Ruard - Your question about the natural insect lead me back to the vice. Once I had a photo of the natural insect I just had to try for a better imitation. Here is my attempt (nymph style). Most fly patterns call for a white line down the back, but the photo seems to suggest a golden color, which is why I chose to use gold silk.
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Isonychia Nymph
Hook - Daiichi Model 1710 (Size 12)
Thread - Pearsall's Napels Silk (Antique Gold /31)
Tail - Burnt Turkey Tail Fibers (Natural)
Lateral Line - Twisted Dubbing Loop formed using the tying silk (tyed in at both ends and the middle)
Body - Beaver Dubbing (Isonychia color)
Legs - Burnt Turkey Tail Fibers (Natural)
Head - Pearsall's Napels Silk (Antique Gold /31) - darken top side with a brown sharpie to complete the fly. The photo was taken before I darkened the head of the fly.
Notes About the Pattern Design:
The Isonychia bicolor nymph is noted to be a very powerful swimmer. For this reason, I tyed the fly with a longer tail, in hopes that it will allow greater movement of the fly in the water. I know the natural insect only has three tail fibers, but I haven't met a fish who can count, so I ignored this fact.
The tail and legs are all formed using natural turkey tail, which has been "burnt" in a dilute bleach solution to remove the tiny barbules that hold the fibers together. I chose this material since it had similar markings to the natural insect (mottled) and it had a similar shape (wide flattened) to their legs. I clipped off the fine ends of the fibers to get the width I wanted. The legs were formed by tying in the turkey tail fibers by the thin end (to allow greater movement).
I noticed that the tail fibers have a bit of a red cast to them and the head of the fly is dark like the body. Once the fly was finished I added these colors with sharpie markers. You can fine-tune the color combination based on local insects.
The lateral line is formed by making a dubbing loop using the tying thread. To form the lateral line, I dubbed the rear half of the body of the fly and then twisted the silk dubbing loop into a tight cord and pulled it over the back of the fly. Tye the dubbing loop in at the middle and then pull it back out of the way while you form the front half of the fly. Once the thorax and legs are all in place, twist the dubbing loop and pull it over the front half of the body. Secure the dubbing loop and then whip finish a head.
I think the body of the fly needs to be just a bit plumper. The natural insect appears to have more of a plump cigar shaped body. The body shape can easily be adjusted by forming an underbody, or using a bit more dubbing.
(Credit for the Isonychia bicolor photo & Kudos go to Jason Neuswanger a.k.a. "Troutnut"/Source:
http://www.troutnut.com/specimen/477)
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Troutnut has a great video of the Isonychia nymph swimming around (
Click on the link and then scroll down to the middle of the page -
http://www.troutnut.com/videos)