The difference between the Blue Winged Olive Dun & the Tiny Blue Winged Olive can be extremely confusing to anyone who isn't an expert in aquatic entomology. Many fishermen mistakenly assume they are the same.
The insects are similar in color, but are of different biological Families. The BWO Dun is in genus:Ephemerella(as is the Hendrickson), is larger and prefers runs, riffles and pocket water.
The Tiny BWO is in genus:Baetis and range from size 18 to MUCH smaller. The Tiny BWO prefers the tails of gravel runs and heavy vegetation, one of the reasons they are so numerous in spring creeks.
If there is one latin phrase you should learn when it comes to mayflies, it is Baetis. When you hear fishermen or shops say the Beatis is hatching, it is the Tiny BWO. NOW, if they say it is a size 14 Beatis that is hatching, it is the BWO Dun that you should be fishing.