Mesh fly screens for windows sound like an excellent idea but they are not without problems! Firstly, as window handles are usually deeper than the frames and protrude past the frames, they can be difficult to attach without the window handles poking out! And secondly, don't forget that you will have to pull them away from the self adhesvive "velcro" type self adhesive touch and close tape, with they are attached, every time you wish to open or close the windows, which can be a bit of a nuisance!
Thirdly, although they usually come with strips of self adhesive "velcro" (touch and close tape) with which to attach them, these strips are usually EXTREMELY narrow, so it is VITAL to ensure that surfaces are scrupulously clean before you even attempt to apply the "velcro"! If you fail to do so, the "velcro" will not stick properly and will pull away from the frame.