What is meant by a spey dressing in fly fishing?

Spey dressing of Salmon Flies was born in 1850's by Fly Fishers on the River Spey in NE Scotland. They are sleek, attractive and usually colourfully dressed on long shank salmon hooks. They were created to drop quickly down in fast water, hence the low set wings, long flowing hackles that extend well beyond the hook's bend, sparse slim body finished with a neat head. Their remarkable success has carried on through generations of fly fishers of Atlantic Salmon and Steelheads.