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I help with my local online biodiversity citizen science recording in central lowland France (Obs'37). Recently we've had a lot of Chrysotoxum posted and I wanted to double check that the 2010 edition of van Veen's guide to hover flies was the best resource I could be using for this genus. I know they've been a bit of a nightmare in the past. I also wanted to know how far one can assume that if one has Chrysotoxum low down on grass then it is C. arcuatum, and if it is up on a leaf in a bush then it is C. cautum? Is that consistent enough to be diagnostic? Thanks in advance for your input and engagement.