If you find them crawling around on cement, brick, ironwork, or wood materials they're probably harmless spidermites. Harmless to you or I, but can be disastrous for evergreens if enough of them accumulate. 
We have them a lot here on our sidewalk, I just make a point to squish them. I guess a holdback from my childhood, when I'd try and get the longest red smear I could. =P
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~*Taceas*~
rain@mainecoon.net
"And shepherds we shall be, for thee my lord for thee. Power hath decended forth from thy hand so our feet may swiftly carry out thy command. And we shall flow a river forth to thee and teeming with souls shall it ever be. In nomine Patris, et Filii, et Spiritus Sancti." - The Boondock Saints