Wbranch on Jan 18, 2014January 18th, 2014, 7:59 am EST
I'm pretty sure there are no current flies tied specifically to imitate the Water Penny. I really don't know if they are a significant food source to trout. If I was to tie one I would just use a pair of those round Velcro tabs about 1/2" in diameter and put one tab on the bottom of the hook shank and add some glue and put the other half on top and press them together so the Velcro hook and loops mesh and the glue is distributed well. After the glue dried I could color the top and bottom with various waterproof pens and trim the diameter to match the size of the natural.
LowBudget on Jan 20, 2014January 20th, 2014, 6:28 am EST
Like many flies, my musings on a water penny larva fly was more about a fly for the fisherman than perhaps for the fish.
Now that I've had time to think about one, I am thinking about a round tab of brown leather (I have some from an old jacket) over a bit of light brown / white mohair mix.
I could envision a trout downstream of a disturbance of the stream bottom, caused by wading fishermen or a herd of deer, slurping up some random water pennies dislodged from their attachment to gravel and rocks.
Adding a pattern on top made by waterproof pens as Wbranch suggests would be a nice finishing touch.
Low Budget Fly Tying and Fishing author of "Behind the Fly"