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The NBN Gateway maps are to some extent a function of the distributions of both diptera and dipteriists and for some flies this is fairly obvious (for example the big cluster in the midlands in this map https://data.nbn.org.uk/Taxa/NBNSYS0000030349
is probably just the result of a particularly active recorder in Nottinghanshire!). However, the map for N. ferox does seem to show a pretty clear bias towards the south-east of England and an avoidance of the highland areas in the south-west and in Wales. I would expect this to be fairly rare in Cornwall but with global warming they might be becoming more common.