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 certain band of insects has earned itself a notorious reputation. First off, they buzzzzz all around our picnics and patio parties. They lap the ice cream, dive into cola, and tickle through the hair on our arms. Sometimes they bite, too. Indoors, they behave no better. And they also may carry diseases. So you can hardly be faulted for wishing all flies could be driven from the planet. Particularly since insect expert Erica McAlister1 reports there are an estimated 17 million flies in the world—that’s per person! The good news is that the vast majority of flies go mostly unnoticed and quietly go about doing important work for our planet.
Importance of flies
Flies are second only to bees and a few wasps as pollinators of plants. In some cases, they’re the primary pollinators. For instance, the world would have no chocolate without midge flies to pollinate cacao flowers!2 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 themselves food for other wildlife.
In the following, you’ll read about their abilities, behavior, and much more. For instance, did you know they can fly both forward and backward, hover, and turn on a dime? Have you seen them flip upside down to cling to your ceiling and wondered how they did that? You’ll find the answer here. You’ll also notice that not all flies are House Flies, Musca domestic, and horse flies; they’re diverse in appearance, with several species 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 our planet. More than 120,000 fly species have been identified so far. They include robber flies, crane flies, flower flies, soldier flies, and fruit flies. Mosquitoes, gnats, and midges are flies, too. More than 16,000 species make their home in North America. About 91 percent of all flies living around human habitations are House Flies because the smell of our food and garbage attracts them.
The world’s smallest fly is Euryplatea nanaknihali, discovered in Thailand in 2012, which measures a minuscule 0.0158-inch (0.4 mm)—smaller than a grain of salt. At the other extreme is the giant Midas Fly, Gauromydas heros, 2.8 inches (70 mm) long, which inhabits Brazil, Bolivia, and Paraguay.
Flies belong to the order Diptera (DIP-ter-uh), a word from the Greek dipteros for “two wings.” As the name says, flies have only two wings, which sets them apart from almost all other flying insects. Dipterans are called “true flies” because there are other insects with the word “fly” in their name but aren’t actually flies. Think of butterfly and dragonfly; they aren’t 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.
A fly has three body sections: head, thorax, and 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.
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 contains only 100,000 neurons, compared to 100 billion for humans, but it has strong systems for detecting and remembering smells. Flies also can make decisions and 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) and are sensitive to any movement around them. Most species also have three simple eyes, or ocelli, which are arranged in a triangular pattern on their “forehead.” They sense light rather than 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.
Flies have two antennae, which are covered by sensory organs for detecting odors. Depending on the species, they may be long or short. Those with long antennae tend to be small and delicate, and many of them produce 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 upper and lower lips (labrum and labium), most have maxillae (“pincers” to hold and manipulate food), two appendages called palpi (sensory organs), and many have mandibles (strong jaws for cutting and sawing).
Flies take in their liquid food in one of three ways, depending on the species: by sucking, sponging or lapping, or rasping-sucking. House Flies, for example, 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 rasp-like mandibles to saw through skin and suck up flowing blood (the males 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, although they may bite if handled.)
The thorax is the middle section, and it’s all about movement: attached 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 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 a butterfly’s. Different species have unique patterns of colorful spots, swirls, and stripes, which may be a way they 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. 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 way when a fly walks.
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, also, 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. So how, then, you may wonder, do those 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.)
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 sixteen 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. Instead, 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. They can feel touch through scattered hairs on the body.
Flies undergo complete metamorphosis, which means they progress in four stages, from egg to larva to pupa to adult.
Females 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 he’s the most suitable suitor. First, he taps her on the abdomen with his foreleg. Then he shows off by “singing” to her by vibrating his 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.
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 ten or more generations of House Flies in a season.¹ The 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 and, depending on the species, will shed it (molt) three to eight times.² When they’ve molted for the last time, they move away from their food source to pupate within their hardened outer skin or in a cocoon made of soil or silk, or both. When they emerge, they’re fully formed adults and soon ready to start mating.
The complete lifespan of deer flies and horse flies is two or three years, but only thirty to sixty days of that is spent as an adult. House Flies go through their entire life cycle in about thirty days, spending about twenty-five days as adults. Fruit flies spend a week or two as larvae and pupae, then forty to fifty days as adults. Most flies overwinter as larvae or pupae. The adults usually die, although some 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 game. (It would only take 1,200,000 mosquito bites to totally drain our blood, 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 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.
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. Some, like crane flies, don’t eat at all as adults.
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 may see feasting on rotting carcasses and garbage.
Flies and disease
There’s no denying that despite their benefits to the world, flies are disease vectors. Flies that feed on garbage and carcasses inadvertently collect disease organisms on their bodies as they do their daily business. Those pathogens are then transferred to whatever 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 U.S. 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.
|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, with true flies, the word “fly” is separate from their common name. For example, there are the Bee Killer Fly, Robber Fly, Tabanus Horse Fly, Mosquito Hawk Crane Fly, and House Fly—while, with other insects, “fly” is used to form one word. But 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, and some gnats. Mosquitoes stab rather than bite.
|6 A male horse fly, Hybomitra hinei wrighti, was clocked at 90 miles per hour (145 km/h) while pursuing a female, according to Wikipedia.com.
|7 Floreano, Dario; Zufferey, Jean-Christophe; Srinivasan, Mandyam V.; Ellington, Charlie, Flying Insects and Robots, Springer Science & Business Media, 2009.