Building on the comment of @Weather Vane: First, my qualifications to answer this -- mosquitoes love me; I am the best mosquito repellant at an outdoor dinner a fellow diner could have!
I wear my husband's old white cotton shirts when gardening. They are much too big for me, and hence very loose and blousy. White, cotton, looseness, all help with heat. A neck-scarf with sewn-in gel pack fresh out of the freezer also helps with the heat. This means the shirt usually does not stick to my back with perspiration, and if it does, there is always a fresh old shirt and a fresh neck-scarf gel-pack combo. One layer is enough, for me. I live in Virginia, about 20 miles from Capitol Hill.
We don't have mosquitoes yet, so you probably live in a warmer area -- but I hope not with tougher mosquitoes!
As an aside, in the arctic summer, two loose layers are a necessity.
@Weather Vane comments that in scorching weather, two layers are necessary. I have not felt the need for two layers, but I am not out in blazing sun at high temperatures. I can believe that in blazing sun and high temperatures, two layers, both loose, would be better than one.