Wingcase

Other fly parts:
hackle/wing/tail/
wingcase/legs/body/head

antron wing case (emerging pupa)

They are usually a bit darker and bulkier than the rest of the body.  They are on top of the insect where the developing wings of the natural insect are encased.  The larger the wings of the adult, the larger the wingcase.

Wingcases are most pronounced on mayfly and stonefly patterns.  They are tied with a variety of materials, both natural (feather, fur, hair, etc) and synthetic.

folded quill wingcase (stonefly nymph)