We went forward on this early September day with almost no expectations. The river had been quiet these last weeks—few hatching mayflies and fewer rising trout. Normally, at this time of fall creeping in, there are little blue-winged olives on the water every day. These little flies do not hatch in huge numbers, but their numbers are enough to get even the wariest trout to feed at the surface.

But not today.

We arrived at the junction, Galen and I, to see dimples on the flat water below the riffles: a sign of small trout on the rise. This is odd, I think; trout feeding in bright sunshine, but on what? Inspection shows not an olive in sight. A closer look reveals tiny black ants, little black bodies with clear, glassine wings at a size 22 or 24. There are thousands of these little creatures riding the surface, few flying off, vulnerable to the hungry mouths of trout lurking just below the surface. In the air, flitting about over the river, are a few large, juicy flying ants exactly like the one in the photo to the right. They are about a size 14 to 16, so much larger than the little black ants on the water. I wonder what kind of trout-feeding activity there would have been if there were hundreds of these larger ants on the water?

Anyway, Galen and I fished the hatch—if it can even be called that, since ants are land-born insects—until about 7 p.m. when the river shut down as night closed in and the air temperature dropped. While there were a lot of trout feeding, rises were sporadic with no sign of large fish. In the end, we each landed one small brown. The next day, I received an email from a friend explaining that he found the same tiny black ants several miles down river from where we had been fishing. The difference was he moved a few large trout.

Ants come in all sizes, from tiny number 22s up to about a 14. There are little reddish-orange ones, tiny black ones, big black ones, tan ones and the so-called “honey ants” that anglers eagerly wait to appear along Idaho’s famous Henry’s Fork of the Snake River each fall.

Almost 20 years ago on September 11, 2001, to be exact, I had the opportunity to go to West Yellowstone, Montana to fish the rivers in that area. Part of that trip included several mornings on Henry’s Fork looking for those honey ants. My friend that arranged that trip told me about honey ants well beforehand and with so much enthusiasm that I tied about two dozen in anticipation of some great dry-fly action. Despite all the pre-trip hype, we didn’t see a single honey ant during the week or so we fished the Henry’s Fork. We did go back the next year and had a morning with honey ants. When these ants are on the water, there is a feeding frenzy with slashing rises; the trout are fairly easy to hook but not land. By fall, the Henry’s Fork is full of rooted aquatic vegetation so every trout that is hooked immediately dives into to the lush weed growth and is lost.

Flying ants are a seasonal phenomenon and seem to appear mostly in September. Unlike aquatics insects, which emerge pretty much on schedule each year, depending on the weather, ant “hatches” are not predictable; here one day, gone the next. Despite quite a bit of research, I found very little information on where these ants come from and how they end up on rivers. The fact that these little ants are on the water in such large numbers, over several miles of river, for a day or so, is a phenomenon that is just inexplicable—at least to me.

As the trout season begins to wane and aquatic insect hatches decline, terrestrial insects become more and more important to anglers. There are beetles, caterpillars and, of course, a variety of ants. And while completely unpredictable, ants will be on the river when you arrive one day. So make sure that you have several in a variety of sizes and colors in your fly box!