Select the wing-membrane's material in the Material Room, then attach a Simple_Color node to the Displacement channel.

Set the displacement strength to a fairly low value ... 0.001 or so. You only need to displace the opposite sides of the polys enough so that they no longer occupy the same space.

Don't forget to enable displacement in your render options, also.

The displacement trick only works with the Firefly render engine. The old P4 renderer doesn't support displacement.