Basic Fly Fishing Entomology | The Bugs You Need to Know & Must Have Flies

Basic Fly Fishing Entomology | The Bugs You Need to Know & Must Have Flies

Fly Fishing Entomology is one of the many things in fly fishing that has been WAY over complicated.  Fly fishing entomology really boils down to a 3rd grade science quiz.  Don’t make it more complicated then it needs to be!

Though this is over simplified and we will not cover the various species in each category.  Even so, you will be able to “match the hatch” by understanding and matching the common characteristics of each insect  you see on the water to the flies in your box.

Fly Size

For practical purpose, fly size is rated from #2 (large fly) to #24 (small fly) in increments of 2…. 2,4,6, 8, 10 and so on…

Fly Types

There are a variety of different life stages and insects we try and replicate with artificial flies.  Nymphs are subsurface insects & organisms that trout eat. Dry flies are used to replicate the mature stages of stream born insects or anything that floats and that trout might eat (grasshoppers, ants, etc).


Midges are the smallest and hardiest of the stream born insects.  The hatch in a variety of water types, temperatures and are very common on tailwaters & best fished in winter & low/clear water conditions. #18-24.

Midge Nymphs

Must Have  Midge Nymphs:

jujubee Midge
jujubee Midge
Zebra Midge

Midge Adult Dry Fly

Must Have Midge Adult Dry Flies:

griffith’s Gnat


Mayflies come in a variety of different sizes but the body structure remains consistent.  Distinguishing marks of the nymphs are three tails and a streamlined body. The adults fly gracefully and look like a sailboat when sitting on the water. You will hear a variety of insect names tossed around such as Blue Wing Olives (BWO), Pale Morning Duns (PMD), Tricos, Drakes, Quills etc. These are all types of mayflies. #10-24

Mayfly Nymph

Must Have Mayfly Nymphs:


Phesant Tail


Mayfly Adult (Dry Fly)

Must Have Mayfly Dry Fly: 

Parachute Adams


Caddis are a common stream born insect we see in the summer months once water temps hit 50 degrees.  The nymphs will often develop in a casing much like a caterpillar and swim to the surface (emerger) once they have developed and water temps are correct. The adults look like moths in flight and when resting have a tent shaped wing. #12-20.

Caddis Nymphs (larva)

Must Have Caddis Nymphs:


Graphic Caddis
Deep 6 Caddis


Adult Caddis (Dry Fly)

Must Have Caddis Dries:

Puterbaugh Caddis
X2 Caddis


Stoneflies are distinguished in the nymph stage by two tails and two antennas. They can range from size 4 (salmon fly) all the way to size 20 (yellow sally). Unlike other insects mentioned, stoneflies crawl out of the river and break out of the exoskeleton which you will see along the bank during stonefly hatches. The adults have two sets of wings.

Stonefly Nymph

Must Have Stonefly Nymph


Rubberlegs Stonefly Nymph


Stonefly Adult

Must Have Stonefly Dry


Crystal Stimulator


Other Stuff Trout Eat

Aside from the hatches mentioned, there are other food sources that either live in the river or fall in the river but don’t grow wings and fly such as…

  • WormsleechesCrawdadsminnowsfreshwater shrimp




Must haves flies in the “other stuff” Category…

San Juan Worms, Barr’s Slumpbuster, Buggers, Amy’s Ant

It’s time to nail down your 2024 fly fishing trip with Minturn Anglers.

About the Author

Justin Nolan

More Articles