No insects are more associated with fly fishing than are the mayflies (order Ephemeroptera) so we will handle tying their life cycle first in this series. Mayflies get their name from their ephemeral nature - they do not feed as adults and live from a matter of minutes to maybe a few days once they emerge from the water. Like most all beings that reproduce sexually, they begin life as a fertilized egg and from this they develop. As an aquatic nymph, to grow to the size they will be when they leave the water, they need to shed their exoskeleton (molt or ecdysis) repeatedly. Eventually, it is time for them to emerge from the stream and become an aerial sub-adult which us fly anglers call a dun and scientists call a subimago. They will remain in this stage until they molt one last time into a reproductive adult, the spinner or imago stage. At this stage, they return to the water, often in great numbers, to reproduce and die. They will spend somewhere between a couple of months to a couple of years as a nymph and from a few hours (or less) to a few days as an aerial insect. The name Ephemeroptera suits them well.
Rather than rehash a lot of ground I have covered elsewhere, I will link to previous posts on angling entomology. In previous posts, I have covered some of the Latin - and if you really need to know Latin. I have covered the life cycles of the four main aquatic insect groups - mayflies, caddisflies, stoneflies, and midges - a good prerequisite for this post and others coming up in the Tying the Life Cycle series. And I have covered, in some detail, what I think are the hatches someone who fishes my homewaters of the Driftless and someone that would like to "chase hatches" in Wisconsin should know more about.
Previous Angling Entomology posts:
I am going to try to do this as Latin-free as I can though I will reference a few family names and common names of species (i.e. Ephorons or white flies, Hexes for Hexagenia spp.) when those are names commonly used by fly anglers.
Nymphs are typically divided into categories based on their morphology and habitat into clingers, crawlers, swimming, and burrowing mayfly nymphs. Their names are quite informative as to what they do and where they live. Do you really need to tie your nymphs to match the naturals' relatively small differences? No, probably not.
Clingers cling to rocks in fast moving riffles so they are dorso-ventrally flattened (compressed top to bottom) so they can live in the slower currents created by the friction between water and the rocks they live on. This boundary layer is surprisingly calm but it is but a couple of millimeters in height so some insects have evolved to occupy this space. The species you are likely to be familiar with are those in the family Heptageniidae such as March Browns, Light Cahills, Quill Gordons, and others. If you think it is important to create a more accurate clinger nymph (it is probably not...), flatten the underbody weighting or add some mono to the sides of your hook to widen the body. Flymen Fishing Co. even make bead heads specifically to imitate clinger heads which is almost certainly unnecessary but maybe that is your aesthetic.
Crawler nymphs crawl along substrates in (generally) slower moving water compared to the more flattened and typically more stoutly built clingers. Crawlers are some of our best hatches - the Hendrickson, sulphurs: pale morning and pale evening duns. These are generally represented by probably the most reliable of the old reliable nymph patterns - the gold-ribbed hare's ear (GRHE).
Swimming nymphs swim which means that these nymphs typically live in slower currents where these relatively small bugs are able to swim against the water current. Baetis spp. are what make up much - but not all - of the hatches anglers group together as "Blue-winged Olives" or "BWOs". The pattern most associated with swimming nymphs is the Sawyer pheasant tail nymph with its slim body and lack of legs. The American version of the pattern is better as a general mayfly nymph. Sometimes these are referred to as "minnow nymphs" for their swimming style. Isonychia are a another swimmer that have a unique trait of swimming towards shore or midstream rocks to emerge, much like stoneflies generally do.
And I think you are probably sensing a trend here, burrowing mayflies burrow into (relatively) smaller substrates and comprise some of the largest mayfly species such as the Hex (Hexagenia spp.), Eastern Green Drakes, Brown Drakes, and our late-summer Ephoron hatch. Burrowing mayflies are rarely out of their burrows except when hatching and are generally relatively easily differentiated from other groups by their prominent abdominal gills (others have abdominal gills but burrowers take it to the extreme...) and their front legs that often protrude forward. Imitations of burrowing nymphs tend to use some marabou, ostrich herl, game bird aftershaft feathers, or other similar materials to imitate the gills. A standard imitation is something like Barry Ord Clarke's (The Feather Bender) burrowing mayfly pattern.
Some people get hung up about nymph fly patterns but I am not one of those people. In recent years it seems more fish are caught on Perdigons and other not particularly realistic looking flies than on more exacting imitations. What "Euronymphing" should teach us is what really matters when fishing nymphs is that it is not the pattern we are fishing but getting that fly deep and being able to detect strikes that is important. Trout are not all that discerning, particularly in faster moving water where they simply can not afford to be all that picking. If they are holding in fast water, it is because of the food supply in faster water - these fish are almost always actively feeding.
If one wanted to be a little more specific about things to consider when tying mayfly nymphs, here is a short list of things I have given some thought to in the past.
Most aquatic and aerial species are counter-shaded meaning that they have bellies that are lighter and match the sky when viewed from below and their backs are darker and camouflaged to the environment they live on, swim over, or fly over when viewed from above. This is true from tiny insects to penguins to Blue Whales. Nymphs you are fishing below fishes eyes' should be darker and those you are fishing above their eyes (higher in the water column) should be lighter.
While I know fish do not count tails (mayflies have 2 or 3 tails, depending upon the species), I have never understood the super bushy tails of flies like the gold-ribbed hare's ear (GRHE) which to my eye looks more like an extended body rather than tails. While I do not necessarily tie them with 2 or 3 tails, I do try to tie the tails so they do not increase the length of the nymph and look more like tails.
Like all insects, they have 6 legs but again, I do not think that the fish are counting legs. Swimming mayflies swim with their legs held into their bodies which is what Frank Sawyer was attempting to imitate with his incredibly simple and durable pheasant tail nymph. In other groups, I think that we should be imitating the movement of the legs more than the legs themselves.
Nymphs get larger throughout the course of the year. In mid-summer after most mayflies have hatched, smaller nymphs may be more effective and patterns more true to the adult size should be used the closer you get to that species emergence. Many of the insects are much more active as they get closer to their emergence date.
If you grab a rock in a riffle, you can tell the mayflies that are getting ready to hatch by looking for darkened wing pads on the nymphs. I generally try to imitate this with a dark wing case on my nymphs - peacock herl and turkey feathers are great materials for this.
If you REALLY want to know about the mayflies of Wisconsin, download the book for free.
The duns are what most of us think about when it comes to fishing mayfly imitations - though I might argue for many hatches, their spinners provide more important and dependable fishing than does their dun stage. Duns are the stage produced by the final molting of the nymphs and most - but not all - mayflies do this on the water's surface. Duns are not yet reproductively-viable, rather they need to go through another molt to become the reproductively-viable imagos or spinners in fly fishing language. As such, there is not much reason for duns to spend a lot of time on the water. However they do need to dry their wings before they can leave the surface of the stream to fly to streamside vegetation where they "hang out" until they shed their exoskeletons one last time. This tends to take longer when the air and water temperatures are lower.
Duns have upright wings that tend to be more darkly veined compared to the spinner stage which is typically less stoutly built. Fly tyers have devised a myriad of ways to imitate the duns' upright wings ranging from a simple hackle collar to much more specific imitations such as "Wally-wings" or using wing burners to get a more realistic mayfly wing. You may be sensing a trend here - I do not think it matters much. In fact, my favorite mayfly dry fly imitations are the sparkle dun and comparaduns which imitate the upright wing with a single bunch of deer hair or CDC fibers. The Sparkle Dun imitates the nymphal shuck which is still attached to the emerging dun. This is to imitate a more helpless - thus more vulnerable - individual.
As I had written before, there is no better book on dry fly design than Datus Proper's What the Trout Said (see Fly Fishing Books, Part I) and unless something better comes along, I will stand by that assessment. Proper and Vince Marinaro, his mentor and author of A Modern Dry Fly Code and In the Ring of the Rise, both emphasized how distinct and sizeable the wing on a dry fly looks from below. Marinaro devised the thorax dun which is tied quite differently from the clipped hackle thorax fly that most of us tie today. Proper, working from Marinaro's thorax dun, devised his "perfect dun" with many of the same characteristics but a bit of a simpler tie.
Dry fly patterns are a compromise between looking realistic, behaving realistically on the water, ease of tying, how they float on the water, and probably a host of other factors. In heavy water, a Wulff or heavily-hackled Catskill or parachute patterns work well. Those same flies on flat water are likely less effective. The Sparkle Dun, which has long been my mayfly imitation of first choice, is great on flatter water but on riffles it may not float as well as you would like.
Spinners are the reproductive stage of mayflies - they are now truly adults and as adults they have sex and die and that is it. For some mayfly hatches, these are the most important stage to imitate. While we often think of these as those spent wing flies; until the point that the female deposits her eggs and (eventually) dies, some spinners are quite robust. For example, the March Brown is a very stout spinner that will ride the surface for along distance before ever becoming the spent-wing spinner we are more accustomed to. For others mayflies, like Tricos, the duns may hatch at night and it is the spinners returning to the stream to mate that we are imitating most of the time. For Ephorons, it happens so quickly that the spinner is often trailing the shuck of the dun as they return to the stream. And for the Hex hatch, the duns and spinners overlap significantly.
These examples give some reasons why it is important to know something about the hatches if you plan to fish them. For different mayflies, different stages are more or less important.
Emergers and Cripples
While there are distinct stages - the nymph, sub-imago (dun), and imago (spinner); the process of moving between those stages takes place over time. And for some species, particularly under some conditions, the transitions may be lengthy and sometimes they "get stuck" in a transitional stage. Fly anglers generally refer to these as emergers and other names. Most typically, emerger fly patterns represent the transition from a nymph to the dun stage and cripples often refers to individuals "stuck" in transitional stages. Because the transition from a dun to a spinner (typically) occurs on streamside vegetation, we do not typically see mayflies stuck in this transition. However for some mayflies, like the Ephoron, this transition essentially occurs nearly all at once and we typically see the spinners carrying around the molted exoskeleton of the dun stage.
Imitations for emergers very often are tied on curved hooks to put their abdomen below water's surface, have trailing shucks to represent being stuck in their nymphal shuck, and/or shorter, not yet fully dried wings. Lawson's Mayfly Cripple is probably the best known cripple pattern and it is quite a simple one. Many other emerger or cripple patterns use CDC, often in a loop wing, to imitate a wing that is not fully unfurled and dried. Others are essentially floating nymphs to imitate a nymph that has risen from the bottom to the surface but has not yet transitioned to the dun (subimago) stage. In pretty much all cases, the theory behind fishing these patterns is that they represent an easy target for trout, one that is unlikely to fly away before the fish can intersect your imitation.
Oddities (sunken or clumped spinners, eggs, and other such rarities)
There are a number of other ways to fish mayflies that I am going to group together as "oddities". The oddest of the oddities to me was from an article in Fly Fisherman Magazine about fishing using mayfly egg imitations tied as tiny balls of dubbing on very small hooks (#26 and smaller). Can not say I have ever tried it nor have many, I assume. Maybe we are missing out on some of the greatest fly fishing ever. Probably not...
Sunken spinners is not exactly a new idea, certainly many anglers' dry imitations have sunk and the angler found fishing the spinner subsurface was an effective strategy. It remains a pretty minor technique but fishing weighted spinner imitations is certainly used by some anglers. Another potential avenue is to imitate clumps of spinners that get caught in eddies and other areas of streams. I often hear or read of warmwater anglers matching popper color to some of the large burrowing mayflies at times of the year.
Wrapping it Up
Mayflies spend most of their lives in the nymphal stage but they are more vulnerable as they transition from an aquatic nymph to an aerial dun (subimago) and then spinner (imago). Then as a semelparious species, they return to the stream to lay eggs and die. At this point, they have returned their nutrients to the stream, whether or not they are eaten by fishes.
Most anglers will find a few fly patterns that they like and find work well for them and then stick with them. My personal choices are typically some old standards - it is hard to beat a pheasant tail or GRHE nymph and increasingly Perdigons and other slim, simple nymphs. Most often, my dry fly of choice is a sparkle dun or a parachute. Simple spent wing spinners or a parachute - maybe with a very short, bright colored post - are my standards for imitating spinners. And floating nymphs and loop wing emergers work well when not much else is. Although most of the time, I am fishing mayflies dead drift, there are times when a moving fly work quite well. Nymphs moving to the surface to hatch can be imitated with a twitched nymph or swung wet fly or soft hackle. And there have been times when I have seen trout looking for mayfly dry flies that are showing a bit of life - be observant match what you can see the trout eating. Unfortunately, finding and fishing mayfly hatches other than BWOs and late summer's Tricos are getting more and more difficult around the Driftless.