The premier insect event on the North Branch of the Ausable is Brown Drake emergence and (3 days later) spinner fall. This year I have arrived at a satisfactory spinner pattern.

Hook: 2X long #12 or #10 (Mustad 94831 or Daiichi 1280)
Thread: 6/0 olive or tan
Tail: moose body
Abdomen: deer hair (short, coarse)
Wings: saddle hackle

The tails and body are like Roberts Drake. I use moose for the tail because pheasant tail barbs stick together and always look like one fiber.
Deer hair is hollow so this fly floats without a lot of fussing with floatant. Dubbed bodies always get waterlogged.

The wings are a “widespread fiber spinner wing” from a book by Vince Marinaro. For these, you wrap a saddle hackle (less the fuzzy lower part) around the hook, then force the fibers to the desired position with figure 8 thread wraps.

Marinaro made a “slant tank” to observe the appearance of real flies and of imitations from below the surface. He concluded that this was the most realistic wing.

Ephemera simulans is a burrower that lives as a nymph in the silty margins of the stream. Why we have it but not Hexagenia limbata, I cannot explain.

Update 6 June 2014 : I am reading Hatch Guide for Upper Midwest Streams by Ann R. Miller. In her discussion of the Brown Drake Nymph, she says “Search out their habitat of sandy-gravel stream bottom – not silt!“. So that is the reason we have Brown Drakes but no Hex – the North Branch does not have enough silt.