The following fly is intended to imitate a crane fly larva. As you see, these larvae can be rather large and would be quite a meal for most trout. I'm guessing it's why they like them so much. This particular pattern is really a mash-up of Frank Sawyer's Killer Bug, the Utah Killer bug from Tenkara Guides, Jason Klass's version of the killer and also Walt Young's "Walt's Worm". So thanks guys.
For a hook, I go a little longer than most and use a Dai-Riki #285 size 12. Start by mashing the barb and then get the hook secured firmly in your vise. If you have a rotary vise, make sure everything is aligned for rotary use.
This pattern should be fished deep and .02 lead free wire is about the right size for this hook. Start the wire just above the hook point. I like to make 8 or 9 wraps but how heavy you want to make the fly is really up to you.
I'm going to use 140 Denier fluorescent pink UTC thread here but any heavier pink thread will do. Start your thread leaving a little space behind the hook eye and take a few wraps rearward before snipping off the tag. Adding a small drop of Zap-a-gap to the hook shank and then sliding the wire wraps forward locks everything in place. With that done, give the weight a light coating of thread wraps ending at the front of the wraps.
Jamieson's Shetland Spindrift wool yarn in a color called "oyster" is the final material needed for this pattern. Snip about an 8 to 10 inch length and then cut one end off square and neat. With a pinch wrap, secure this end to the hook shank just in front of the weight. Taking wraps rearward secure the yarn to the top of the hook shank all the way back down into the bend and then all the way forward again. Repeat this process, not going back quite as far each time. This should result in a nicely tapered underbody.
If you have a rotary vise, throw a couple of half hitches on to secure the thread then position it in the bobbin cradle. Get hold of the yarn and twist to tighten it. Begin making adjacent wraps up the hook shank. You may need to occasionally twist the yarn some more as you go. Keep making turns all the way up the hook shank to just behind the eye. Remove your bobbin from the cradle and tie off the yarn securely before cutting the tag end free.
Use some good tight turns of tying thread to cover the yarn butt and produce a small head on the fly. Do a 5 or 6 turn whip finish and then snip or cut your tying thread free. I like to put a couple of drops of head cement on the wraps as insurance against unraveling but it probably isn't necessary.
For a fly that uses so few materials and is so easy to tie, it's amazingly effective. In faster water I prefer adding a second weighted fly to the rig like a Copper John as opposed to using split shot, but again, the choice is yours. I almost feel guilty when I fish with these things but it usually doesn't last long. The Copper John generally works pretty well also.