The first stage in the life cycle of a caddis fly, is the larvae. These are found in both still and moving water. Depending on family and species, the larvae is either a case building larvae, net building larvae or a free living larvae.
The case building larvae builds a protection around the body of sand, small stones and/or plant materials. As the larvae groves, many species will leave their old case to build a new one. During this time they are vurnable and an easy target for the fish.
Net building larvaes use silk thread to spin cases and/or nets that are used to catch food.
A free living larvae is what it sounds like - a larvae that doesn't build a protection or use silk thread to catch food.
Most often you will find the caddis larvae on the bottom and they will only accidently drift down stream in moving water or in search of new feeding grounds. Those living in still water, will only move across the bottom. It is necessery to fish your imitations of the larvae as close to the bottom as possilbe.
Many anglers would say that an imitation of a caddis larvae are among the best, if not the best, trout fly around.