Ephemera danica. E. vulgata. E. lineata. I'm lucky enough to have all three of the large Mayfly species, living in and around this region of France.
Whilst in England, E. danica is the most common species, in this area E. vulgata seems to be more common, or at least the one I see the most. E. lineata is not over common but found often enough if you are looking for it, and know what to look for?
I don't profess to know a great deal about Entomology, but enough to get by, and more importantly what to look for when trout are feeding.
For a long time during my early fishing years, I would tie on a dry fly at any sign of rising fish, now however I can read the rise forms better. Trout seem to spend a long time taking emerging insects on my local river, and it takes a very big hatch of Mayflies to make them lose all there caution, and take the fully emerged duns!
I expect this is down to were the most food is found, and how much effort they have to excert to eat it. It may also have something to do with sunlight too, in this region we get many bright, hot days during the summer, perfect weather for big fish stalking, but not good for hatches or for trout taking insects off the surface.
However when your in that cold chalk stream water, on a red hot day, with fish bulging on the surface, its all good, oh yes, all good!!