The EPT Factor

A food web exists beneath the rippled current of a trout stream far more diverse than anglers may realize. The functioning biological systems of a trout stream are often hidden from view unless we are willing to explore the wetted depths with a mask and snorkel or with kick seines.

Fueling this sub-aquatic food chain is biofilm colonizing almost all available sub-surface areas where this organic film can feed on sunlight and dissolved nutrients. More complex organisms then feed on this biofilm and pass the nutrients up the food chain to the next higher level organism. Eventually, the suns energy from the bio-film works its way up this food chain to the larger benthic macroinvertebrates in which fish feed on. This food chain is circular in nature where the higher organisms return the nutrients such as nitrogenous waste back into the system fueling the cycle all over again.

John P. Newbury fishes still water

Taxonomists have divided the known biological world up into eight categories from Domain, kingdom, all the way down to Genus and species. For a fly angler wanting to know how to choose the right fly, it is essential to know at least the family orders of the three primary aquatic insects that make up the bulk of a trout's diet. Familiarity with the three dominant taxonomic families is a must when choosing the right fly.

Only three families of aquatic insect are of significant importance to the fly fisher to learn: Ephemeroptera, Plecoptera, and Trichoptera AKA Mayflies, Caddisflies, Stoneflies, and Midges (EPTD). Each of these families of insect has its own unique life history and critical role within the aquatic food system.

Each member of EPT is not hard to recognize throughout all the individual life stage. Mayfly adults have a distinct pair of upright wings with a larval stage known as a nymph. Mayfly nymphs are further broken down based on body shape Caddis adults have tent shaped wings, the larval stage has tubular shaped bodies and often build external shelters that they carry around with them. Stonefly adults have a two pair of wings that fold flat over the back, and the larval form is often large and robust crawling about the rocks in search of food.

Keeping a simple collection of fly patterns to imitate the EPT categories is the very foundation of a quality working fly box. It is not hard to get drawn into an endless array of fly patterns filling the bins of a well-stocked fly shop, each fly pattern calling out its special fish catching allure. Expert fly anglers know it is easy to get carried away with the variety and stick with a small collection of well-chosen fly patterns to get the job done. The reason only a few patterns are all that is needed is that they imitate the four families of aquatic insects very well in shape size and behavior.

Nymph Shapes of the EPT group.

I have selected a handful of fly patterns to serve as a foundation in which to imitate the EPT families. These fly patterns are based on the general size shape and color of each family with enough impressionistic qualities effectively to mimic more than one aquatic insect family. Each season, I narrow down my collection of patterns based on general effectiveness experienced throughout many seasons. New trends emerge from the vice that simply outperforms old flies from previous tying seasons. I also noticed that when I emphasize tying flies with an impressionistic quality, the better my angling days.

Good science has no need to reinvent the wheel as the wheel is perfect in its intended purpose, to roll. The same holds true for many fly patterns invented long ago. There is no need to reinvent something that is already perfect for its intended purpose. However, like the wheel, we can alter it to suit our intended use without changing its core functionality. When choosing a collection of fly patterns to imitate the EPTC group, I looked at the so-called wheels of fly design that have already been rolling to select the tried and true flies as the foundation of a high-quality fly box. Having chosen a core group of fly patterns, I can then alter their size and color to imitate even more possibilities. In most cases, I have stripped the fly down to its most common elements, to emulate the basic shape, size, and color of the specific nymph I am imitating.

Ephemeroptera - The Mayflies



ephem·er·al | \ i-ˈfem-rəl  , -ˈfēm-; -ˈfe-mə-, -ˈfē-  \

Definition of ephemeral

1: lasting a very short time ephemeral pleasures

2: lasting one day only an ephemeral fever

Merriam Webster

Ephemeroptera = short lived with wings.

While the name suggests, ephemeroptera, mayfly’s are short-lived often less than two days. However, it is only the winged adult phase that mayflies only live this brief time. Nymphs of mayflies can live up to a year or longer during the sub-adult larval phase AKA nymph.

Mayflies have an incomplete life cycle with only a larval and adult stage and lacking a pupal stage. However, mayflies undergo two further stages of reproductive maturity with the first adult phase called the sub-imago or dun, the winged but not yet sexually mature and the imago stage or spinner, the final reproductive stage in the insects' life cycle. As mayflies emerge from the water, they will gather in and around the riparian canopy to undergo one more molt to become imagos or commonly called spinners and mate. Essentially, the dun stage is only used to transport the nymph from the bottom of the stream to the tree canopy to mate.

Historically, fly fisherman favored fishing during a mayfly “hatch,” the transitory period between nymph and dun and imitate a dun (sub-imago) with a dry fly. When the nymphs transition into duns, a period of predatory vulnerability exists while the emerging insect floats on the water surface. Depending on environmental factors such as weather, water temps, cloud cover all play a role in how long insects may drift along before taking flight into the bankside foliage. These extended drifts make for rewarding dry fly fishing opportunities when you happen upon it.

Today’s anglers know that a dry fly hatch can be a short and unpredictable event with being at the right place and right time essential for dry fly fishing success. Fishing with nymphs offers anglers an unlimited opportunity to present a food item to a fish during the rest of the day. Famous rivers that were historically known for having regular daily hatches are seeing less reliable hatches and dry fly fishing taking a back seat to fishing nymphs or emergers. Many factors may be conspiring to the unreliability of daily hatches such as increasing angler hours/visits and climate change. Heavy angling pressure may be driving the fish from surface feeding and solely concentrating on nymphs and emerging duns sub-surface.

Below are examples of the mary shapes of ephemeroptera larvae we should be aware when tying imitative patterns.

Minnow Shaped

Minnow shaped nymphs such as Baetis species are agile swimmers. Baetis actively swim to the surface and back to the bottom repeatedly for several days prior to emergence. When tying patterns that imitate baetis nymphs, bear in mind the shallow taper and slim body. Keep these nymphs thin and streamlined.

BONY Armored Crawlers


Bony armored crawlers such as the western green drake Drunella grandis, crawl freely amongst the substrate hunting for food.

Strongly defined tapers and heavy thoraxes are design considerations when tying crawlers such as the Western Green Drakes. The gills are also very active on the Naturals some a well dubbed and roughened with a wire brush can make the appearance of lively gills.



Members of the Crawler group are active crawlers and swimmers but prefer to remain amongst the cobble. Many begin the emergence process underwater where a spot of color enhances the fly and rapidly swim towards the surface.

Flathead Clingers


Clinger nymphs, as the name suggests, cling firmly onto rock and woody debris. It is now thought that the shape of clingers is not only about remaining firmly attached to the substrate, but rather, the thin profile allows them feed in the interstitial spaces between rocks that other nymph shapes cannot exploit. I suspect the large gills also aid in breathing within those murky, silty spaces.

Members of the clinger group are easily imitate with most patterns covering other shapes already such as the Body Glass Clinger Drake nymph.

Plecoptera - The Stoneflies

Stonefly Shapes

Most stoneflies exhibit an elongated cylinder shape. When tying stoneflies consider the shape you are trying to mimic. When in doubt on how fat to tie your abdomens, its always better to error on the thin side allowing the nymph to swim nicely. Too fat or heavy a fly can complicate your presentations and cause an unnatural drift.

Trichoptera - The Caddisflies

Of all the members of the EPT group, caddis are the only family to undergo a complete life cycle: larva, pupa, and adult. The pupal form is the transitional stage between either a sheltered dwelling of free-living caddis larva and the winged adult. Free-living caddis larva only build protective shelters of debris or stones that they seal themselves inside during pupation. Caddis pupa are only available at the time of emergence and in most cases, emerging pupa is the most vulnerable stage of the life cycle that available to trout. 

It has been debated over the decades the theory that caddis inflates their exoskeletons with a gas bubble to emerge to the surface. While I have not seen this phenomenon in person, I have seen tiny flashes of light near the surface of the water like that of the paparazzi shooting a movie star walking the red carpet. I have surmised that I was witnessing the glints of light as caddis emerge encased in this so-called gas bubble. Because of this phenomenon, I fish with caddis emerger patterns that imitate this gas bubble to some degree at or very near the surface instead of presented deeply. When fishing deeper in the water column, I use much less flashy, more opaque caddis pupa imitations because they have not begun to exhibit this gas bubble yet.

I have observed several caddis species whose exoskeleton would exfoliate in the process of emergence. The chitin of the pupa would be somewhat transparent allowing the more colorful adults’ abdomen to show through. Patterns such as my Lucent Sedge imitate this occurrence. Either way, I strive for a bright yet transparent effect when designing caddis pupa patterns.

Free Living Caddis larvae

Caddis larva are broken down into three distinct groups, the free living, net spinning and case/shelter building groups. Each of these groups have distinct behavioral characteristics important for the angler and fly tyer to understand. Free living caddis are predatory in nature, do not build shelters and crawl around the rocks in search of other macro invertebrates in which to feed upon. Net building caddis spin funnel shaped webs and feed on detritus filtered from the current captured in the nets that they construct. Shelter building caddis build protective shelters that they carry around with them as the crawl amongst the cobble feeding on the bio-film that grows on the rocks. 

It is interesting to note that free living caddis larva species emerge in small numbers throughout the season and are always available as opportunistic prey items throughout the day and long into the evening. Emergence activity of the free living groups will typically be in small but predictable numbers all season long. This makes them a top choice for the angler to imitate when no specific hatch activity can be observed. I generally prefer to start my angling day with a heavy Czech nymph probing the depths hoping to take advantage of opportunistic feeding activity and switch to a caddis emerger pattern when I actually detect emergence activity.

Cased Caddis


Some smaller shelter building caddis use a survival by numbers strategy for success. This means that they are typically found in very large quantities and emerge en-mass to avoid being heavily preyed upon in order to propagate the species. A very well known instance of this phenomenon is the Mother’s Day Caddis hatch of Brachycentrus. This hatch usually only lasts about 10 days or less just prior to runoff season. Clouds of caddis fill the air overwhelming predators both above and below the water through sheer confusion and overwhelming numbers.

Anglers should practice the habit of turning over rocks and observing the general size shape and color of the caddis found. Kick net sampling is another effective method of gaining an understanding of the diversity of caddis groups that are present in your water.