Ian,
From another post you made I gather you are new to caravanning. Most, if not all, newer vans as well as having window blinds also have flyscreens either incorporated into the blinds or on a separate roller system between the blind and window. A lot of vans also have a sliding flyscreen on the doorway.
I don't know how easy it would be, or how much it would cost, to retro fit these to a van not already equipped with them. They are handy though, even in this country, you can open your window and keep out the flies and wasps.
Robert