Geez guys. Foam for a Griffth's Gnat? Sacrilege!
I'm going to disagree until someone does a side by side test. I think the peacock herl underbody is a key ingredient of a Griffith's Gnat. There are only two elements to this fly other than the hook and thread, and both are necessary. Adding foam is unnecessary.
On another BB, there was a question by a beginner fly tyer about material substitutions. I listed a few times like jungle c*ck eyes, cdc fibers, seal or polar bear fur, etc that one should try to use when called for in a pattern. These materials have special properties that cannot be duplicated by other materials or synthetics.
I placed peacock herl into that category. Peacock herl has a special sheen and reflectivity that I think is unique, and I think it is one of the reasons that the Griffith's Gnat works so well. I don't think a foam can duplicate the spicky appearance of the fibers or the sheen of natural peacock herl.
Foam cannot duplicate the reflectivity of Peacock Herl.
Foam cannot duplicate the "buggy" shaggy nature of the herl body.
The Griffith's Gnat simulates the clustering of mating adult midges with multiple bodies at odd angles and the hackles simulating the legs and wings of the cluster touching the surface.
This is what we are tying to imitate:
So we can catch this:
Note in the photos above that some of the midges are UNDER the film. The beauty of a Griffith's Gnat is that you can make it sit lower in the film by cutting off the bottom fibers flush withe the hook. This is a trick that I use when they wont take the standard higher floating pattern. Trout are dumb but they know that a ball of midges can't fly off and the ball of midges partially submerged is easy pickings.