When does the Caenis hatch?

June–September, typically 18:00–21:00. It starts from about 12°C, peaks around 18°C and tails off by 24°C.

What flies imitate the Caenis?

Caenis Spinner, CDC F-Fly White, Last Hope, typically on a #22 hook.

How big is the Caenis?

Spinner stage: 3–5mm. That matches a #22 hook.

Mayfly · Aquatic insect

Caenis

Caenis horaria

Size

3–5mm

Hook size

#22

Hatch window

Season June–SeptemberPeak time 18:00–21:00

Water temperature

18° peak

12°24°

How fish feed on it

Millions of them, all identical, all tiny, all at once. Trout feeding on a caenis fall are not selecting — they are grazing — and the more precisely you match the insect the more you compete with the other million. Contrast beats accuracy here, which is the opposite of almost every other rule.

Flies that imitate this

Caenis Spinner

CDC F-Fly White

Last Hope

Ecology

Rivers & stillwaterLimestone loughsChalk streamsTolerates poor water

Where you'll find this

British IslesPresent
IberiaPresent
The AlpsPresent
ScandinaviaPresent
Continental EuropePresent
Atlantic fringePresent

Present throughout

Identification

Caenis horaria is a species of mayfly in the genus Caenis .

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