Last year, I was fishing on Oregon’s East Lake. I was out there around noon in anticipation of the Callibaetis hatch. I was fishing down with an intermediate line with a nymph and picking up fish. There were rises with increasing frequency moving my way some 100′ away.
I glanced down at the surface before me and saw a few little glowing balls. When I looked closer, I could see a medium colored body of an insect beneath this glowing orb and angled slightly downward. With increasing numbers more and more glowing dots appeared just an inch or so below the surface. They hovered there right below the surface and the fish started rising all about me. I continued with the deeper nymph and caught fish, but it was obvious the fish were keyed, for the next fifteen minutes on the hapless nymphs about to emerge right below the surface. And then the hatch was all about me with duns everywhere. So, this pattern is an experiment waiting for June on East Lake or some similar body of water with a Callibaetis hatch.
Ok, update (4-13-2008) :
“Early in spring, gasses begin filling the void between the exoskeleton and the body within. As the pressure builds, the exoskeleton starts to swell and the nymph becomes unnaturally buoyant. The exoskeleton stretches thin and radiates a shimmering glow as light reflects from the taught skin and interior gasses. Perhaps to lessen the uncomfortable pressure, the nymph starts crawling upwards. This isn’t happening to a lone individual, but to dozens, hundreds, or perhaps thousands of nymphs at the same time. Up the reeds, and up the rocks, up the stumps, and even up the legs of wading anglers these nymphs migrate toward the sun.” (http://www.flyline.com/entomology/callibaetis/)