Bath and Body Works sells a skin product called Warm Sweet Vanilla. It''s a spray on like perfume. This stuff reminds you of something strippers wear BUT IT WORKS on GNATS! It doesn't kill them but it does keep them away! Just spray it on the bill of your hat and on the band around your ears. Try it on the ankles, I'll bet it works!
Another product that works reasonably well is Absorbine Jr. Folks around here have used it for years. Good ol listerine works just as well and costs far less.
I've never any of this stuff north to try on black flies but all three are worth a shot!
The best is the warm sweet vanilla, Just make sure your wife knows where you've picked up the scent!!!!!