If you have a shingled house with a 1-2 inch space under some of the shingles (which usually happens in any shingled house) the bats will crawl right up in there and sleep all day. At dusk we'll see them drop out of the shingles and start fluttering about. When we turn the porch lights on, the bugs all gather in the light beams and the bats fly back and forth through the bug-gathering, having themselves an insect massacre. Usually by late July we don't have any more mosquitoes near the house (most mosquitoes usually only fly a few hundred feet from where they hatch.)
I remember when I was a kid, we caught a bat (I think it got in the house somehow.) Dad had it trapped in a transparent container so we could look at it. Up close you could definitely tell it was a mammal, furry and kind of like a mouse. To think, they actually give birth to live young (pups) and nurse them with mother's milk.
According to this internet site, they are the only mammals that can fly (flying squirrels don't count because they don't have the ability to gain altitude - only glide.)
http://icwdm.org/handbook/mammals/bats.asp