!!  Our best guess is that either they are coming in through the chimney (the flue has been open because we've had fires in our fireplace lately due to the cold weather) or that they are getting in through our windows (the seals aren't great on all of them).  The weirdest part is just that there are so many of them.  Sick.  But at least they've been easy to kill!  Rolled up newspaper is my new best friend.  If anyone has any ideas about why and how in the world these gross, unwanted guests are getting in, let me know!