Have to admit I may have missed the point of the original question there DP – not a first! Still not sure if it refers to different species or variations in the same species.
I don’t think there’s likely to be any speciation - neatly illustrated and discussed
here - in Britain as the habitat is pretty uniform .Variation in individual species does exist though and I suppose that offers an evolutionary advantage in itself.
One of our of the monitors from the group I’m with sent me some pictures of vivid orange and red coloured heptagenids from the Afon Tyweli which I reckoned to be a Photoshop wind up until I rang him. Since then I've seen an equally deep red hept in the Afon Cneifa myself. I was surprised at the size of these nymphs, you'd think they'd stand out like a sore thumb to the trout but they'd obviously survived well.
I've also noticed variation in BWO nymphs too, some are jet black, and others have lighter and blotchy black/white markings. Whether this is variation or the result of instars is open for debate.
I haven’t spent any time on the adults really so I couldn’t say if there’s much variation in colour amongst those – I suppose there is to some extent, wing patterns certainly aren’t uniform in distinct species.
Haven’t seen a red BWO yet though!
May Fly Colours
to explain it well but in my rivers may flies are same every year....so if it is white one year it is white next year on same river
Linear Mode
