What Causes Circular Dry Patches On Legs?

I have circular dry patches on my legs. They are very dry and itchy. Some are as big as a nickel. I've been having these patches on my legs for 3 years now. They don't itch all the time but every so often I get the urge to itch them. Each place that the patches are seemed to be the same areas where a mosquito has bitten me. What could this be and how can I treat it?