From Fling and Puterbaugh The Basis Manual of Fly-Tying where the focus is on imitating nature
Simple Caddis Larva
Hook: #12 – 13 wet (here #16 scud/emerger)
Thread: olive (or desired color)
Rib: fine or medium copper
Body: wool to match thread color (here Crewel yarn in light olive)
Thorax: Peacock herl
Notes:
Lay down thread base
Tie in wire rib
. 
Tie in wool for body; touching wraps forward to thorax; tie off
Spiral wrap rib over body (optional to counter wrap); tie off
. 
.
Tie in Peacock herl behind eye; optional to create a herl rope; touching wraps to eye; tie off
Whip finish; head cement