This happened yesterday when I got a call to "remove a bug that's bothering a guest". Okay, not really my job, but I was curious to see what kind of silly person couldn't remove a harmless bug themselves. And I was curious to see what the bug was. I grabbed a small jar and a net and headed off.
What I found was completely unexpected. No interesting spider. No giant centipede. No weird beetle. Just 50 MILLION FLIES, all congregated in one corner of the room near the floor. Okay, so maybe there were only hundreds of flies. But it seemed like more.
Well, no. At least, I think there is a more parsimonious explanation. After I removed the flies I did a little research. I assumed that they were house flies, but I think they may have actually been something called cluster flies. These insects look a lot like house flies (the difference is just a few golden hairs) but their life-cycle is different. They feed on earthworms, staying out of sight in the soil most of the time, and hatch out at the end of summer. As the weather cools they tend to emerge en masse and head into structures where it's warmer, usually congregating on the interior of the sunniest wall while trying to get inside the wall. Yesterday was much, much cooler than the preceding week. It's the end of August. They were on the interior of the warmest, west-facing wall. Check, check, check. The only thing I didn't discover was what their exact range is, but I think the mystery is solved. Unless anyone else has a better explanation for me?
Thanks to google image search for the fly pictures.