I’ve read JC’s “Flies Only” series several times and have learned a lot that’s not available anywhere else.
Many of the images illustrate his research on how to imititate a mayfly dun. His photos clearly show that when the dun has fully emerged, the body is above the film and does not show until the trout gets very close.
Many dry flies imitate bugs floating much lower than a fully emergered mayfly dun. We specifically design flies with bodies that are below the film.
The parts of the fly below the film are much more visible than those above and show much more color. Put some flies in a glass of water and see for yourself. Depending on the position of the light, some colors show up very well.
JC’s slant box duplicates a trout’s eye view more realisticly than a fly floating in a glass, but you can see that there are conditions where the trout can see the body color very clearly.
When almost all the light is coming from directly above the fly, all you see is a dark sillouette. As more of the light comes from below or the sides, the color appears.
Since we fish in varied conditions, bright sun overhead, cloudy, low sun, under trees or other dark shadows, I think that the fish can sometimes see the dry fly body colors, so I like to carry a variety.
I wish that it didn’t matter, so I wouldn’t have to worry about size, pattern and color, but sometimes it does.
BTW - I had much more luck early yesterday evening on a #10 orange stimulator and a #12 bright yellow trude, than I did with flies that more closely matched the size or color of the caddis and mayflies that were hatching. However, as the sun set, a switch to a small, brown Quigly Cripple was the hotter ticket.
Doesn’t really prove anything except - sometimes the fish want something and sometimes they want something else.