These are really cool nymphs. They have a goose biot tail and wing, tain and brown wire thorax, rubber legs, and a partridge hackle. 
All the ingredients of a great fly - they get down deep and look very similar to our Yellow Sallys that permeate the mountainside in our VA spring and summer. 
Start with the 16s in mid/late spring and as the temperatures rise go with the 14s. 
Unless of course there's high water - then start right off the bat with the 14 as it will sink a little deeper.
Tied on a 3XL hook.