Gnats are flies: fungus gnats, midges and no-see-ums are all small true flies (Diptera). Fungus gnats flutter weakly near houseplants and don't bite (UC IPM). Midges swarm near water; chironomids don't bite, but biting midges (no-see-ums, about 1/16 inch) do (Missouri Dept. of Conservation; Clemson HGIC). Phorid flies are humpbacked and run over surfaces instead of flying (University of Kentucky). House flies are much bigger (6–7 mm) and strong fliers (UF/IFAS).
Gnats vs flies: is a gnat a fly?
Yes. Are gnats flies? Every insect people call a gnat — fungus gnats, drain flies, midges, no-see-ums — is a true fly, in the order Diptera, with a single pair of wings. The Missouri Department of Conservation puts it simply for midges: "being true flies, midges ... have only one pair of wings". So gnats vs flies isn't really a contest between two groups; it's the difference between small, delicate flies and the bigger, sturdier ones like the house fly.
What is a gnat?
What is a gnat? Not a scientific group. "Gnat" is a loose everyday name for many kinds of small fly. Clemson Extension notes that two quite different flies are both "referred to as gnats" outdoors — biting midges and eye gnats — and indoors the word usually means fungus gnats from houseplants, or drain flies. The University of Kentucky lists fruit flies, fungus gnats, drain flies and phorid flies as the small flies found most often in homes.
Flies vs gnats: the house fly comparison
The clearest flies vs gnats comparison is with the house fly. A house fly is 6–7 mm long, grey with four dark stripes on its thorax (UF/IFAS) — two or three times the length of a fungus gnat, which is commonly 1/16 to 1/8 inch (UC IPM). UC IPM contrasts the two directly: compared with "more active species such as the common housefly", fungus gnats are "relatively weak fliers and usually don't move around much indoors". House flies breed in manure, garbage and compost; fungus gnats in damp potting soil. See how long house flies live for the house fly itself.
Gnats vs midges
Gnats vs midges is the outdoor version of the question, and the answer depends on whether they bite.
Non-biting midges
Chironomid midges are the clouds of small flies dancing over a pond or a lawn at sunset. The Missouri Department of Conservation describes them as looking "a lot like mosquitoes: small and dainty, rather soft-bodied, with long, narrow wings and long, skinny legs". Most are under 3/8 inch long; males often have feathery antennae, used to hear the high-pitched wingbeat of females. Their swarms are made of males, often in the early evening, gathered above a marker such as a bush or a post. Adults rarely eat and live only a few weeks at most, and they don't bite people. Their larvae, the red "bloodworms", live in the mud at the bottom of lakes and ponds.
So midges vs gnats indoors? Midges don't breed in houseplants or drains, so a swarm in the garden won't become an indoor infestation. Two easy ways to tell a midge from a mosquito: midges have no scales on their wings, and at rest they hold their front legs forward and up, while many mosquitoes hold their hind legs out and up (Missouri Department of Conservation).
No-see-ums vs gnats
The "gnats" that bite are biting midges, family Ceratopogonidae, genus Culicoides. Clemson Extension: "Biting gnats, also called sand gnats or no-see-ums, are in fact biting midges." They're around 1/16 inch long, so small that "most humans never see the midge as it feeds". They breed in the mud around swamps, marshes, ponds and streams; females need a blood meal for their eggs, feed early and late in the day and leave an itchy, raised spot. Ordinary screens don't stop them — Clemson recommends 20×20 mesh or smaller, and a fan, because they're "small enough to be blown off course by air movement". That's the whole no-see-ums vs gnats difference: no-see-ums bite, fungus gnats never do.
Phorid flies vs gnats and fungus gnats
What are phorid flies?
What are phorid flies? Small flies of the family Phoridae, also known as humpbacked, scuttle, coffin or sewer flies (University of Kentucky). The common household species are Megaselia scalaris and Dohrniphora cornuta (NC State Extension). They are about 1/8 inch long on average and black, brown or yellowish.
Phorid fly identification
- The hump: seen from the side, a clear hump on the thorax — the reason for the name humpbacked flies (NC State; University of Kentucky).
- The run: they "run rapidly over surfaces when disturbed, rather than flying away" (University of Kentucky) — scuttle flies.
- The wings: heavy veins near the front edge, with three or four fainter veins running across the wing — NC State calls this the easiest identification feature.
- The legs and eyes: long, flattened hind legs, and no red eyes (NC State); Colorado State notes a small head and brown eyes, unlike the fruit fly's large head and red eyes.
Phorid fly identification versus a fungus gnat therefore comes down to behaviour and build: a phorid runs, a fungus gnat flutters; a phorid is humped and stocky, a fungus gnat slim and long-legged. That's phorid flies vs fungus gnats in two words — running versus fluttering. And for phorid flies vs gnats in general, the breeding site is the other big difference: no phorid fly comes from a well-kept houseplant.
Where do phorid flies come from?
Where do phorid flies come from? Moist, decaying organic matter. NC State lists pet waste, the lining of drain pipes and trash bins, moist food that has dropped under appliances, rotting fruit and vegetables, garbage disposals, dirty mop heads, septic systems, potted plants and compost. The University of Kentucky adds standing water from leaking appliances, and notes that breeding sites can be "buried well below ground". The University of Maryland warns that very large numbers on a ground floor "may indicate a broken drain". NC State suggests starting the search in the kitchen — rotting bags of potatoes and onions — then plumbing, crawl spaces and wall voids; tape or a clear bowl over a drain shows whether flies are breeding in its biofilm.
Unlike fungus gnats, adult phorid flies live about one to two months (University of Kentucky). For what cold does to them, see do phorid flies die in winter.
Gnats vs mosquitoes: a quick comparison
Fungus gnats and midges both look like small mosquitoes, so the quick test is behaviour: a mosquito comes to you, often with a high whine near your ear, and the female bites (CDC). Fungus gnats stay near their pots and never bite (UC IPM); non-biting midges have no wing scales and hold their front legs up (Missouri Department of Conservation). Mosquitoes breed in standing water and rest indoors in dark, humid places like under sinks and in closets (CDC). For how long they live and why only females bite, see the mosquito lifespan calculator.
Identification table: gnats, flies, midges and phorid flies
All six insects at a glance, using the same sourced descriptions as the identifier above:
| Fungus gnat | House fly | Midge | Biting midge | Phorid fly | Mosquito | |
|---|---|---|---|---|---|---|
| Size | 1/16–1/8 in (about 2–4 mm) | 6–7 mm (about 1/4 in) | Most under 3/8 in (under 1 cm) | About 1/16 in (1.5 mm) | About 1/8 in (3 mm) | Not given in our sources |
| Body and wings | Slender, mosquito-like, long legs and antennae | Stout, much bigger than a gnat | Mosquito-like but no scales on the wings; males have feathery antennae | Tiny fly — 'most humans never see the midge as it feeds' | Humped back, small head, long flat hind legs | Long legs; scales on the wings; long biting mouthparts |
| How it moves | Weak, erratic flight near pots and windows; runs over the soil | Fast, active flier | Males dance in swarms, often in the early evening above a marker | Small enough to be blown off course by a fan | Runs rapidly and erratically over surfaces instead of flying | Flies to people; a high whine close to your ear |
| Where you see it | Houseplants, windows, computer screens | Kitchens, bins, around food | Near lakes, ponds and other water | Near swamps, marshes, ponds and streams | Bins, drains, damp floors, ground floors, anywhere in the building | Indoors in dark, humid places; outdoors near standing water |
| Bites people? | No | Not covered in our sources | No | Yes — females, early and late in the day; itchy raised spot | No | Yes — females |
| Breeds in | Moist potting soil and organic matter | Manure, garbage, compost, rotting food | Larvae (bloodworms) at the bottom of lakes, ponds and damp places | Mud around marshes, ponds and streams | Decaying organic matter — drain linings, broken pipes, rotting potatoes, pet waste | Standing water in containers |
"Not given in our sources" means exactly that: we didn't find an extension or academic source for that detail, so the identifier doesn't use it.
How this identifier works
Each answer adds points to every insect whose sourced description matches it. Flight style, location and bites count most (3 points each), then size and swarming (2 each), for a maximum of 13.
| Clue | Weight | What it points to |
|---|---|---|
| How it flies | 3 points | Weak → fungus gnat; fast → house fly; swarm → midge; runs → phorid fly; whine → mosquito |
| Where | 3 points | Plants → fungus gnat; water → midges and mosquitoes; food, bins or manure → house fly or phorid fly; drains → phorid fly |
| Bites | 3 points | Yes → biting midge or mosquito; no → fungus gnat, midge or phorid fly (house fly bites aren't covered in our sources, so they score nothing) |
| Size | 2 points | Tiny → biting midge, fungus gnat or phorid fly; house-fly size → house fly or a large midge |
| Swarms | 2 points | Yes → midge (males dance in swarms — Missouri Dept. of Conservation) |
Confidence is "high" when the best match scores 9 or more and leads by at least 3; "medium" with a lead of 2; otherwise "low". "Not sure" answers add nothing and cap confidence at medium. A trait our sources don't describe never earns points — mosquito size and house fly bites, for example — so the result never rests on a guess. Each result tells you what to check for breeding and links to the page that covers that insect.
Frequently asked questions
Are gnats flies?
Yes. Gnats — fungus gnats, drain flies, midges and no-see-ums — are all true flies (Diptera) with one pair of wings. "Gnat" just means a small, delicate fly.
What is a gnat?
An everyday name for many small flies, not a scientific group. Indoors it usually means fungus gnats or drain flies; outdoors, biting midges or eye gnats (Clemson HGIC; University of Kentucky).
What's the difference between gnats and midges?
Midges are one kind of gnat. Non-biting midges swarm near water and don't bite; biting midges (no-see-ums) are tiny and bite. Houseplant gnats are fungus gnats, a different family.
Do midges bite?
Chironomid midges don't bite people (Missouri Dept. of Conservation). Biting midges, or no-see-ums, do — females feed early and late in the day (Clemson HGIC).
What are phorid flies?
Small humpbacked flies (Phoridae), about 1/8 inch long, that run rapidly over surfaces instead of flying. They breed in decaying organic matter such as drain linings and rotting produce (University of Kentucky; NC State).
Where do phorid flies come from?
Moist, decaying organic matter: drain and bin linings, rotting potatoes or onions, food under appliances, pet waste, septic systems and broken drain pipes (NC State; University of Maryland).
How do I tell phorid flies from fungus gnats?
Phorid flies are stocky and humped and run across surfaces; fungus gnats are slender, long-legged and flutter near houseplants.
Are no-see-ums gnats?
Yes, in everyday speech. Clemson Extension: biting gnats, sand gnats or no-see-ums "are in fact biting midges" (Ceratopogonidae).
Is it a gnat or a mosquito?
If it comes to you, whines and bites, it's a mosquito (or a biting midge). Fungus gnats stay near pots and don't bite.
How can I keep no-see-ums out?
Use very fine screens (20×20 mesh or smaller) and a fan on porches — they're small enough to be blown off course (Clemson HGIC).
The identifier gives a best match from sourced descriptions, not a lab identification. For a stubborn problem, your local cooperative extension service can identify a specimen. We give no pesticide advice, and for fleas on pets please talk to your vet.