Plural of “fly”

The plural of fly is flies.

Why

"Fly" comes from Old English "fleoge," the insect name, a word distinct in origin from the unrelated verb "fly" (to move through the air), which comes from Old English "fleogan" — two separate Old English words that happened to converge on nearly identical modern spelling. As a consonant-plus-y noun, the insect name takes the standard -ies plural.

The insect noun and the verb of flight are historically distinct words that simply look alike today, which occasionally causes confusion, but their plural and conjugation forms stay cleanly separate in practice: "flies" as the plural insect noun, and "flies" as the third-person singular verb ("the bird flies") — identical spelling, but grammatically unrelated uses that context sorts out immediately.

Usage note

"Flies" is standard for the insect in every context, and the identical spelling also serves as the verb's third-person singular form, a coincidence that rarely causes real ambiguity since the noun and verb rarely occupy the same grammatical slot in a sentence.

Entomologists distinguish true flies, insects of the scientific order Diptera with a single pair of wings, from many other winged insects colloquially called "flies," including mayflies and dragonflies, which belong to entirely separate insect orders despite sharing the common name. The word's coincidental identity with the flight verb has inspired its own wordplay over the centuries, most famously the pun "time flies like an arrow, fruit flies like a banana."

Common real mistakes

The consonant-plus-y misspelling "flys" is the primary error, though it's worth noting this insect name should not be confused with "flies" used as a verb, or with the unrelated noun "fly" meaning the front opening of a pair of trousers, which is typically not pluralized at all in that specific sense. Anglers and fly-fishing writers should also distinguish literal insect "flies" from artificial fishing "flies," hand-tied lures designed to resemble the insect, a specialized craft vocabulary that pluralizes identically to the literal insect sense.

The wider pattern

Under the same y-to-i-plus-es mechanism that governs "spy" and "sky," the noun "fly" pluralizes to "flies" — and the fact that it's spelled identically to an unrelated verb meaning to travel through the air is a handy reminder that English plural rules key off spelling alone, indifferent to whatever other word a form happens to resemble.

Examples

• Flies swarmed around the picnic table within minutes.
• The barn was full of flies during the hot summer months.
• She swatted at the flies buzzing near the window.
• Fruit flies multiplied quickly in the unwashed bowl.
• The fishing lure was tied to resemble local flies.
• Horses swished their tails to keep the flies away.
• Scientists study fruit flies extensively in genetics research.
• The screen door kept most of the flies outside.
• Several flies had gotten trapped between the window panes.
• The exterminator treated the kitchen for flies and other pests.
• The tackle shop sold hand-tied flies for trout fishing.
• Researchers bred thousands of flies for the genetics experiment.
• The exterminator identified three separate fly species in the attic.
• Fly fishing enthusiasts tie their own flies by hand.
• The greenhouse struggled with an infestation of whiteflies and fruit flies.
• She swatted away the flies buzzing near the fruit bowl.
• The barn owner sprayed repellent to keep the flies away.
• Naturalists documented dozens of fly species along the riverbank.
• The lodge kept screens on every window to keep out flies.
• Trout rose eagerly to the flies drifting on the current.
• She studied how flies navigate using visual landmarks.
• The barn swallows fed on flies swarming near the pond.
• She tied new flies for the upcoming fishing trip.
• Researchers released sterile flies to control the population.
• The stable owner sprayed the horses to keep off flies.
• She caught several flies in the kitchen with a swatter.