Basic Entomology for Fly Fishing

Free Guide to Fly Fishing Entomology

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This is a free part of our fly fishing entomology course.  If you like what you see, check out our full entomology course here.  In order to take your fly fishing to the next level, you need an understanding of basic entomology (the study of insects) and how it applies to fly selection. This will cover the basic bugs found in typical trout waters and help you understand how to catch fish with this knowledge including what flies to use.

First off, don’t get overwhelmed and think you need to learn every bug on the water and their latin name, species, and genus just to catch more fish. The real key with any fly fishing entomology lesson is to equip you with the knowledge you need to identify the family of bug, and the stage that bug is in. Once you know that, you then see if that bug is on the menu for trout in that stage and match your fly accordingly. It’s really rather simple when you have the basic knowledge.

Secondly, let’s remember the biological classification order and understand that for most fly patterns, we imitate the family or even the order of the insect, not every single species. This is especially true with a fly like the Parachute Adams Dry Fly as there is no such thing as an “adams” insect, it is meant to imitate a wide variety of mayflies (Ephemeroptera). That greatly reduces the amount of knowledge you’ll need in your head at any given time to understand the basic entomology for fly fishing.

Here are 8 simple categories that you can memorize that will help you identify insects on the river:


Most Important to Memorize

  • Midge Patterns
  • Mayfly Patterns
  • Caddis Patterns
  • Stonefly Patterns

Slightly Less Important to Memorize

  • Terrestrial Patterns
  • Scuds & Sowbugs
  • Annelids (Worms)
  • Damselflies, Dragonflies and Water boatman

Now, without any more introductions here is the Basic Entomology Guide for Fly Fishing Part 1.

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Step 1 – Identify the Bug and The Stage

The first step in improving your fly fishing with basic entomology is to be able to identify what you are looking at. Let’s address 2 of the major categories of insects that fly fisherman mimic with fly patterns in this first part of basic entomology for fly fishing.  In addition, I will address the stages of the insects and help you identify the stage the insect you are looking at is in.

Midges (Chironomidae)


A close relative to a mosquito and often mistaken for them, midges are present in a river year round and will always be on the menu for trout. At times trout will look for bigger meals when those bigger meals are more prevalent, but it’s never bad to throw on a midge.

Here’s some videos on what midges look like up close:

How To Identify a Midge: Easy answer? If they look like a mosquito, but they aren’t biting you, they’re midges. You’ll often see them buzzing around and above your head in huge swarms.

Longer answer: Midges have 2, long and narrow wings without scales, are rarely bigger than a size #16 and the males have long feathery plumose antennae (often imitated with cdc puffs). Their wings are layed back along their body (different than mayflies and caddis), without a tent shape (like caddis) and most of the time these are mistaken as mosquitos.

Lifecycle of a Midge

Midges go through 4 stages, and fly fisherman really only care about 3.  Larva, Pupa/Emerger, Dry/Adult.  They have a spinner form, but I don’t think anyone really imitates it.  Let’s address what the characteristics of these stages are to help identify the stage of a midge for fly fishing.

Midge Larva/Nymph


Midge larva are little worms that have segmented bodies and are often red due to their diet.  your class midge nymph pattern such as a black beauty, is an accurate imitation.  Midge larva are the standard “nymph” form of a midge and are fished sub surface throughout the water column.

Midge Pupa

midge-emerger-iconMidge pupa are the emergent form of the midge patterns.  The key difference between a midge larva/nymph and a midge pupa is the air bubble they use to emerge and the U-shape they often make while just below the water’s surface.

Often, midge patterns with beadheads imitate a pupa because the flash of the bead imitates the air bubble.  Great emerger patterns or pupa patterns for midges include the Top Secret Midge, UV sparkle midge emerger, Smokejumper, and mercury midge.

Midge Dry/Adult


Midge Dries or Adults are the final stage for midges that fly fisherman care about.  They are fished with a dry fly imitation.  Midge adults are small and often difficult to fish.  They appear to be mosquitos on the water and some of your best chances of imitating them are to use a cluster pattern like a griffiths gnat.  When midges mate, they all gather together on and above the water in a cluster.  The cluster is a perfect meal for a trout and worth the effort of a rise, whereas a single midge adult is less appealing to trout (though not always, as is a smokejumper pattern imitates a single midge).  This isn’t always the case but often a good strategy to use when fly fishing with midge dry fly patterns.

Here is a quick 15 second video of a midge cluster on the water to get an idea of why a griffiths gnat  or crackleback patterns works.

Mayflies (Ephemeroptera)


Mayflies are arguably the most elegant insect of the fly fishing world.  They vary in size and color and are a staple in the trout’s diet.  As fly fisherman, we imitate this order with patterns like the parachute adams, adams dry fly, pheasant tail, comparadun, and the mercury baetis.

How to identify a Mayfly Dry/Adult: Easy answer is if the fly has wings that post straight up like a sailboat, it’s likely a mayfly.  They emerge/hatch during the morning and evening hours most of the time (though sometimes more)  and you will see trout slowly and methodically eating something on the surface.  Good chance it’s a mayfly.

Mayflies have 2 wings that sit vertical, and have transparency to them and often detailed designs.  the nymphs have long legs, short antennae, and have 2 or 3 tails.

Life Cycle of a Mayfly

Here is a quick video on one of the many species of the mayfly nymph up close.  This video and many more are included in our entomology course:

Mayfly Nymphs


Mayfly nymphs are broken down into 4 groups:  Swimmers, Clingers, Crawlers, and Burrowers.   Most mayfly nymphs are imitated with the same patterns, but fished at different depths and different styles.  Swimmer nymphs for example can be stripped like a streamer to get a strike, burrowers, are often drifted along the bottom.   Identifying which bugs the fish are eating is almost always a process of elimination.  However if you use a seine to see what’s in the water and choose the most prevalent, you have better chances then just guessing. In addition, a lot of mayfly nymph patterns mimic all of these types with moderate accuracy so there isn’t always a need to decipher mayfly behavior patterns except on the most selective tailwaters.

Mayfly Emergers


Mayfly emergers are likely one of the most important stages for trout fisherman to know and identify.  Emergers will have a shuck behind their bodies and will be fresh on the water or just below the surface.  It’s easy to tell if a trout is feeding on an mayfly emerger.  All you do is look to see if you can see the trout’s mouth.  Often, they will rise, you will see their tails but never their mouths or heads pop our of the water.  This is because trout are feeding just below the surface and feeding on emergers.  depending on the voracity will help determine the kind of emerger they are feeding on (mayfly, midge, caddis etc).

Mayfly Adults


The adults are most often fished and easily identifiable.  They will have their wings intact and will be floating well on the water.  The adults rarely spend much time on the water however, which makes this less important of a stage.  Even though most of our fly patterns we use as fly fishermen are tied as an adult, I believe they imitate the spinner form as well and the fish are rarely too selective to only eat spinners vs the adults that are on the water or just recently emerged.

Mayfly Spinners


Likely one of the most important stages to pay attention to and the easiest to identify.  These are mayflies that are falling back to the water after mating.  They don’t really move on the water as most are dead or close to dead and they are easy prey for trout.  The key to identifying these is simple.  You’ll see them on the water with wings fallen to the sides.  The recommended patterns do a great job of imitating a mayfly spinner.

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