Buzzers or chironomid nymphs are the trout's most common form of food.
The nymphs leave there larval cases and stay at the bottom, until eventually
finding there way to the surface. This is where the evening rise occurs and counts for most of the trout being caught.
The most favoured patterns are the Coves pheasant tail, Buzzer pupa (below) 
Montanna nymph, Horn caddis (above) and the Dragonfly nymph.
The hook should really not be larger than a size 10.
Try this popular Buzzer below.

Hook. 10-16
Thread. Black
Tail. White fluorescent floss
Body. Black floss silk
Rib. Silver wire
Hackle. None
Wing. None
Thorax. Bronze peacock herl
Breathers. White fluorescent floss