Fly Hatches on
Eugene P. Macri Jr.
Aquatic and Environmental Scientist
Fly hatches on Falling Spring (Falling Spring Run) are different than most
freestone streams. Because Falling Spring Run is a limestone spring creek the diversity of the hatches will be
limited by the extremely coldwater conditions found there. Furthermore, smaller streams have less diverse
substrates and current speeds and this also limits the diversity of the invertebrate population in the
Fly hatches on spring creeks tend to have longer durations than on freestone streams because usually the insects
build up larger populations in the invertebrate community compared to freestone populations of the same
species. Furthermore, the hatches may be spread out more during the season and on some streams the hatches
will be more intense.
Let's start with the mayflies (Ephemeroptera) and discuss one of the more important hatches on Falling
The Blue Winged Olives
The Blue Winged Olives are an important group of mayflies on Falling
Spring. Most of the Blue Winged Olives are Baetis tricaudatus Dodds if you are interested in the
species. I should know because I've done all the research on these spring creeks.
This mayfly will be from sizes 14-22 on the stream. The early season
varieties will surprise you with their size. As the season progresses the mayflies get smaller. The
Blue Winged Olives have a number of different broods per year on the stream. They tend to come off in cloudy,
and some times nasty weather. They are often hard to see because of the back ground of the sky without a lot
They also are substrate and current specific so they may emerge on one
section of a riffle and not another. They do not come off in a blizzard. They usually emerge off and on throughout
a time period and through out the year.