They appear to be made of something like Spandex: I don't think they would stop a mosquito if the fabric were against the skin. The DG gaiters I have don't lie on the skin however, they sort of blouse out except where the elastic at the top gathers them in. If one were to tuck ones pants into the top of the gaiters I suspect that it would take a pretty tough mosquito to get through either the gaiters or the junction.
May I walk in beauty.