Learning entomology, even in basic terms, is very important in helping a fly fishermen become more successful. Knowing what insects are hatching, what stage the trout are taking, or having a good idea of what to expect, can be the difference in a frustrating or successful night on the water. Understanding entomology will also help to identify rising fish forms (check out the section on “feeding patterns”), where to look for fish during certain hatches based on what types of bugs they are, or knowing what to select from your box with an understanding of how to drift or present that particular fly.
The Life Cycle of a Mayfly A mayfly has four life cycles that should concern fly fishermen. The nymph, emerger, dun, and spinner. All of these cycles represent important stages that trout feed on. The nymph takes up the majority of a trout’s diet. This is when the mayfly is in its beginning stages. A nymph will molt several times spending its time beneath rocks and buried in the stream bed growing to size in preparation to hatch. Trout feed on nymphs when they begin to move out from under rocks, and drift down river as time nears for them to hatch. Nymph fishing is usually best early morning and midday when the nymphs begin to move and drift downstream. Fishing nymphs that are the predominant hatch occurring or one that will be on the water in the near future will increase nymph fishing success.
An emerger, at times not considered a stage in a mayfly’s life cycle, is when a mayfly begins to either rise to the surface, crawl onto shore, or shed its skin beneath the water in preparation to hatch (they hatch several ways depending upon specific bugs). This is a very important stage, because the mayfly is very vulnerable. Many mayflies are eaten when they are an emerger because they are still subsurface, making this an important stage to understand. Trout feeding on an emerger can usually be seen visually as a dry fly feeding fish. Many people mistake a trout feeding on an emerger as a fish taking a dun (adult mayfly). This can easily be distinguished by watching the way the fish is feeding. If the trout is rising dorsal fin, tail, then kicking down either creating a bulge or splashy rise it is usually feeding on an emerger. Large trout feeding on emergers usually just create a bulge. The key is to see if the fish is showing its head, and if it is not then it is usually feeding on an emerger just subsurface.
The dun or adult mayfly is the stage most fly fishermen dream about. Seeing a hatch of mayflies, or being in the middle of one is a wonderful experience. These bugs hatch either under the water and swim to the surface, hatch on top of the water, or crawl to shore or a nearby rock to shed its shuck. At this time mayflies can be seen in the air, riding the water’s surface, or fleeing to a nearby tree to await for its reproductive organs to develop. When the mayfly is fully developed it will mate preparing the female for egg laying. Once the mating process is taken care of the females will fly to the water and deposit their eggs. Some will fly up and down off the water depositing eggs or even dive underwater to lay eggs which settle to the bottom adhering along the stream bed. These eggs will be the next generation for the following year (a few mayflies will take longer or shorter to develop). Fishing mayfly duns is a tremendous and exciting experience. The last stage of a mayfly is the spinner. After it has mated and deposited eggs it become weak, and eventually dies. Most spinners fall in the evening which in turn represents the term “evening spinner fall”. Some mayflies can be found spent during the morning hours, Tricos being a good example. Spinners are an easy meal for trout. They are completely dead and flat on the water. Trout don’t have to expend any energy to feed on this stage. You will often see trout gently sipping spinners off the top. Large trout especially understand what it is to use as little energy as possible to feed on these easy targets. Spinners should be fished with the least amount of drag possible (no drag at all is best). Devoting a fly box to just spinners is not a bad idea.
Four types of mayflies -Tips on where and how to fish them There are four different types of mayflies: clingers, crawlers, burrowers, and swimmers. Each different type of mayfly includes a variety of mayflies and hatches. It is important in understanding the four stages of mayflies especially on large river systems.
Clingers like fast moving water and that is why they are built flat and broad to hang onto rocks in fast currents. So mayflies such as March Browns and Light Cahills which are clingers can be found in areas of hard pushing riffles and runs. If you find an area of river or stream that has an abundance of fast moving water, this is where you may find your best fishing with clinging type mayflies. Clingers also tend to jump out of their shucks faster then some other mayflies. This makes them a good choice to imitate with a slow swung wet fly rising steadily to the top. These mayflies tend to be a little more sporadic of a hatch, yet can still offer excellent dry fly fishing.
Crawlers are usually found in areas of soft riffles and runs during their stage as a nymph. These bugs such as Hendricksons and Sulphurs usually have a hard time hatching from their shucks. The crawler family represents the largest family, and offers the best fishing with dead drifted emergers just in or below the surface film. The duns of these bugs can hatch in prolific numbers sometime blanketing the water. They also fish well tied with trailing shucks.
The burrower family represents some of the larger mayflies such as Green Drakes, Brown Drakes, and Hexagenia. These mayflies are found in better numbers in and around silt. Areas of rivers with a good soft bottom will harbor these mayflies best. These mayflies need the silt to burrow and live as a nymph. These hatches attract some of the most anglers, and are usually very prolific but short lived. The spinner falls of these mayflies are very often outstanding fishing. Swimmers such as Isonychias like a combination of large soft moving pools, riffles, and runs. These mayflies fish excellent as a fast swung wet fly or even as a dun in an area of swift moving water. These bugs can swim very fast so don’t be afraid to swing these wet flies quickly, and use heavy tippets as strikes can be vigorous. These mayflies can hatch both in sporadic and heavy numbers.
Other insects and their importance Caddis, stoneflies, midges, and terrestrials all play a big factor along with the mayflies in a trout’s diet. Caddis and stoneflies like mayflies go through an important life cycle which when understood will help every fly fishermen become more successful.
The four stages can be found in a caddis that can in a mayfly. Nymph (often called a pupa for a caddis), emerger, dun (adult caddis), and spinner (spent caddis). Caddis pupa are basically the same as a mayfly nymph and can be fished in the same manner (dead drifted near the bottom). Caddis emergers can be fished just below or in the surface film. Caddis emergers can also be swung, as caddis swim to the surface at a fast pace. Trout can often be seen exploding with an escaping caddis emerging from the trout’s chase. Caddis duns can also be fished with a dead drift or skated presentation. These flies tend to hop on skitter on top of the water. Larger trout usually don’t expend the energy to chase adult caddis. Spent caddis are very important, as with mayflies they present an easy meal for trout.
Stoneflies come in a variety of sizes and colors. They can be anything from a size 2 or 4 up to a size 18. They are an important bug especially in the nymph stage. Stonefly nymphs present a good majority of the fish caught during the trout season. They are often large easy meals for trout. Stoneflies usually aren’t known for being fished in the emerger stage, because they crawl out of the water to hatch. They can be fished in this stage especially at night with a large wet fly. Fishing these wets behind boulders and in seams where struggling emerging nymphs fall off of rocks or drift into awaiting trout can be very rewarding. Adult stoneflies can also be extraordinary fishing either late evening, nighttime, or early mornings.
Midges are important especially to smaller streams with less mayfly activity. They can present some of the best fishing when other bugs are not around. Fine tippets and long leaders are usually a must to fish these tiny flies.
Terrestrials are also an important food supply for trout. On smaller streams summertime terrestrial dry fly fishing can be great especially under overhanging trees. Ants, beetles, crickets, and inch worms are some of the important terrestrials to have in your box. Many times even on prolific large rivers, terrestrials such as flying ants can steel the show from the many mayflies covering the water.
Overall, learning entomology is very important to fly fishermen of all skill levels. Whatever level you take learning entomology too, it will help you to become a more successful and less frustrated angler. Getting out on the water, turning over a few rocks, and having a good book on entomology will dramatically increase your odds