Mayfly Genus Epeorus (Little Maryatts)

This is a pretty clear photo of an Epeorus nymph clinging flat against a log. The big mound of debris on the bottom center of the picture, attached to the light rock, is a structure that's been puzzling me. I've found several in one section of the river. It's big, like 2 inches long and maybe 1/2-3/4 inch wide/tall, and hollow, like some sort of coccoon or something. It's clearly a structure built by some sort of little creature, but I'm not sure what.

In this picture: Mayfly Genus Epeorus (Little Maryatts).

This log houses several Ephemerella mayfly nymphs and, on the top right, an Epeorus mayfly nymph.

In this picture: Mayfly Species Ephemerella invaria (Sulphur Dun) and Mayfly Genus Epeorus (Little Maryatts).

There are a couple Epeorus mayfly nymphs clinging to this rock. One interesting thing I've noticed is that even though the stream has a lot of clingers, they rarely show up in my photos, and I've started paying more attention to the river bottom while I'm fishing I've noticed that clinger nymphs generally see or feel me coming and scurry to the undersides of their rocks before I can get in picture range. I think I have the same problem with stoneflies.

In this picture: Mayfly Genus Epeorus (Little Maryatts).