Fly Fishing

Understanding Mayfly Entomology = Catching More Fish

Mayflies make up an extremely important part of most freshwater ecosystems and provide trout with an all year round food source. It would be arguable to say mayflies are the most important aquatic insect food source for trout and could vary by geographic location but they are certainly in the top five. If you were to look into any fly fisherman’s fly box, regardless of what part of the world they fish, they would almost certainly have flies that are either an impressionistic or imitative mayfly pattern. 

A basic understanding of mayflies (and other aquatic insects) is an extremely underrated piece of knowledge and skill that all fly fishermen can benefit greatly from. A lot of us understand the basics of how mayfly nymphs emerge and magically transform into flying fly insects, and while you don’t require a degree in entomology, going that little bit further with your understanding will provide results on the water. 

In this article we will be looking at identifying mayflies, their life cycle, the different and most common species of mayfly, what flies imitate them and when to fish these flies.

Identifying Mayflies

Identifying mayflies (at all stages of their life cycle) is the logical first step to understanding these insects better. There are over 3000 mayfly species worldwide, approximately 700 of those species are present in North America. New Zealand however has a much smaller list of mayfly species with only 40 different species, all of which are endemic to NZ. Obviously, we are not going to cover the identification of 3000 different species, luckily for us, they all have a number of common identifiable traits.


Mayflies hatch from eggs to become nymphs. The majority of a mayflies life is spent in its nymph form which is most commonly about one year though there are exceptions to this, some species are only in the nymph stage for a matter of weeks while others remain in the nymph stage for up to two years.

Mayflies can be easily mistaken for stonefly nymphs though there are some key differences to set them apart.

  1. If it has three tails it is certainly a mayfly. However if it has two tails it may still be a mayfly.
  2. If it has one “joint” in its legs it is a mayfly, if it has two “joints’ in its leg it is a stonefly.

Adult and Sub Adult (Dun)

The rule of the tails and legs still apply to the adult forms while also having a slender elongated body with upward-pointing wings.

A unique characteristic of the mayfly is the two adult phases that it goes through. This will be discussed further in the life cycle below. However, there are two key differences between the adult (Imago) and the sub-adult (Sub-imago or Dun) is the adult’s wings are transparent whereas the sub-adults wings are opaque.

Life Cycle

The life cycle of a mayfly is rather simple and consists of six phases including the egg phase up until when the mayflies short adult life is terminated and lies spent on the surface of the water after laying its eggs for the next generation. This process has sustained the existence of the species for over 100 million years. The stages of life are as follows…

  1. Hatches from an egg into a nymph (Nymph Stage)
 This first stage ‘the nymph’ is important for anglers to understand. This is the stage the mayfly is in for 99% of its life. Nymphs when first hatched are very small (only just visible to the naked eye) and can grow up to 35mm depending on species. Mayfly nymph fly sizes range from #10 to #26, the most common mayfly nymph size fished on New Zealand’s Central Plateau rivers are sizes #12 to #18. A good idea to find out the size of the nymphs in your area and rivers is to go and lift some rocks, you will find a range of sizes and colours of nymphs. These will give you a good idea of the sizes the trout will be eating.
  2. Emerges from the bottom to the water’s surface (Emerger)

This is a very brief stage of the mayflies life though is an important stage for feeding trout as on this journey they are often devoured by hungry trout. A split forms in the thorax of the nymph and it makes its way to the surfaces being lifted through the water column on an air bubble (some species swim to the surface also). Through this split thorax, wings begin to emerge and the nymph is beginning the moulting process.

Nymph to Dun Emergance

  3. Molting into the sub-amigo phase (Dun)

This is what most fly fishermen know as the Dun stage of its life. Once through the film of the surface, the emerged nymph moults and the Dun phase of its life begins. You may sometimes see these hatches in large number, the telltale sign of the mayfly is the upward-pointing wings like a sailboat while the mayfly struggles to take flight. Once airborne, the dun usually seeks out the river or lakeside vegetation.


  4. Molting into the Amigo phase (Adult Spinner)

After what is usually a few hours in the Dun stage of its life, the mayfly again moults into its full adult stage. During this stage of its life, it is focused solely on mating and laying eggs. This stage usually lasts no more than a day (varying slightly with each species). Once mating has taken place the female mayfly returns to the water to lay eggs, at this point it is often seen dancing around the surface of the water, once again becoming a potential meal for trout.

  5. The end of the mayflies life (Spent Spinner)

As the adult mayfly has no mouth or body parts capable of feeding, it relies solely on energy stores from its nymph stage. As such, after the mating and laying of eggs is complete, the mayfly ends up spent without energy and lifeless on the water’s surface. An easy meal compared to a lively mayfly or caddisfly.

Spent Spinner Photo from Fly Tyer Magazine

Mayfly Patterns & When To Fish Them

All of what has been spoken about so far is useless (to a fly fisherman) if we are unable to put it together onto a hook at the right time. When selecting or tying a pattern it is worth noting the difference between imitative and impressionist mayfly patterns.

An imitative pattern is a pattern that is designed to look near identical to a mayfly or even a particular species of mayfly in order to fool a trout. Whereas an impressionistic pattern is more of a generalised pattern that imitates a number of mayfly species or other types of nymphs of a similar shape. 

Generally speaking, it is more common to get impressionistic patterns in the nymph and emerger stages as at this stage in their life they share a number of qualities in terms of shape and size with a range of other aquatic insects. Swinging a wee wet fly such as a March Brown during any nymph hatch is likely to produce results. As for nymphs, a fly with the correct profile, size and colour should create a sufficient impression to fool wary trout. Visit my article Three Key Considerations While Fly Tying for more on the ideas behind impressionistic tying.

As the mayfly progresses through its life cycle and features become larger and/or more distinctive it is more important for flies to be more imitative. Many anglers have experienced the frustration of casting the wrong fly to rising fish for your fly to be rejected while they continue to rise (meanwhile you are left to figure out exactly what they are taking). In this case, the trout have become so tunnel-visioned on the particular hatching insect that they fail to see or care for any other offerings.

Barry Ord Clark's Spent Spinner (Imitative)

Below is a list of some mayfly patterns for each life stage…


  • Sawyers Pheasant Tail (Imitative)
  • American Pheasant Tail (Imitative)
  • Hare n Copper (Impressionistic)
  • Frenchie (Impressionistic)
  • Hares Ear (Impressionistic)
  • Improved Pheasant Tail (Imitative)
Due to most mayfly species spending over a year in the nymph stage, mayfly nymphs are a great all year round nymph pattern to be fished at all times of the day. I often fish impressionistic mayfly patterns all year round as it also closely matches a number of other nymphs that trout may be feeding on as well as mayfly nymphs.



  • March Brown Wee Wet (Impressionistic)
  • Baetis Flymph Emerger (Imitative)
  • Iron Blue Dun Wee Wet (Impressionistic)
  • Barry Ord Clarkes Foam Mayfly Emerger (Imitative)
Mayfly emergers can fish well all year but really come into their own during a mayfly hatch. Understanding rise forms can help your know whether to fish a dry fly or an emerger. When you see fish rising but only see their backs breach the surface, this is when to try an emerger as it tells you the feeding is going on just below the surface. Swinging wee wets work great if fishing downstream whereas a dead drifted emerger sitting in or just below the surface film is most effective for upstream presentations.


Dun (sub amigo) and Adult Spinner

  • Adams Mayfly (Imitative)
  • Light Hendrickson (Imitative)
  • Sparkle Dun (Imitative)
  • CDC Mayfly Dun (Imitative)
  • Wally Wing Mayfly (Imitative)
  • Extended Body Mayfly (Imitative)
These are a good choice when you see fish rising to small sailboat looking flies on the water’s surface. These are mayflies upward-pointing wings as they leave as the flies emerge from their nymph form. The recently hatched mayfly struggles on the water’s surface until they are ready to take flight. In this stage, their wings have a more opaque appearance.
The more developed extended body mayfly is seen often in the evening, dancing across the water’s surface to lay eggs after mating. Expect this mayfly surface activity later in spring and early summer.
Extended Body Mayfly

Spent Spinner

  • Rusty Spinner
  • Wally Winged Spent Spinner
  • Poly Wing Spinner
A spinner fall can make for a spectacular sight send trout into a feeding frenzy. If you are lucky enough to be on the water during a spinner fall, be sure to tie on one of these patterns.

Wrap Up

I’ve been lucky enough to fish during some spectacular mayfly hatches and having some basic knowledge of mayflies has definitely served me well. With the number of species across the world in the thousands, it’s needless for an angler to become too preoccupied learning each individual species. Far more can be gained by flipping some rocks and understanding the size and colours you want to be imitating in the rivers and lakes you are fishing.

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