All about flies – pests at picnics, but vital pollinators


Jump down to: Background  •  Physical description  •  Head, Brain, Eyes, Antennae, Mouth   •  Thorax, Wings, Legs  •  Abdomen  •  Senses  •  Reproduction, Life cycle, Lifespan  •  Behavior, Habitat, Diet, Predators  •  Flies and disease  


A band of notorious insects has earned themselves a lousy reputation. First off, they buzzzzz themselves all around our picnics and patio parties. We catch them lapping up the ice cream, skating across the Jello, diving into cola, and tickling through the hair on our arms. Sometimes they bite, too. Indoors, they behave no better. And they also may carry diseases. You can hardly be faulted for thinking all flies should be driven from the planet! But learn all about them, and you’ll appreciate their importance.

It may surprise you that the bothersome ones represent only a tiny number of flies. Insect expert Erica McAlister1 reports there are an estimated 17 million flies in the world—that’s per person!—but the vast majority go unnoticed.

Flies are second only to bees and a few wasps as pollinators of plants. In some cases, they’re the primary pollinators—without midge flies to pollinate cacao flowers, the world would have no chocolate!2 Egads! Flies are also recyclers of plant and animal matter into organic nutrients. Entomologist Brian Lessard3 says “. . . if we lived in a world without flies, our streets and parks would be full of dead animals, rotting leaves and logs and nasty surprises left by dogs.” As if that wasn’t important enough, flies are predators of undesirable insects and are themselves food for other wildlife.

Flies are intriguing in other ways. They’re champion aerialists: They can fly both forward and backward, hover, turn on a dime, and flip upside down to stand on a ceiling. And, they’re diverse in appearance, with several families resembling bees and wasps.


Flies first appeared on earth over 200 million years ago during the Upper Triassic Period. Today they live everywhere, except the Polar Regions, and are among the most successful insects on earth.

More than 120,000 fly species have been identified so far, and include such groups as robber flies, crane flies, flower flies, soldier flies, horse flies, fruit flies, and others. Mosquitoes, gnats, and midges are flies, also. More than 16,000 species make their home in North America. In the world, about 91 percent of all flies around human habitations are House Flies, Musca domestica.

Flies are highly diverse in appearance and size. The world’s smallest is Euryplatea nanaknihali, which was discovered in Thailand in 2012 and measures a mere 0.4 mm—smaller than a grain of salt. The largest is the giant Midas Fly, Gauromydas heros, up to 2.8 inches (70 mm) long, which inhabits Brazil, Bolivia, and Paraguay. 

A fly that looks like a honeybee standing on a white flower.

The Drone Fly, Eristalis tenax, is a hover, or flower, fly. It feeds on nectar and is the most widely distributed hover fly in the world. Easily mistaken for a honeybee. (Jean and Fred / Flickr; CC BY 2.0)

Flies belong to the order Diptera (DIP-ter-uh), which is a word from the Greek dipteros for “two wings.” And, just as the name says, flies have only two—their forewings—which sets them apart from almost all other flying insects. Dipterans are known as “true flies” because many other insects have the word “fly” in their name, like dragonfly and butterfly, but only they are true flies.4

Diptera is divided into two suborders. The first is Nematocera (nee-maw-toe-SER-a), a word from New Latin nemat + cera, for thread-like horn. It consists of mostly aquatic flies, such as mosquitoes, crane flies, and midges, which are delicate-bodied, slender, usually small, and have long antennae with at least six segments. The second suborder, Brachycera (bra-KISS-err-uh)—from New Latin brachy + cera, for “shortened horn”—consists of heavier-bodied flies with antennae of usually no more than three segments.

Physical description

A fly has three body sections: head, thorax, abdomen. Its shape is typically slender with long legs. An external skeleton (exoskeleton) made of chitin, which is tough and flexible, encases the body to provide support and protection. There are bristles and hairs on the body; some flies have more than others.

Most flies are a shade of brown, gray, or black. There are some pale ones, though, and bright ones, too. For example, flies in the family Bombyliidae (bom-buh-LIE-uh-dee) look much like bees. Flower flies have striking black-and-yellow patterns on their abdomen. And, long-legged flies, family Dolichopodidae (doll-uh-co-PODE-uh-dee), are iridescent green or orange.

Illustration of a fly with arrow pointing to different parts of the external anatomy.

Fly external anatomy. I: head; II: thorax III: abdomen. — 1: prescutum 2: anterior spiracle 3: scutum 4: basicosta 5: calypters 6: scutellum 7: wing vein 8: wing 9: abdominal segment 10: haltere 11: posterior spiracle 12: femur 13: tibia 14: spur 15: tarsus 16: propleuron 17: prosternum 18: mesopleuron 19: mesosternum 20: metapleuron 21: metasternum 22: compound eye 23: arista 24: antenna 25: maxillary palps 26: labium 27: labellum 28: pseudotracheae 29: tip. (Fiestoforo / Wiki; CC BY 4.0)


The fly’s head is large and movable and contains a brain, two compound eyes, three simple eyes (usually), two antennae, and mouthparts. Parasitic flies, Ormia ochracea, also have unique hearing organs they use to locate singing prey, such as crickets.


The brain is minuscule and contains only 100,000 neurons (compared to 100 billion for humans), but it has systems for detecting and remembering smells. And, not only that, but flies can make decisions, and they appear to “think” before they act. 


Two compound eyes occupy most of the head. Together they have an almost 360-degree field of view and can see colors. They’re composed of up to 3,000 or more individual lenses (called ommatidia) that are sensitive to any movement around them. Most species also have three simple light-sensing organs called ocelli (oh-CELL-ee), arranged in a triangular pattern on their “forehead,” which don’t transmit images. Some flies have eyes that reflect colors. Those of blowflies and fruit flies, for example, reflect red, a horse fly’s eyes reflect bold metallic colors, and the deer fly, Chrysops relictus, has gold-green eyes with dark-red patches.


There are two antennae, and they may be long or short, depending on the species. Flies with long antennae tend to be small and delicate, and many of them have aquatic larvae. They include mosquitoes, midges, and crane flies, among others, and are grouped in the suborder Nematocera. Those with short antennae tend to be heavier-bodied, but not always, and are in the suborder Brachycera; they include house flies, fruit flies, and blowflies. Sensory organs for detecting odor cover the antennae.


Flies don’t chew; they consume their food in liquid form. All have a labrum and labium (upper and lower lips), most have maxillae (“pincers,” to hold and manipulate food) and two appendages called palpi (sensory organs), and many have mandibles (strong jaws for cutting and sawing).

Close up of the head of a robber fly in the family Asilida, showing its sharp proboscis.e

Robber fly (Asilidae sp.). A formidable predator, notice its large compound eyes, and needle-like proboscis used to stab prey and inject a paralyzing venom to subdue it. © guraydere / Shutterstock)

Flies take in liquids in one of three ways, depending on the species: By sucking, sponging or lapping, or by rasping-sucking. For example, House Flies sponge up liquids with their labium. Fruit flies stab into fruit with their proboscis and suck the juices. Female deer flies, horse flies, biting midges, and others5 associated with painful bites use their rasp-like mandibles to saw through skin and then suck up flowing blood (males, on the other hand, suck nectar).

Female mosquitoes stab through skin with a syringe-like proboscis and then suck blood. Robber flies stab prey, inject a combination of venom and digestive enzymes into it (to subdue and liquefy the hapless creature), and then drink the liquid. (That sounds like a pretty awful way to kill something, but robber flies are beneficial as predators of pest insects. Fortunately, they aren’t aggressive toward humans and don’t sting, but may bite, if handled.)


The thorax is the middle section, and it’s all about movement—attached to it are the insect’s wings and legs.


While most insects have four wings, flies have just two, the front ones. They’re delicate and appear to be transparent, sometimes with a few markings. However, scientists have discovered that in front of a black background, they glow with rainbow colors on a par with those of a butterfly. Different species have their own unique patterns of spots, swirls, and stripes, and that may be a way flies communicate with each other and identify other species. The colors aren’t pigments, but a reflection off of the microscopic structures in the wings.

Apparently, flies see their lack of hind wings as a glass-half-full, as it were, and simply ramp their wing speed up to 1,000 times per second, when necessary. That makes them the champion wing-beaters of the world!

Although they don’t use that adaptation to fly forward particularly fast—top speed is about 5 miles per hour (8 km/h)6—they harness it for a tactical advantage. They can fly in any direction, turn 90 degrees in milliseconds, go forward and backward, hover, and flip upside down to stand on a ceiling. Fruit flies under study were seen to fly nearly upside down, and other species may do it, too. Dario Floreano, who studied insect flight control and how it might apply to robotics7 has this to say about flies’ aerial agility, ” . . . (they’re) “arguably the most agile objects on earth, including all things man-made and biological.”

Why the hind wings don’t develop isn’t known. But, they don’t leave an empty spot; in their place are tiny club-shaped “halteres” that oscillate with the wings and behave like gyroscopes to keep the body level. It’s speculated they may also be used in some fashion when a fly walks.

Picture-winged Fly standing on a green leaf.

Picture-winged Flies, Chetostoma curvinerve, are fruit flies with a noticeable wing pattern. (gailhampshire / Flickr; CC BY 2.0)


Flies have six long legs, three on each side, and they’re composed of many human-like structures: hip bone, femur, tibia, and tarsus. Feet, too,  and they’re equipped with tiny claws that play an important role: A particular talent of flies is their ability to walk up walls, cling to window glass, and hang upside down. They manage to do that using large, hairy footpads made up of short hairs called setae (SET-ee). The setae are spatula-shaped and produce a “glue” made of sugars and oils. How then, you may wonder, do six tiny tootsies become unglued when the fly wants to, well, fly? It turns out that each glued foot uses its two claws to help it get unstuck. (With the right equipment, scientists can see the gluey footprints left behind.)

Illustration of a fly's internal anatomy with labels pointing to different organs.

Fly internal anatomy. (Daktaridudu / Wiki; CC-BY-SA 4.0.jpg


The abdomen is the back section of the body and the largest. Within it are the rearmost parts of the nervous, digestive, and circulatory systems, in addition to the excretory and reproductive systems.

The entire cavity is filled with clear “blood” (called hemolymph) that doesn’t carry oxygen. Instead, flies breathe through a series of 16 respiratory openings called spiracles, eight spaced along each side of the abdomen. (The thorax has another pair.) Attached to each one is a tube-like trachea that networks throughout the body to passively deliver oxygen to cells and collect carbon dioxide to be expelled.

Flies don’t have a heart or blood vessels. In what’s called an open circulatory system, blood circulates from the dorsal vessel, a pumping organ, to the insect’s head and then freely floods back through the body, bathing all the organs in nutrients and taking up waste along the way.

The digestive system is essentially a long tube that stretches from the mouth to the rectum. It consists of several sections (guts), including a stomach. Nutrients are extracted from food and passed through semi-permeable membranes into the blood-filled body cavity.


Flies have five senses: sight, hearing, smell, taste, and touch. Their eyesight is excellent, and most also have color vision. Their antennae have hair-like sensory nerve cells that detect odors and sound waves. The maxillary palpi (part of their mouthparts) have taste-sensors. Scattered hairs on the body can feel when they’re touched.


Flies undergo complete metamorphosis, which means they progress in four stages, from egg to larva to pupa to adult.

Female flies announce their sexual willingness by releasing enticing chemical odors (pheromones). Some species have complex courtship rituals. Take fruit flies, for example. A male must follow a sequence of steps to accomplish a seduction. Although the female has released “come hither” pheromones, he must convince her that he’s the most suitable suitor. First, he taps her on the abdomen with his foreleg. Then he shows off by “singing” to her with his vibrating wings. His final show of ardor is to touch his mouthparts to her genitals. If these actions don’t take place in the proper order, the poorly organized little guy is plain out of luck.

Dozens of mosquito larvae hanging from the surface of water and others swimming around.

Mosquito larvae in water. About 100 eggs are laid at a time on the surface. (© Ajintai / Shutterstock)

Females lay their eggs in masses of dozens to hundreds in moist places, such as fruits, vegetables, fungi, carrion, manure, and, with parasitic species, inside their hosts. A single House Fly, Musca domestica,4 may lay 500 or more eggs in her lifetime, and there may be as many as 10 or more generations in a season.¹ Eggs hatch in warm weather, anywhere from half-a-day to weeks later, depending on the species.

Video: Female Picture-winged Fly, Pseudotephritis approximata, depositing eggs in rotting wood. (Katja Schulz / Flickr: cc by 2.0)


Fly larvae are usually soft-bodied and legless, and except for aquatic species, they have poorly developed heads. There’s a lot of variation in their coloring. Aquatic species breathe through a tube located at the tip of the abdomen, raising it above the surface of the water, like a snorkel. Non-aquatic species, which is most of them, live in either the soil or decaying matter or are parasites of other wildlife.

Larvae feed nonstop. As they get larger, they outgrow their skin. Depending on the species, they shed it (molt) three to eight times.² When they’ve molted for the last time, they migrate away from their food source to pupate in either their hardened outer skin or a cocoon made of soil or silk, or both. When they emerge from pupation, adults are fully formed and soon ready to start mating.


The complete lifespan of deer flies and horse flies is two or three years, but only 30 to 60 days of that is spent as an adult. House Flies go through their complete life cycle in about 30 days, spending about 25 days as adults. Fruit flies spend a week or two as larvae and pupae, then 40 to 50 days as adults. Most flies overwinter as larvae or pupae, and the adults die. Some adults survive by hibernating in a warm place, including homes or attics from which they may make a surprise winter appearance.


There are some exceptions, but most flies are active in the daytime. Adults can easily be spotted as they go about their daily business. Some buzz around flowers, sipping nectar. Others perch on foliage waiting for prey to pass by. Sometimes, as with mosquitoes, humans are among their prey. (It would only take 1,200,000 mosquito bites to drain our blood totally, so be very careful out there!)    If Mosquitoes Love You, It’s in Your Genes     


There’s hardly any environment or organic matter that flies don’t live in or on. The larvae of aquatic species are found in lakes, ponds, rivers, puddles, swamps, marshes, birdbaths—even brackish water. Some larvae burrow into sediment on the bottom, while others live in water-plant stems, leaves, and roots. Non-aquatic species live in soil, in fungi, on plants, under bark, in garbage, and in dead or alive animal tissues. There are even some, the larvae of the Petroleum Fly, Helaeomyia petrolei, that live in pools of crude oil, feeding on insects that fall into them.

Food sources

Both male and female adults feed on a wide variety of foods, including nectar, sap, living or decaying plant or animal matter, other insects, and dung. Females of some species, such as mosquitoes, drink blood. Hessian flies eat grass. There are some, like crane flies, that don’t eat at all as adults.

Golden-backed Snipe Fly, which has a beautiful golden thorax, standing on a green leaf.

Golden-backed Snipe Fly, Chrysopilus thoracicus, found in wooded areas of the eastern US. Adults alleged to be predatory. (Katja Schulz/Flickr; CC BY 2.0)


Larvae feed on a wide variety of foods, both plant and animal, with some being parasitic. The larvae of many species look like white rice and are the roiling masses of “maggots” we find feasting on rotting carcasses and garbage.


Birds, bats, insects, and spiders prey on adult flies. Larvae are prey for fishes and other aquatic life.

Flies and disease

There’s no denying that in spite of their myriad benefits to the world, flies are disease vectors. Flies that feed on garbage and carcasses inadvertently collect disease organisms on their bodies as they go about their daily business. Those pathogens are then transferred to whatever else the flies touch or feed on, including our foods. That’s a potential problem if the fly that landed on a hotdog at a picnic previously foraged in a dead animal.

Flies can also spread germs to humans when they bite. Fortunately, most people in the US don’t develop symptoms greater than short-lived pain or an itch. We also have accessible medical care should we need it. But, in many parts of the world, fly-transmitted diseases take a tragic toll, causing unimaginable human suffering and millions of deaths each year.

*Top photo: House Fly, Musca domestica. (© Cherdchai Chaivimol / Shutterstock)
1 Curator of Diptera, Museum of Natural History, London, and author of The Secret Life of Flies, published in 2017.
2 We have certain species of flies in the family Ceratopogonidae, called Chocolate Midges, to thank for this. They inhabit Central America, South America, Africa, and Asia.
3 Watch his interesting TEDx talk.
4 Curiously, the common name of a true fly is usually separate from the word “fly”—for example, Bee Killer Robber Fly, Tabanus Horse Fly, Mosquito Hawk Crane Fly—while, with other insects, it’s combined to form one word. The Monarch Butterfly isn’t a butter fly, and the Golden Dun Mayfly isn’t a may fly.
5 Types of flies that bite: deer, horse, black, stable, snipe, sand, yellow, biting midges, some gnats. Mosquitoes stab rather than bite.
6 reports, however, that a male horse fly, Hybomitra hinei wrighti, was clocked at 90 miles per hour (145 km/h) while pursuing a female.
7 Floreano, Dario; Zufferey, Jean-Christophe; Srinivasan, Mandyam V.; Ellington, Charlie. Flying Insects and Robots. 2009. Springer Science & Business Media