Invertebrates

Fly

Diptera · also called flies, true fly

Not Evaluated omnivorecarnivoreherbivore Temperate grassland Temperate forest Rivers & wetlands

A fly flies on half the usual equipment. Where most flying insects have four wings, a true fly has two, with the back pair shrunk into a pair of tiny vibrating knobs that work as gyroscopes. The result is the most agile flight in the insect world — and an order that includes both the animal that has killed more humans than any other, the mosquito, and the fruit fly that built modern genetics.

Overview

Flies are the order Diptera, around 160,000 named species, on every continent including Antarctica. The defining feature is a single pair of flight wings, the hindwings reduced to knobbed stalks called halteres. Flies have large compound eyes, mostly sponging or piercing mouthparts, and a life cycle with a legless maggot and a pupa. As a group they eat almost every kind of organic matter.

The halteres

The halteres beat in time with the wings and act as vibrating gyroscopes, feeding the fly constant data on how its body is rotating. This is what lets a fly hover, fly backwards, land upside down, and complete a banked turn in under 50 milliseconds. A fly also tastes with its feet, so it walks over food to sample it.

Useful and deadly

Flies are the second most important pollinators after bees — biting midges are the only effective pollinators of the cacao tree. Blowfly maggots are primary decomposers of carcasses and a forensic clock for timing death. But mosquitoes, tsetse flies, sandflies, and blackflies transmit malaria, dengue, sleeping sickness, and river blindness, which makes flies collectively the deadliest animals to humans.

Status

Flies as an order are Not Evaluated. Most are abundant, but broad declines in flying-insect biomass in Europe and elsewhere include hoverflies and other beneficial flies, and some specialists tied to old forest, particular wetlands, or natural dung have declined.

Fly: common questions

What makes an insect a true fly?

Having a single pair of functional wings. Most flying insects have two pairs, but in the order Diptera the hindwings are reduced to halteres, small drumstick-shaped organs that act as flight sensors rather than for lift. This is the defining feature, and it is why entomologists write the names of true flies as two words, house fly, horse fly, fruit fly, while insects called flies that are not Diptera, such as dragonflies, butterflies, and mayflies, are written as one word. Other clues to a true fly are large compound eyes, short antennae in many groups, sponging or piercing mouthparts rather than chewing jaws, and a life cycle with a legless maggot and a pupa.

Why do flies land on food and walk around on it?

A fly tastes with sensory hairs on its feet, so walking over a surface is how it samples whether the surface is food. Many adult flies also cannot bite or chew; they have a soft, spongy mouthpad and can only take up liquids, so to feed on solid food such as fruit, sugar, or a carcass they first spit out saliva and digestive enzymes onto it, let that dissolve some of the surface, and then sponge up the resulting liquid. Repeated landing, tasting, regurgitating, and sponging is why houseflies are effective at moving bacteria from waste onto food, since they carry pathogens on their feet and mouthparts and in their gut.

Are flies good for anything?

A great deal, most of it unnoticed. Flies are the second most important pollinators after bees, and some crops, including cocoa, rely on them. Blowfly and other maggots are primary decomposers of carcasses and dung, recycling nutrients and cleaning the landscape, and medical maggots are used to clean wounds. Hoverfly larvae eat aphids and are valued in pest control, and many flies are food for birds, fish, bats, and spiders. Flies are also central to science — the fruit fly Drosophila has been a workhorse of genetics for over a century, and forensic entomology uses fly development to time a death. Only a small fraction of fly species are pests or disease carriers.

Why is a fly so hard to swat?

Speed and sensing. A fly's compound eyes and its visual processing run far faster than ours, so a hand moving toward it appears to the fly in something like slow motion, giving it plenty of warning. On detecting a looming shape, the fly's nervous system very quickly works out which way the threat is coming from and pre-positions its legs for a jump away from it, then launches, all within about 100 milliseconds. The halteres let it stabilise and steer the escape in mid-air. To beat a fly, aiming slightly ahead of where it is sitting, so your hand arrives where it jumps to, works better than aiming straight at it.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 10, 2026. Last updated September 10, 2026.

Quick facts

Scientific name: Diptera
Diet: Almost everything organic, since across the order flies eat nectar, sap, rotting fruit and flesh, dung, blood, fungi, plant tissue, and other insects, with the adults of many species sponging up liquids through a soft mouthpad and the maggots feeding as scavengers, parasites, or predators
One pair of wings, and a gyroscope: The order name Diptera means two wings, and it refers to the single pair of flight wings; the hindwings have shrunk into a pair of tiny knobbed stalks called halteres that beat in time with the wings and act as vibrating gyroscopes, feeding the fly constant information on rotation and letting it make split-second corrections
Extreme aerial agility: Flies are among the most manoeuvrable of all flying animals, able to hover, fly backwards, land upside down on a ceiling, and execute a banked turn in under 50 milliseconds, and a housefly's escape response, triggered by looming movement, plans an evasive jump and takes off in around 100 milliseconds
Tasting with the feet: A fly carries taste receptors on the pads of its feet, so it can tell whether it has landed on something edible simply by standing on it, which is one reason flies walk over food, and the same pads bear sticky, clawed structures that let the fly grip smooth vertical surfaces and hang upside down
The most important pollinators after bees: Flies, especially hoverflies, bee flies, and midges, are the second most important group of pollinators worldwide, and some crops depend on them almost entirely, with the tiny biting midges being the only effective pollinators of the cacao tree and therefore of chocolate
Nature's recyclers and forensic clock: Blowfly and flesh fly maggots are among the first and most efficient consumers of carcasses and dung, returning nutrients to the soil, and because different species arrive and develop on a body in a predictable sequence, forensic entomologists use them to estimate how long a person or animal has been dead
Disease carriers and a model organism: Mosquitoes, tsetse flies, sandflies, and blackflies transmit malaria, dengue, sleeping sickness, and river blindness, making flies collectively the deadliest animals to humans, while at the other extreme the fruit fly Drosophila is one of the most important research animals in the history of genetics
Range: Every continent including Antarctica, where a wingless midge is the largest purely terrestrial animal, and almost every habitat from the deep-sea shore and hot springs to alpine snowfields, deserts, caves, and the insides of buildings, with larvae developing in fresh and salt water, wet soil, dung, carcasses, fungi, plant stems, and the bodies of other animals.
Conservation status: Not Evaluated ≈ Not assessed as an order of around 160,000 named species, and most flies are abundant, but there is broad evidence of decline in flying insect biomass in Europe and elsewhere that includes hoverflies and other beneficial flies, and a number of specialist species, such as some hoverflies tied to old forest or particular wetlands, are threatened, while a few dung and carrion flies have declined with changes in livestock farming and the use of veterinary drugs in the wild