Another abundant food form on the Sky. Case making does not occur until the final instar. At this time the abdomen becomes grotesquely enlarged. The family generally known as purse-case caddis, construct portable, bivalve silk-cases of sand. They continually enlarge their cases to accommodate the growing abdomen. Many microcaddis overwinter during the final instar. When a caddis leaves the pupal case and rises to the top it is functionally an adult.
After hatching, microcaddis drift and struggle great distances. In slow currents, trout will drift with them before the take. They may produce, at times, the same "smut" rise often attributed to chironomid and simulids. Although they are small, local concentrations may produce prodigious hatches. The cased larva are about 5mm long, drift freely in the current and are found in drift samples. Dr. Norman Anderson or Oregon has recorded their association with ranunculus and their high-drift rate on the Metolius River.*
* Taken from The Purse-Case Hydroptilids section
(page 11) of Darrel Martin's Micropatterns, Tying and Fishing the Small Fly.
Heh there, what type of caddis is it that leaves the trail in the river bottom as it is crawling towards the shoreline. I see these millions of times every year, but have been told that trout don't key in on them because of the case.
These are trails from the case builders. The Caddis fly has over 12,000 species in North America, more than all the mayfly and stonefly species combined.
It is not possible to identify which species have left the trails you are seeing but it makes more sense to id the metamorphic state they are in which is the Larval stage.
Caddis flies go through full metamorphosis, three stages of life to imitate. The larval stage can be broken down by three basic behavioral characteristics, case building, net spinning, and free living. The case building caddis use a myriad of material, both organic vegetation and rock or sand, to make their homes which creates the trails you are seeing when dragged behind.
I'm not sure about trout feeding on these cases but there are a number of fly patterns out there to imitate cased Caddis. It hard for me to imagine a trout not eating one when they move around the way they do.
The free-living larva are voracious predators that feed on other small insects. Free living caddis live in riffles and fast currents, and because of their roaming life style, are available to trout year round. The net spinning caddis larva build stationary homes with catch nets. These nets are situated on the top of rocks in the main current to catch pieces of floating vegetation, and small organic debris, which the larva feed on.
The Pupa stage of the caddis fly is rarely seen by the fly fisherman, because of its fast transition to the adult stage. The pupa are fully encased inside a sealed cocoon on the stream bottom until they are ready to emerge. At the time of emergence, they break open the cocoon and swim very quickly to the water's surface, where they explode into the air as a full formed adult. This rapid transition can cause trout to go wild. Trout may be seen leaping from the water as they chase the caddis flies from the stream bottom into the air.
The adult caddis flies mate soon after emergence in trees and bushes along the stream. They return to the water to deposit their eggs, usually at dusk. Caddis can lay their eggs in two different ways; one is called ovipositing, where the female flies over the water's surface, releasing her eggs. Caddis may also dive under water or crawl down plants and rocks, depositing their eggs on the stream bottom. If you notice caddis on your waders while you are fishing some night, the green stuff on your boots the next morning will be caddis eggs.