Film Flies: The Five Stages of Insect Emergence and the Best Flies to Imitate Them

This article was originally published in the May 2008 issue of Fly Fisherman magazine.

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Big fish, it seems, are smarter than small fish. Big fish somehow know that the fly is a sham and there’s an angler on the other end of the line.

But “smarter” is not exactly the right word—especially when a fish’s IQ is in the single or low double digits. Scientists have calculated that a trout has an IQ of 6. If a small brown trout has an IQ of 6, a big brown trout would also have an IQ of 6.

So, it really isn’t a matter of getting smarter. What it is—really—is a matter of energy conservation.

Let’s say a small brown weighing 4 ounces (still with an IQ of 6) eats an insect with a food value of 40 calories. The fish gets 10 calories of energy for each ounce of body weight (40 calories/4 ounces = 10 calories/ounce). But when a brown of 5 pounds (and its sterling IQ of 6) eats the same insect, it gets only half a calorie of energy for each ounce of body weight (5 pounds = 80 ounces; 40 calories/80 ounces = .5 calorie/ounce). The little fish gets 20 times the overall food equivalent from eating the same 40-calorie insect as the big trout.

Thus, little fish can afford to tear around and grab all sorts of things to see if they are good to eat—they jump out of the water to catch flying insects and actively chase nymphs. The daily caloric intake required by a little body is met with little effort.

Big fish, on the other hand, have significantly more body mass to move, which requires much more energy—in our case above, 20 times more energy—so the big fish has to eat 20 times as much food as the small fish to get the same caloric value per ounce of body weight.

Therefore, big fish must feed differently than small fish. If they did not, their energy expenditure to catch the food would surpass their energy intake, and voilà, they would be on a diet.

Thus, big fish tend to focus on the easiest-to-catch and most abundant food items. During a heavy hatch, this is typically emerging insects. The reasons are simple: (1) the insects must pause in the surface film for an extended time in order to complete metamorphosis; (2) this causes all the emergers to be concentrated in the surface film; and (3) you can’t run with your pants down—the emergers are trapped in the film and have no way to escape.

Nymphs are scattered around in the water column and adults are constantly flying off. But emergers can’t do anything except stay where they are and take it. So, while small fish are grabbing nymphs, nipping at emergers, slurping adults off the surface, and jumping out to catch them in the air, the big guys sip emergers out of the surface film with hardly a ripple.

Why do they feed so delicately? We—as “smart” humans—immediately suppose it’s so anglers won’t notice them. With an IQ of 6? Give me a break.

It’s all about the conservation of energy: eating subtly takes less overall energy, therefore big fish use a different feeding mode than small fish. There are plenty of exceptions to this, but the general rule is that big fish use cur-rents more efficiently than small fish.

Stream fish tend to feed territorially. For this reason, the biggest fish select the best feeding stations and guard them against smaller intruders. The abundant surface drift makes eating more predictable.

On the Henry’s Fork (and other similar streams), big fish tend to park near the shore, where rocks, logs, and protrusions of the bank establish clearly defined feeding alleys. The center of the stream also has good feeding lanes, but fewer of them and they are usually more dispersed than those near shore.

Harriman Ranch

One spring afternoon I fished the PMD emergence in the middle section of the Harriman Ranch on the Henry’s Fork. As I eased along the bank, the little insects fluttered off the currents and smaller fish splashed and grabbed at them. But that day I was searching for snouts. It was tough work ignoring the fish that splashed all over the river, while watching long and hard for signs of big fish in the feeding lies along the bank.

I noticed a small disturbance about 100 feet upstream and a few inches from the bank. I sat down and watched. The disturbance came again, and then again and again in a deliberate cycle. I knew it was a big trout, tight to the bank and feeding. I walked a wide berth around the fish and quietly waded into the river about 100 feet above its lie. It was still feeding.

Then began the most agonizing five minutes of any fishing experience—moving like a heron to get into casting position without spooking the fish. As I did, I continued watching the water, noting the insects in the various stages of emergence. As always, there were plenty of emergers and stillborns in the drift.

As part of my stalking ritual, I have developed the habit of watching the drift, and tying on the fly as I wade closer. This forces me to stop and pay attention as I get the fly out of the box, tie the knot, dress the leader, make certain the hook point is needle sharp, and so on. This is my way of slowing to a crawl—something that I am not known for in other areas of my life.

The fish was still feeding when I finally got to my desired position 30 feet up- and across-stream from it. It would be an easy cast.

The morning air was still and the currents were sliding smoothly along as the fly dropped about 5 feet upstream from the big trout and a little beyond its feeding lane. I gently pulled the rod back a few inches to drag the fly into the fish’s feeding lane; then I dropped the tip and allowed the fly to ride along just like the naturals. The fish came up right on time and slurped in the tiny fly.

When I tightened, I immediately knew I’d made a big mistake: this was not a 4-pound fish that could potentially be handled on the 6X I was using. This was an 8-pound fish, and it tore off like an enraged water buffalo.

I high-sticked him, hoping the leader wouldn’t get caught in the aquatic vegetation, but that didn’t matter. The surge was so strong that the tippet made an audible slicing sound as it unzipped the surface film like a banana.

And then the fish was gone.

Ah, but what a ride. I’d fooled the big fish, or rather, I’d clearly understood what was happening and applied that information to a successful conclusion. Well, not a totally successful conclusion—but at least my disappointment in losing the trout was tempered with the excitement of the moment.

I can still close my eyes and see that big fish bolting out of the shallows, and feel the numbing realization that I was hooked to a fish that was going to work me over like a terrier worrying a rat. Such are the ways of fishing the film.

Feeding Rhythms

Big fish feed rhythmically. Rises are typically metro-nomic, not because they are trying to time their rises, but because of the way big fish use currents to help them feed. During an emergence, they hold in shallow water where there is a moderately slow current—enough current to carry the food along in a regular fashion, but not enough to make the swimming difficult.

That fish I lost on Henry’s Fork was downstream of a small bank protrusion, lying just at the inside edge of the current tongue. The currents concentrated the insects, and set up the perfect feeding conditions.

The fish turned its pectoral fins like the flaps on an airplane and tipped its body up into the slight current. The moving water pushed the fish downstream and up to the surface. When the fish’s nose hit the film, it barely opened its mouth and allowed the water to run through and out its gills. The little emergers just ran down the fish’s gullet. It turned its head down, and with one wiggle of the tail slid forward to its feeding lie, and then repeated the process. A fish feeding in this manner expends almost no energy; the current does most of the work.

This rhythmic feeding pattern is another reason why big trout feed largely on emergers. When a fish gets to the surface, there better be something to eat, and that something is an emerger, the most concentrated of the stages in a mayfly hatch. Big fish quickly become trained on emergers, and anglers often wade away, shaking their heads and mumbling, “Too smart for me.” (What does that say about the angler’s self-admitted IQ?)

Film Flies

The emergence process is the key to fishing the film. Only three groups of insects produce consistent surface-emergence activity: mayflies, caddis, and midges. They all emerge in almost exactly the same way.

Stage 1. Whether nymph or pupa, the insect pokes the top of its thorax into the surface film, and then its back splits, allowing the adult to emerge. Without this mechanism, most of the smaller species would not be able to break through the tough meniscus.

At this point, the insect is in stage 1 of emergence and is nicely imitated by the Low Rider, which my son Jason designed specifically for this stage.

For film flies, profile is everything. The profile—or visual footprint—at this stage should mimic the nymph or pupa tucked up tight on the underside of the film with its back protruding through the surface. The Low Rider uses the 3/4 hackling technique that places the hackle above the body of the nymph (like a parachute hackle) so that the body lies just under the surface. The delicate proportions make the fly look as natural in the film as possible.

Stage 2. The insect continues its emergence by pushing its back up through the hole in the film. At this point it enters stage 2 of emergence and is best imitated by a Parachute Floating Nymph. This is my favorite imitation during Baetis emergences. I use René Harrop’s dubbing ball technique to form the emerging wing mass and add a para-chute hackle (two or three turns only). You can also mimic the emerging wings with a small loop or peg of foam in place of the dubbing ball.

Stage 3. The insect pulls its head out of the shuck, followed almost immediately by the legs. At this point it enters stage 3, which is matched perfectly by the universal emerger: a Parachute Adams (or other fly with an upright parachute post such as the Klinkhåmer).

All three of the surface-emerging insect groups look the same during this stage. That’s why the Parachute Adams is the world’s number 1 dry fly: it matches any mayfly, caddis, or midge in stage 3.

Most fly fishers think of the Parachute Adams as an adult dun imitation, but in reality it is an emerger. In stage 3 the nymphal or pupal body is just under the film and the legs are spread out on the surface to support the body. The body sticks almost straight up, with the wings plastered tightly along the top of the thorax as they continue pulling up and out of the wing pads.

Light reflecting off the upright body with the wings plastered tight along the top, gives the emerging insect a shining, light-colored look.

Still not convinced? Toss a Parachute Adams in a glass of water and view its position.

Stage 4. The adult crawls forward out of the immature skin and enters stage 4. The immature husk hangs nearly empty from the end of the adult, the wings are inflating, but still rumpled, and the adult body is parked on top of the surface.

At this point, the insect creates the largest surface footprint of the emergence process. The fish have no problem seeing the insect because it is totally in the film. This is the time for Sparkle Duns, Cross Dressers, or Sparkle Caddis Emergers. These flies nestle in the film, with the tail of sparkling fibers forming the empty husk.

Stage 5. The insect completes the extraction process and begins stage 5, the final hardening of the wings necessary for the full adult stage. This is the time of the Wet/Dry Fly, Quigley Cripple, and other similar sub-adult imitations.

The emergence process is not without peril. In preparation for emergence, the immature insect secretes a molting fluid between its old and new skins. This allows the adult to slide easily out of the nymphal or pupal husk. If the fluid does not sufficiently coat the adult, the bug can get hung up in the process and eventually drown.

In some hatches, especially PMDs and a few others, these “stillborns” are incredibly abundant and become the favorite targets for trout. I developed the Wet/Dry Fly during such a hatch on central Wisconsin’s Tomorrow River in May 1973, and it is still my favorite pattern for stage 5.

Regardless of the fly you use, if you don’t understand one simple fact, you will fail: Emergers are mired in the film and do exactly what the film does. They do not swim or crawl or get blown over the surface. They drift like dead spinners.

The Nemesis

Enter, stage left, the film fisher’s biggest nemesis: drag. On a scale of 1 to 10, drag is 1,000, and everything else—including the fly—is 5 or less.

All fly shops carry collections of emergers that work just fine—patterns that throw fish into a feeding frenzy—or at least that’s what you’ll be told. But fish any of these flies with a bad case of drag, and none will be effective. You must whip drag before you can be effective with film flies. But that’s a story for another day.

Film Fly Recipes

Emerger Stage 1: Low Rider (Mayfly)

• HOOK: Targus 100 or other standard dry-fly hook, size to match natural.
• THREAD: 6/0-10/0 depending on hook size, color to match body.
• TAIL: Wood duck or mallard flank feather fibers dyed to color of natural’s tails.
• BODY: SuperDry or other fine dubbing, color to match nymph or pupa.
• HACKLE POST: CealFiber or poly yarn, color to match top of thorax of natural; folded forward tight along the top of the thorax and tied in at the head after winding the hackle.
• HACKLE: Color to match legs of natural, wound parachute style; stroke the hackle fibers out to the sides before folding the hackle post forward to tie it down.
• HEAD: Butt end of hackle post, trimmed to the same length as the hook eye.

Note: For caddis and midge imitations, tie the fly without a tail on a Targus 2457 scud hook.

Emerger Stage 2: Parachute Floating Nymph (Mayfly)

• HOOK: Targus 100 or other standard dry-fly hook, size to match natural.
• THREAD: 6/0-10/0 depending on hook size, color to match body.
• TAIL: Pheasant tail fibers.
• BODY: SuperDry or other fine dubbing, color to match nymph or pupa.
• WING BALL: SuperDry dubbing to match color of wings; a small loop of foam or foam peg may be substituted for the dubbing ball.
• HACKLE: Color to match legs of natural; wound parachute style around the base of the wing ball.

Note: For caddis and midge imitations, tie the fly without a tail on a Targus 2457 scud hook.

Emerger Stage 4: Cross Dresser

[Imitate stage 3 (not shown) with a Parachute Adams or Klinkhåmer. THE EDITOR.]

• HOOK: Targus 100 or other standard dry-fly hook, size to match natural.
• THREAD: 6/0-10/0 depending on hook size, color to match body.
• SHUCK: Dun or amber CealFiber or Z-Lon.
• HACKLE: Dun.
• BODY: Peacock herl.

Note: Leave this pattern full-hackled or trim it on the bottom to better suggest an emerger stuck in the film.

Stage 5: The Wet/Dry Fly

• HOOK: Targus 100 or other standard dry-fly hook, size to match natural.
• THREAD: 6/0-10/0 depending on hook size, color to match body.
• SHUCK: CealFiber, Z-Lon, or similar sparkle fiber.
• BODY: SuperDry or similar fine dubbing, color to match adult.
• HACKLE: Dorsal marginal covert feather to match color of adult wing.

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Gary Borger was a Fly Fisherman contributing editor and author of several books including Naturals, Designing Trout Flies, and Presentation. He is professor emeritus of biology at the University of Wisconsin Campus in Wausau.

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