Well its making me sadder and sadder to see people tying very skinny caddis fly larva. They are small BUT they build up a protective shield the size a 0.10 dim and the biggest I have saw was a body was bigger the a 0.05 Canadian nickel. The protective shield is made out of small twigs, sand, rocks (small) and other small things on the bottom of the water that there body can stick on to. The reason they do that are because of 1. give them a little protection from other aquatic animals. 2. It gives camouflage in the water.
I figured this all out one day while i was laying on my dock at Wellman Lake Duck Mountain. I saw about 8 big green caddis walking(mainly green, but you could see i little bit of sand color mix in). If you closed you eyes for 5 seconds and opened them. You could not see them till they started to walk over the lake bottom. As soon a I saw them moving the were really noticeable. So I picked one up (it fell like your hand was touching a rock with a bunch alga on it) and gave it a little squeeze and sure enough half the caddis came out of its protective shell. This got me thinking of a fly.
The fly was a size 8 nymph hook 3x long. I call this fly a mohair caddis. First you put lead at the back of the hook.(just to the bend of the hook. Then you tie on your thread. Make sure that lead is on good. Put on some head cement then your good to go. Then tie your thread to the back of the hook. Tie on 4" of bright green 2 1/2" chenille olive mohair and then two 1'' pieces of brown mohair. Then go up the hook with your thread. Make sure you have enough room for your head. Then take your green chenille and make the body of the fly fat and a bunch of different shapes. Then take the brown mohair put one on top and one on bottom and tie them down and then your green mohair and go around the body not to close. Like how you would put copper wire on a wolly bugger. Then tie the head of the fly and your done. Then use toothbrush or what I use a piece of velcro on a popsicles stick and make it have a really buggy look. The uglier the fly the better. Trust me these caddis am talking about, are very ugly.
I think this kind of caddis is called a Hydropsyche Caddis. (Also known as a net build caddis) No to sure please correct me if am wrong.