Last summer I noticed minuscule reddish bugs smaller than a pin head crawling around on my 2 front windowsills, and now they're back! When I smush them, they leave a little reddish/orangish smear. I can continually vacuum my windows & windowsills, but they keep coming in. My place was built around the early 1970's I think, so the sills are old. What are they, and how can I get rid of them for good? Thanks.