Had a couple of sessions on a res I fish. Small red ants, more the colour than size and shape. Not been up fishing last few years but I know where the nests are but you need the wind in the right direction, the fish loved them. I used a 16s F type fly.
I was catching on the River Avon on them two weeks ago. I couldn't work out what they were taking, thinking the fish were taking midge, then a bigger fly came down and I scooped it out and bingo it was a flying ant and that was it sorted.