Unlike other aquatic insects where the "nymph" and/or adult may be food for fish, there are 3 stages of  the life cycles of caddis that are important in fly fishing: larva, pupa and adult.  Caddis larva, the initial underwater "nymph" stage, are often a large component of benthic communities.  They are common bottom fauna in most freshwater lotic and lentic environments.  Pupa, the 2nd underwater stage, are often more available to fish and hence of more interest to the fly fisher who attempts to imitate caddis. 

Types of Caddis

    Caddis can be "lumped" into 3 groups: "casebuilding",  "free-roaming" & "micro caddis."  Life cycles typically include 5 larval instars and one generation per year.  As the larva become pupa, they typically create pupal cases or nets and become attached to rocks, vegetation or other submerged objects.  Development of the final adult form takes about 3 weeks in most species.  The insect then cuts itself free, crawls from the water or swims to the surface, and the adult emerges from the pupal shuck.  It is at this time in the cycle that caddis are at greatest exposure to predation by fish, and hence the point at which the fly fisher's attention is most often drawn.  Adult caddis are know for their characteristic "skittishness."  A nickname is "sedges", and unlike mayfly adults, spend little time sitting on the water.  Hence, they are not as easily available to fish.