I think this is a bristle fly but I am more interested in what appear to be spores on it, or maybe it is just dust or mildew? This fly has been stationary on this fly screen for over a week. The first two photographs are the most recent while the second two are from a week earlier. I don't see any difference in the amount of 'dust' on the fly but it is still on the screen despite the wind we have had recently. I did not really take much notice when I first photographed the fly but I have put more effort into the more recent photographs to see if I could get more detail. Does anybody have any idea what is happening please? Many thanks.