Turton says (1836 - The Angler's Manual or Fly-Fisher's Oracle) "It is the best fly that can be used for trout and greyling in dark waters"
Pritt says (1885 - Yorkshire Country Flies) "one of the daintiest morsels with which you can tempt a trout and one of the most difficult to imitate successfully"
Lister says (1898 - List of artificial flies) 21. Dark Watchet (iron Blue Dun) "Feather from back of merlin hawk. Head, orange silk. Body, orange a purple twisted together with a little mole's fur. A very small fly but a favourite on cold blustery days. Hook 16"
I say (don't you just like it when you have the power of the pen) - "Fished early in the season when the the days are cold and blustery and the small olives are hatching, it can produce some very special surprises"
This is one the very early flies and it still stands the test of time. Great on the coloured waters that we have in the north of England. I tie mine slightly different:
Hook: Tiemco 103BL
Thread: Uni Thread 6/0 Orange
Rib: Uni Thread 6/0 Purple
Dubbing: Natural mole
Hackle: Jacdaw
(I tie in the orange thread as you would a normal silk body - down and back - catching the purple silk in 2 turns from the end and bringing it back to the eye in turns that will let me place the purple thread between each turn. I then lightly dub the purple silk and take it up towards the eye BETWEEN each turn of the orange.

Enjoy