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Is it only the location that allows this to be separated from the two similar species?
L. borealis and L. subcentralis are the other 2 which have the pale lunar crescent, they are the 2 which have the Scottish distribution, possibly even a northern Scottish distribution. I dont have the book to hand to check, its in another house.
From memory I can't really comment on the appearance of subcentralis but L. borealis has a generally quite dark brown rear portion to the wing, at least it does in the photo in Barnard and Ross. The above caddis would seem to match lunatus better in that the pale crescent has a dark edge rather than being placed in a solid dark area of wing.
Its a very valid question all the same as I think there is some variability in the appearance of these species and between the sexes. Also, the distributions may be larger than recorded, I think I might have even recorded borealis here in north Antrim where the book says it isnt recorded.
I'm always very cautious about location based IDs. Once you get beyond the 'popular' groups like birds and lepidoptera there is significant under-recording so ranges could well be badly known.
That said although I do have some caddis literature they are way beyond my skills so I'd certainly not dispute your ID.
Lat/Lng: 55.2108, -1.6042
OS grid ref: NZ252907