Elk Hair Caddis is an excellent Dry Fly.  Tied in sizes from 12 to 20.

Great searching pattern.  The insect is very tolerant to pollution.

 
Orvis Guide on Dry Fly Fishing

Below are the materials needed to tie the Elk Hair Caddis.  All of them can be purchased directly from Orvis

Orvis Dry Fly Hook

Dry Fly Hackle (Rooster Neck)
Color to match body

Dubbing Fur
Kit or to match natural

Elk Hair