Caddisflies are of the order Trichoptera, any of a group of moth like insects that live near lakes or rivers. There are some 14,500 described species. Caddisfly larvae are aquatic and the adults are considered terrestrial.
Also called sedge-flies or rail-flies, the adults are small moth like insects with two pairs of hairy membranous legs. The aquatic larvae are found in a wide variety of habitats such as streams, rivers, lakes, ponds, spring seeps and temporary waters. The larvae of many species use silk to make protective cases, which are often strengthened with gravel, sand, twigs, bitten-off pieces of plants, or other debris. The larvae exhibit various feeding strategies, with different species being predators, leaf shredders, algal grazers, or collectors of particles from the water column and benthos. Most adults have short lives during which they do not feed.
The life cycle of a caddisfly is somewhat different than that of a mayfly. Caddisflies undergo complete metamorphosis. Their life cycle includes four stages – egg, larva, pupa and adult. Mayflies have an "incomplete" life cycle in that there are only three stages - egg, nymph and adult. Most caddis species produce one generation per year. Caddis larvae build protective structures for the pupae. There are generally speaking, three types of caddis nymphs and they are defined by the kind of "cases" they build for protection...they are case-building, net-spinning and free-living (also a few others that are very uncommon). If you want to know them all, go here...
For more information, please refer to the following links:
Caddisfly by Encyclopedia Britannica
Caddisfly by Wikipedia
Caddisfly by ScienceDirect
For fly fishers that chase trout, the caddisfly may well be the most important of the aquatic insects. The mayfly tends to get most of the attention and the caddisfly is often overlooked.
In an article in Midcurrent, Gary LaFontaine makes a very important comment..."A sad fact of modern fly-fishing is that so much of the lore is geared to one insect, mayflies, that the typical angler has difficulty adapting his methods to the feeding that occurs during a caddisfly hatch. He is conditioned to fish his flies to simulate the typical habits of a mayfly, not a caddisfly."
In his book Caddisflies, Gary LaFontaine notes three signs that indicate when caddis are emerging on rivers, and they are worth repeating. First, trout occasionally are seen leaping in the air. He notes this happens when trout chase emerging caddis pupae; the fish's momentum sometimes carries it right out of the water.
The second clue is that there are no insects on the water. Even during a heavy emergence, adult caddis are just about impossible to see drifting on the surface. They generally emerge and fly off unnoticed. This phenomenon always amazes us. Many times we have held our noses at water level just below a pocket full of trout rising madly to caddis, hoping to see just one adult fly off. Indeed, it is nothing short of a miracle if you do.
Third, LaFontaine writes that most of the feeding trout are bulging and splashing. This occurs as the fish take the pupae from the surface film and turn downward. While this is sometimes true, we find that the riseform is more dependent on the speed of the current the fish is in rather than the food being taken. That is, in fast water bulging and splashing does occur, but in slower water quiet dimples, porpoise rolls, or tails breaking the surface are much more common rise forms. It is important to consider the riseform in deciding what a trout may be taking, but it is wise never to make a judgement based solely on it.
A personal note here...you see that I have referred to Gary LaFontaine several times here in the caddisfly section. There have certainly been a lot of articles in fly fishing literature on caddisflies but, in my opinion, there has never been a fly fisherman who truly understood the relationship that exists between trout and the caddisfly better than Gary. I was honored to know him and have had the opportunity to watch him fish several times. Every single time I saw him on the river (the Henry's Fork) I would get reasonably close, put down my fly rod, sit on the bank and just watch. Gary was a true master...and a great person.