...wish I had more to say about it. I think it's highly likely that no one really knows what the life cycle of this fungus is, but I just edited my comment below with my best guess as to what happens.

Very interesting information. I don't think this is uncommon - I seem to see it several times a year. I see it most often in headwater seepage areas under exposed rocks or sticks. I cannot remember ever seeing it in a crick over a couple feet wide. I also see it on pupae and always thought it was critters which didn't make it.

Just a minor comment that mine were found in a small river about 30' wide and 1' deep.

For all the help and info. on this mystery. I'll have to go check under those rocks again next time and see what's there.

Wow! caddisflies. I wonder if the fried egg looking ones I'm finding have a protective covering for development, or if that is a fungus attack per V's comment on the all white adult -Photo#302289

Caddisfly larvae pupate in their cases (of pebbles, plant materials, etc.), or build new shelters for this purpose. Some do make cases/cocoons of pure silk, but you would not see wings poking out because the pupa emerges from the case before the winged adult emerges from the pupa. Also, the insect's body is flattened as happens with many victims of fungus attack--see the above link to the pathogenic fungi article for examples.

This is bizarre. I wonder how common it is. This location is quite far from the place last year, and I'm pretty sure different waterway. There was a ton of these under the rocks. Does the caddisfly emerge in an infected state, weak, and the fungus overtakes them there under the rock?

I haven't read about the specifics of this fungus, but Lloyd Gonzales gives the names of two fungi that attack caddisflies in a comment on the first image of the four I linked to above. I haven't gotten around to pursuing this yet, and I'd be curious to hear anything you find out if you look these up. The way many pathogenic fungi work is that a spore lands on a healthy insect; the fungus grows inside its body, and then compels the insect to climb up a plant or other object, where the fungus is better able to disperse its spores. The fungus kills the insect and attaches it to the substrate. For whatever reason, these caddisfly fungi compel their hosts to crawl on the undersides of rocks to die. I can only speculate that the spores disperse in water, rather than in the air as is typical. I would further speculate that the spores infect caddisfly larvae, then remain more or less dormant until the host metamorphoses. I would love to find confirmation of this.
[Added later] Actually, it seems more likely to me that the spores disperse on the water surface, infecting adults when they are first emerging from the water.