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I'm not going to list the above generalisation to attempt to steal your thunder but its very difficult to nail this one from the pic. Without length or body colouration its extremely difficult to tell...but it might be Hydropsyche contubernalis which is particularly common in Scotland in late summer.
This one of a dozen or so photo's I have taken over the months of caddisflies in our light trap but have not keep any specimens.
I have just got a copy of the FSC "Guide to adult caddis flies .............. " and thought I would have a go seeing if any were identifiable from just the photo's
I will try and look at them in more detail from now on.
This one will have to be put down as a 'maybe'.
Wildlife of Assynt
Lat/Lng: 58.1, -5.3
OS grid ref: NC0720