Any fly angler worth their salt has their favorite stonefly pattern(s).  I have a few of my own, like my Frontliner and Hoover.  Both have served me, and many others, well when wanting a stonefly or "clinger"-style mayfly nymph imitation.

Many of my friends use other rubber-legged style imitations.  One that is popular among my circle of friends is called a "Turd" and is nothing more than a rayon chenille body and rubber appendages.  It works well.

However, times change and new materials come about.  LW's Polar Fart is aptly named since it's base material is UV micro Polar Chenille, and it's not quite a turd. Simple to tie, this great bug fishes especially well in low-visibiliy conditions or conditions where fish are more apt to strike out of reaction rather than a feeding urge.  However, don't neglect it wherever stoneflies dominate the menu.

MATERIALS:

Hook:  Knapek J (#4-#6), TMC 403BLJ (#8-#12)

Weight:  Black tungsten Slotted Bead, lead wire

Thread: Black 6/0

Tail/Legs: Copper/Brown Speckled Sexi Legs

Abdomen/Thorax: Black UV Micro Polar Ice Chenille

Click photos to enlarge.

Slide bead onto the hook before clamping the hook in your vise.

Match lead wire to hook wire.

Place 15-wraps of lead around the shank and slide the coil tight to the bead.

Starting behind the wire, attach your thread and advance to the bead, then reverse to the bend, and return to just behind the lead.

Monta Fly Co.'s Speckled Sexi Legs is the material I prefer for the appendages.  For the black phase I like the copper/brown legs.

Double a strand of sexi legs and even the tips.  Measure the tips to equal the length of the hook.

Secure behind the lead, at a point slightly longer than which you measured, with a few tight wraps.

Pull the tips to the rear a bit to add tension while your wrap back to the bend.

Clip the excess loop of sexi legs.

Advance the thread to about the 1/3 point on the shank.

Measure the tip of the strand of sexi legs to equal the hook length and secure it to the side of the hook.  Wrap back a ways to form the rear leg. 

Clip the rear leg to be roughly equal to the middle leg.

Repeat on the other side.

Advance the thread to the bead.

Hareline Dubbin's black UV Micro Polar Chenille will be the body material.  This comes in an array of color.  Olive Brown is another color I really like for the Fart.

Tie a strand in directly behind the bead.

Leaving the thread hang in place, carefully wrap the chenille rearward, going between the legs (and using it to "place" the aspect of the legs you desire), and ending at the tail.

Reverse the process, ending at the rear of the bead.  This two-layer wrapping will produce the full-body profile I prefer.

Secure the chenille with a few tight wraps of thread and trim the excess.

Invert the hook.

Pull-back the legs and stray forward-facing fibers...

...and use your thread to secure them to the rear, and to build a base on which to mount the front appendages.

Secure a shorter section of sexi leg over the narrow thread base and to the side of the hook eye.  Make the intial few wraps soft so as not to force the appendage too perpendicular from the hook. 

Repeat on the other side.

Whip finish and you have yourself an LW's Polar Fart!

Bottom View.

Here is the black version and olive brown version side-by-side.  Notice the appendages on the olive brown Fart are a slightly lighter shade.