Thread Yellow 8/0 body thread plus 50 denier tying thread
Materials
Body thread Yellow thread (Champion 8/0)
Tying thread Natural tying thread (Sumo Power 50 denier)
Body Natural brown material (Catgut biothread, size XS)
Tail Natural grey dryer lint (Partridge barbs)
Thorax Natural pine (Pure squirrel dubbing)
Back Brown permanent marker (Thread coated with UV resin)
Legs Natural rubber legs (Teal duck barbs)
How to tie it
Start the yellow Champion 8/0 behind the eye and carry it back past the hook point on the curve, laying a smooth, lightly built-up underbody. The yellow shows through the catgut once wet and is what gives the body its warm, translucent glow.
Tie in a short tail of partridge barbs, no longer than half the shank, then catch in a strand of XS natural brown catgut at the same point.
Stretch the catgut as you wrap it forward in touching turns to the 65% mark. Stretching thins the strand so it segments naturally and lets more of the yellow underbody read through, which is the entire reason for using the material.
Tie in a length of brown thread at the front of the body pointing rearward as the back, then dub a short, buggy thorax of natural pine squirrel over the front 25% of the shank.
Pull the brown thread forward over the thorax, tie it off at the eye, and set a few teal duck barbs down each side as legs, angled back to the hook point.
Give the back a thin coat of UV resin and cure it. The resin hardens the wing case and, with the catgut, gives the fly the wet, glassy look of a real baetis nymph drifting in the column.
Build a small head and whip finish. Keep the whole fly slim. Baetis nymphs are fine and delicate, and a bulky imitation gets refused.