The Ostrich herls from the plume are similar to marabou. The barbs undulate with the current, at least that is what I envision. Mayfly nymphs that are found at several excellent sites like Troutnut.com show the gills down the sides of the abdomen of the mayfly. I have a cursory knowledge of their purpose as propulsion or transference of energy/respiration, but the point is the ostrich wrapped up the shank to form the abdomen produces the feathery suggestion of gills. The gills are on the sides of the nymph’s abdomen, so you could trim the top and bottom of the abdomen, though I don’t think it is necessary.
Ostrich herls are also used as wing components for tube flies along with peacock herl.
SwittersB & Exploring 