I have spent the best summer seasons of my life as a field fisheries biologist collecting data and observing fish and aquatic insects in thier natural habitat. In that time, I have snorkeled numerous rivers counting and identifying fish and benthic macroinvertebrates. My most memorable time was watching insects behaving naturally under water. On a few occasions, I was blessed to have taken a front row seat in witnessing emergence activity and how fish feed during a hatch. In all that time I have spent spent underwater, I have seen very few living nymphs drifting upside down for more than just a few seconds in the current. Living naturals of most species of aquatic insects are remarkably adept at remaining in a dorsal up position despite the heaviest of currents tossing them about. If naturals do get inverted, they are quickly self righted. However, giant Salmonfly nymphs are the most prone to prolonged inverted drifts. More on that later.
I have heard there is a video floating around out there somewhere illustrating jig flies tumbling in the current in all orientations. However, physics will dictate a jig weighted with a slotted tungsten bead will remain in a mostly hook point up position while occasionally tumbling along in the current. This of course depends on current speed, line tension and bead weight.
Because weighted jigs can be prone to a limited degree of tumbling -- depending on where and how you fish them -- I have gone back to the “tied in the round” philosophy adopted almost a half century ago by Major Charles E. Brooks who also observed salmonflies in the Yellowstone River occasionally become inverted and fish avoiding them. Subsequently, Brooks came up with his tied in the round patterns such as the Brooks Stone Nymph.
My pet peeve is focused on perdigon jigs that are tied upside down with the wingcase painted on the side of the fly that is going to drift in a dominant downward position. Brooks observed fish will refuse upside down nymphs. It just makes no sense and entirely unnatural to me to present an upside down fly to the fish based on my field observations and that of my elders.
To ameliorate my pet peeve of upside down jigs, I have omitted the wingcases altogether and have done away with the issue of fly orientation altogether. Problem solved!
Now, let's talk about all those videos documenting unusual hominids meandering around leaving giant footprints in the woods…
You can read a short info on Charles Brooks in the round philosophy here.
Note: Perdigon nymphs are designed to fish the fast turbulent sections of rivers where naturals are most prone to tumbling. So in all fairness, it doesn't really matter how the fly is oriented as the fish only have an instant to decide to take the fly or not. I would just avoid using them in the slower sections of rivers where fish can have a better look.