The Mayfly Life Cycle: Understanding the Complete Metamorphosis

Mayflies are the most important aquatic insect for fly fishers. Understanding their life cycle is essential to success.

The Four Stages

Mayflies undergo incomplete metamorphosis, with four distinct stages:

1. Egg

Duration: Weeks to months, depending on species and water temperature

Location: Laid on water surface, sink to bottom, or deposited underwater

What happens:

• Female drops eggs while flying over water
• Some species insert eggs into aquatic vegetation
• Some wash eggs onto gravel
• Eggs hatch anywhere from a few days to several months later

Fishing implication: Not directly important, but egg-laying activity can bring fish to surface.

2. Nymph

Duration: 1-3 years, depending on species

Location: Bottom of streams, rivers, stillwaters

What nymphs do:

• Crawl among rocks and vegetation
• Feed on algae and organic debris
• Go through many molts (20-40)
• Grow larger with each molt

Nymph types:

• Clingers: Flattened bodies, hold tight to rocks (e.g., March Browns, Heptageniidae)
• Crawlers: Cylindrical bodies, move slowly (e.g., Blue-winged Olives, Baetidae)
• Burrowers: Slender, live in silt (e.g., Brown Drakes, Ephemeridae)
• Swimmers: Streamlined, can swim (e.g., Isonychia)

Fishing strategies:

• Nymphs are available to fish year-round
• Most drift naturally when molting or moving
• Good nymph imitations: Hare’s Ear, Pheasant Tail, Prince Nymph
• Fish deep in faster water
• Dead drift is critical

3. Emerger/Dun

Duration: Seconds to minutes as emerger; hours to days as dun

The emergence process:

• Nymph rises from bottom toward surface
• Nymphal shuck splits open on surface or in film
• Dun (adult) pulls itself free
• Wings inflate and dry
• Dun floats on surface

Emerger fishing:

• Critical stage - trout key on emergers
• Fish feed just below surface
• Patterns: Soft hackles, RS2, Barr’s Emerger
• Fish in surface film or just below

Dun fishing:

• Duns ride surface until wings dry
• Fly off streamside vegetation to mate
• Classic dry fly fishing
• Patterns: Adams, Comparaduns, Sparkle Duns
• Matching size and color is important

Important duns:

• Blue-winged Olive: Size 16-20, olive/gray
• Pale Morning Dun: Size 14-18, pale yellow
• March Brown: Size 10-12, brown/gray
• Green Drake: Size 10-12, green/gray
• Trico: Size 20-26, black/white

4. Spinner

Duration: Hours to days

What happens:

• Duns molt into spinners (sexually mature adults)
• Spinner stage is brief
• Mating occurs in streamside vegetation
• Females return to water to lay eggs
• After egg-laying, spinners die and fall to water

Spinner falls:

• Often occur in evening or morning
• Fish feed selectively on spent spinners
• Wings lay flat on water (distinctive profile)
• Often difficult to see on water

Spinner fishing:

• Patterns: Rusty Spinner, spent mayfly patterns
• Dead drift critical
• Use smaller tippet (6X-7X)
• Fish rising steadily but gently
• Tricos: famous morning spinner fall

Seasonal Timing

Spring

• Baetis (BWO): Size 16-20, multiple broods
• March Browns: Size 10-12, moderate currents
• Hendricksons: Size 12-14, eastern waters

Summer

• Pale Morning Duns: Size 14-18, mornings
• Green Drakes: Size 10-12, western waters
• Tricos: Size 20-26, morning spinner falls
• Callibaetis: Size 12-16, stillwaters

Fall

• Blue-winged Olives: Size 16-20, cloudy days
• October Caddis: Size 8-10 (actually a caddis)
• Mahogany Duns: Size 14-16

Winter

• Midges: Size 20-26, only reliable hatch
• Small BWO: Size 20-24, warmest part of day

Behavior by Water Type

Freestone Streams

• Multiple mayfly species
• Hatches can be intense but brief
• Fish opportunistic but can be selective
• Carry multiple patterns in various sizes

Spring Creeks

• Fewer species but more consistent
• Fish can be very selective
• Precise imitation important
• Longer, more sustained hatches

Tailwaters

• Consistent temperatures year-round
• Mayflies hatch throughout the year
• Blue-winged Olives dominate
• Midge-mayfly combos common

Stillwaters

• Callibaetis most important
• Damsels and dragons (not mayflies)
• Wind-driven hatches
• Fish cruise and feed selectively

Matching the Stage

Rising Fish

Splashy rises:

• Feeding on duns (emerging adults)
• Size 12-18 typically
• Dry fly patterns

Subtle dimples:

• Feeding on emergers or spinners
• May refuse dun patterns
• Try emerger or spinner

Porpoising:

• Feeding just below surface
• Emerger patterns or nymphs

Jumping:

• Not usually feeding (escaping something)
• Or chasing hatching insects

No Visible Rise

• Fish feeding on nymphs
• Nymph deep with appropriate pattern
• Vary depth until you find fish

The Complete Fly Box

For mayfly fishing, carry:

Nymphs:

• Pheasant Tail: Sizes 12-20
• Hare’s Ear: Sizes 12-18
• Copper John: Sizes 14-20
• Zebra Midge: Sizes 18-24

Emergers:

• RS2: Sizes 16-22
• Barr’s Emerger: Sizes 14-20
• Soft Hackle: Sizes 14-18

Duns:

• Adams Parachute: Sizes 12-20
• Comparaduns: Various colors, sizes 12-20
• Sparkle Dun: Various colors, sizes 12-20

Spinners:

• Rusty Spinner: Sizes 14-22
• CDC Spinners: Various colors

Key Takeaways

1. Size matters most: Match the size before color
2. Stage matters: Fish on emergers when trout refuse duns
3. Observation is critical: Catch a bug, examine it closely
4. Carry multiples: Each stage in multiple sizes
5. Be versatile: Learn to fish all stages effectively

Mayflies provide some of the best dry fly fishing of the year. Understanding their life cycle helps you be prepared when the hatch begins.

The next time you see rising fish, observe carefully. What stage are they taking? Match that stage, and you’ll catch fish.