Slide a 5/64 black nickel bead onto a size 16 curved nymph hook and run fluorescent chartreuse thread down to the barb and back. Do not taper this one. Bloodworms are the same thickness end to end, and the tyer wants the body just under the diameter of the bead.
Tie extra small silver wire in on the near side and run it back to the bend, then capture olive buzzer wrap right at the rear and bring the thread forward with smooth even wraps.
Decide your collar now: leave the chartreuse on for a hot spot, or half hitch and change to olive for a natural finish.
Wrap the buzzer wrap forward, doubling or tripling up on every third turn. Buzzer wrap is thin enough that this adds almost no bulk but gives the blotchy, uneven look these larvae actually have.
Tie the buzzer wrap off, trim, then rib the body with the silver wire in even open spirals. Spacing matters more than the exact number of turns.
Build the chartreuse hot spot collar up with a few extra wraps, whip finish and coat the fly. The coating is what brings out the mottled effect under the buzzer wrap.