The 'Killer Bug' is a pattern that brings into question just 'why a fish takes a fly?' Simplicity in fly tying is epitomised by this example. Two materials only, Frank Sawyer designed and used this fly mainly in the pursuit of grayling.
The original yarn used to tie this fly was Chadwick's 477. Once a cheap wool, these cards now (when rarely available) reach staggering amounts of money, with people truly believing that the wool has a specific shade (when wet) that cannot be matched. Whilst my intention is not to further this debate, if you have some original 477 then treasure it, but enjoy using it. However, if you choose not to tie the Killer Bug, because you are without this specific yarn, I think you're missing out on a using a truly spectacular fly.
My chosen substitute is from from Lathkill's. It is similar in appearance to the original yarn and takes on the pink hue which seems to prove so irresistable (to grayling in particular).
It is a truly suggestive pattern. However, I am sure it works best as a shrimp pattern. If you're yet to try it, allow yourself the opportunity to tie such a simple but deadly pattern. You'll find the 'Killer Bug' has earned it's reputation as a deadly pattern!
Hook: Your Choice (This is a Varivas 2120WB #14)
Thread: Copper wire
Yarn: 477 Substitute
I like to experiment with wires of different guage and colour:
An example tied with pink wire.
An example tied with a heavier copper wire and thread head: