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© 2026 Barbless.co. Flow and gauge data from USGS; weather from Open-Meteo. · v2026.10.18.2

  1. Home
  2. Hatches & Forage
  3. Mayflies
  4. Flav

Updated August 25, 2026

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Flav

Hatches › Mayfly

Mayfly

Flav

Drunella flavilinea · species

A western spiny crawler mayfly that matters most when trout key on drifting nymphs and mayfly adults.

Overview

Category

Mayfly

Category

Mayfly

Category

The insect or forage category for this profile.

Size

Generally a medium mayfly; match with medium nymphs and adults compared to tiny olives or midges.

Size

Generally a medium mayfly; match with medium nymphs and adults compared to tiny olives or midges.

Size

Typical size range anglers imitate on the water.

Key stages

Eggs, Nymph, Emerger, Dun / Adult, Spinner, Egg Laying Spinner, Spent Spinner

Key stages

Eggs, Nymph, Emerger, Dun / Adult, Spinner, Egg Laying Spinner, Spent Spinner

Key stages

Life stages most relevant to fly fishing this hatch or forage.

Water types

rivers, streams

Water types

rivers, streams

Water types

Water types where this bug or forage is commonly found.

About Flav

Drunella flavilinea, commonly called the Flav, is a mayfly species in the family Ephemerellidae. iNaturalist and Wikipedia describe it as a spiny crawler mayfly found in western North America, making it part of the broad mayfly mix Idaho anglers may encounter.

Also known as: Flavilinea

Life Cycle

Mayflies spend most of their lives underwater as nymphs before emerging as adults. They then complete their aerial mating and egg-laying cycle, often providing a final meal for fish as spent spinners.
Eggs

Eggs

Female mayflies lay hundreds to thousands of eggs either directly in the water or nearby, which then sink to the stream bottom to develop.

Nymph

Nymph

Mayfly nymphs reside on the streambed, where they feed and grow. They undergo multiple molts during this aquatic stage, making them a consistent food source for fish.

Emerger

Emerger

During emergence, the nymph rises to the water's surface, where it transforms and hatches into its winged adult form, often struggling to break free from its shuck.

Dun / Adult

Dun / Adult

The freshly emerged adult, known as a dun, rests on or just above the water's surface. It's a vulnerable stage before its final molt into a sexually mature spinner.

Spinner

Spinner

After a final molt, the dun becomes a spinner. These mated adults ascend into the air in swarms for their mating flights, completing their airborne phase.

Egg Laying Spinner

Egg Laying Spinner

Female spinners return to the water's surface to deposit their eggs. This critical stage ensures the continuation of the mayfly life cycle.

Spent Spinner

Spent Spinner

Having completed their reproductive purpose, adult mayflies die shortly after laying eggs. They often float lifelessly on the water's surface, becoming easy prey for fish.

Stages
Anglers fish it as a nymph, emerger, dun, and spinner.
Emergence cues
Look for activity during the general mayfly season when water is warming and trout begin taking drifting nymphs, emerging insects, or spent adults; hatches are often most noticeable in softer light.
Behavior
As a spiny crawler mayfly, the nymph is most important below the surface, where trout see it as a drifting food item when dislodged or during emergence. Adults become important when fish switch to duns riding the surface or spinners fallen back to the water.

How to Tie Flav Patterns

Nymph

You're Tying Mayfly Nymphs WRONG ! Try This Instead - Fly Tying Tutorial

Fliesbyaspeen

Materials

  • HookSize 14
  • Thread
  • MarabouDARK BROWN
  • Pheasant Tail
  • DubbingNatural
  • DubbingBlack Pepper
  • WireBrown
  • LegsNatural Brown

Emerger

How to Tie an Emerger Pattern Gilled Flav Emerger

Royce Davis Fly Fishing

Materials

  • dry fly hooksize 16TMC 101
  • Uni Thread8/0rusty dun
  • Antronlight brown, for tail and gills
  • biotsgray olive, for body
  • CDCtwo strandsnatural Mallard

Dun / Adult

Flavinella Comparadun | A Great Dry Fly for a Very Important Hatch! | Fly Tying Tutorial

Fly Fish Food

Materials

  • HookGamakatsu S10 Executive Series Dry Fly Hook - 14
  • Hook (Alternate)Hook (Alternate): Fulling Mill 1310 - Down Eye Dry Hook - 14
  • ThreadThread: SemperFli Nano Silk Ultra Fine 50D 12/0
  • Thread (Alternate)Copper Thread (Alternate): SemperFli Nano Silk Ultra Fine 50D 12/0
  • BodyFulling Mill Flex-Floss - Olive
  • Body (Alternate)Fulling Mill Flex-Floss - BWO
  • WingNature's Spirit Comparadun Deer Hair - Natural
  • Wing (Alternate)Comparadun Hair - Natural

Spinner

Tying The Compara Spinner with Kelly Galloup

TheSlideinn

Materials

  • Thread8/0Waxed Thread
  • MFC Micro Barred FibettsBrown Barred Black
  • ThreadBody
  • Super Fine DubbingThorax
  • EP Trigger Point FibersQuick Silver or Spinner Wing

Egg Laying Spinner

Superfine Sulphur Spinner

tightlinevideo

Materials

  • Dry-fly hooksize 14-16Fulling Mill 5050
  • Thread6/0 or 70-denierRusty Brown
  • Wood-duck flank-feather fibers
  • Superfine dubbingPale yellow
  • Polypropylene floating yarnWhite
  • Superfine dubbingAmber

Spent Spinner

Dave Ties a Compara-Spinner

East Rosebud Fly and Tackle

Materials

  • Dry Fly Ring Eye#16TMC 101
  • Veevus10/0Rusty Brown
  • MicrofibitsWhite
  • McFlylonWhite
  • Sparkle YarnClear
  • Polish Quill
  • Super Fine Dry Fly DubbingRusty Brown

Where It Hatches

Flav

One species, multiple rivers.

RiverJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDEC
  • Big Wood River
  • Henrys Fork of the Snake River
  • Snake River, Osgood Diversion Dam to Henrys Fork

Fly Patterns

  • Flav Sparkle Dun#14-16

Angler Notes

Why it matters
Flavs give Idaho anglers another medium western mayfly to match, especially when trout are feeding selectively on mayfly nymphs just below the surface film or on adults during a hatch.
Imitation tips
Carry spiny mayfly nymphs, soft-hackle or emerger patterns, upright-wing dun imitations, and flush or spent spinner patterns in medium sizes. Prioritize profile, stage, and silhouette over exact color.
Key life stages
nymphemergerdunspinner
Presentation
Start subsurface with dead-drift nymphs near broken current and seams. If fish begin showing near the top, switch to emergers in or just under the film, then to a clean-drifting dun or low-riding spinner when rises become surface-focused.

Identification

Key features
A stout, spiny crawler-style mayfly with a compact nymph body and the classic mayfly sequence of nymph, emerger, dun, and spinner. Anglers usually group it with other western Drunella-type mayflies and match the stage first, then size and tone.
Size
Generally a medium mayfly; match with medium nymphs and adults compared to tiny olives or midges.
Coloration
Usually imitated in muted mayfly tones: olive to brown nymphs, with duns and spinners tied in subdued gray-brown to olive families.
Similar bugs
Green DrakeBrown DrakeFlavilineaPink Albert

Habitat

Water types
riversstreams
Substrate
Rocky, firm-bottom runs and riffled water where crawler-type mayflies can hold in current.
Conditions
Best associated with moving water and clean, well-oxygenated conditions typical of productive mayfly habitat.

Eaten By

Brook Trout
Brook Trout
Brown Trout
Brown Trout
Cutbow Trout
Cutbow Trout
Mountain Whitefish
Mountain Whitefish
Rainbow Trout
Rainbow Trout
Yellowstone Cutthroat Trout
Yellowstone Cutthroat Trout

On this page

OverviewLife CycleFly TyingWhere It HatchesFly PatternsAngler NotesIdentificationHabitatEaten By

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On this profile

  • 3 waters
  • 1 fly pattern
  • 6 predator species
  • 6 tying videos
  • 7 life stages

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