What are they eating?

Down at our local stream's bridge one or two small trout can be seen at almost any time of the day feeding voraciously under the willows. The downstream animal in particular is "lapping" away at the surface rapidly. I would have thought I could have caught it.

I have tried to put a range of flies across its nose: blue dun, beetles, etc. I have also tried nymph below it in the waist deep water. The most it has noticed is to stop feeding, briefly. Last night I went down with a net and took a sample of the scum line they are feeding in. There were lots of "shucks" there; possibly caddis judging by the time of the year and other writers but looking more like mayflies to my scientific (but new to angling) eye.

(Whilst I was there I also took a sample of the downstream glide where large numbers of baitfish (fingerling trout?) regularly leap out of the water. Mainly chronomids there judging by the wrigglers in the water and the plague around my face.)

Next I think I'll try more drys (kakahi queen, royal wulf, grey wulf) and maybe get over to the other bank and dead drift over the feeding fish. I find it hard to get the fly under the willows into the drift.