May suggest you carry a ’ Grey Duster’ , a dry fly, which is used to great effect here in the UK, before, during and after the Mayfly, or I should say, was in my peak fly fishing time during the late 1960s to late 1980s. I am sure those ’ in the know’ these days will still use it.
The Original dressing (source ‘Dictionary of Trout Flies’ by the late A.Courtney Williams’) is:-
Tying silk: brown
Body: Dubbed with light rabbit’s fur, sometimes mixed with a modicum of blue
Hackle: Badger hackle, well-marked with a black Centre and white list
[Note - a larger hook size is recommended for lakes or when there is a full hatch of Mayfly. In smaller sizes #15/16 it is also good when there is a lot of midge activity] Mr A Courtney Williams was almost tempted to be a ‘one pattern’ fly fisherman, it is that good.
A very good friend of mine, Jimmy, only ever fly fished using two dry flies in various sizes - a Grey Duster and a Hackled Coachman ( the Original pattern not the Royal Coachman which I believe is popular in the USA). With those two flies he outfished everybody regardless whether or not we were fishing rivers or stillwaters. I recall one occasion when we were fishing a large reservoir in the mountains of North Wales during a snowstorm. Jimmy was sat there on the bank, covered in snow, puffing at his pipe, intently watching his Grey Duster bobbing on the surface about 10 yards out . He caught the most fish that day!
Sorry I have rambled on a bit but you don’t need to have a wide entomological knowledge. Just put a Grey Duster on!