Disco makes sense (never thought I'd say that!). Do you suppose it's due to the transparency in the midge's soon-to-be-shed casing? I know with caddis, the shuck has a see-thru tone as the adult emerges. I've played with patterns for october caddis a lot and have had success with partial shuck imitations.
I see a lot of casings while on stillwater with a midge hatch going on, I guess I should collect a few samples this spring.