All about flies


Can you think of anything nice to say about flies? Probably not. A band of notorious flies have earned themselves a well-deserved, bad reputation. First off, they invite themselves to our picnics and patio parties. We find them lapping up the ice cream, skipping across the potato salad, swimming in the cola, diving into the Jello salad and tickling through the hair on our arms. Sometimes they bite. And indoors they’re no better behaved. You can hardly be faulted for thinking all flies should be driven from the planet!

It may surprise you then to learn that the bad ones represent only a very small number of fly species. Fly expert Eria McAlister* reports there are an estimated 17 million flies in the world — per person! The vast majority, however, go unnoticed. And, unappreciated: Flies are valuable pollinators of crops and flowers. Flies are recyclers of plant and animal matter into organic nutrients. They’re also beneficial as predators of undesirable insects and are themselves a food source for other wildlife.

Flies belong to the order Diptera (DIP-ter-uh), which is from the Greek for “two wings.” As the name says, Diptera species have only two — their forewings. Diptera is the order of “true flies.” Many insects have the word “fly” in their name, like dragonfly and butterfly, but only Dipterans are actually flies. (By the way, with true flies, the common name and “fly” are two separate words, while with non-Dipterans “fly” forms part of one word. For example, a butterfly, a member of the order Lepidoptera, isn’t a butter fly. And the House Fly, a Dipteran, isn’t a housefly.)

Flies first appeared on earth over 200 million years ago during the Upper Triassic period. They live everywhere in the world, except the Polar Regions, and are among the most successful insects on earth.

Flies are among the champion aerialists of the insect world. They can fly both forward and backwards, hover, turn on a dime and even flip upside down to land on a ceiling (pads on the bottom of their feet enables that little trick.) They’re very diverse, coming in many shapes and range in size from minuscule to two inches long. Several families of flies resemble bees and wasps. So when you see a “bee” or “wasp,” don’t run away, take a second look. It’s just as likely to be a fly.

Image of a Drone Fly, Eristalis tenax, which looks a lot like a honeybee and feeds on flowers.

The Drone Fly, Eristalis tenax, looks a lot like a honeybee and eats nectar. (Gilles Gonthier /  Flickr; cc by 2.0)

More than 120,000 species of flies have been classified by scientists so far. More than 16,000 species make their home in North America. They include families of robber flies, crane flies, flower-loving flies, soldier flies, horse flies, fruit flies and others. Mosquitoes, gnats and midges are also flies.

Physical description
Diptera is divided into two suborders. The first, Nematocera (nee-maw-toe-SER-a), contains mosquitoes, crane flies and midges. These insects are delicate-bodied, slender, usually small and have long antennae (an-TEN-ee) that have at least six segments. The second suborder, Brachycera (bra-KISS-err-uh), includes the heavier-bodied flies, which have short antennae, with usually no more than three segments. (Brachycera comes from New Latin brachy + cera, for “shortened horn.”)

Color illustration of fly anatomy.

Anatomy of the House Fly. 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: maxilary palps 26: labium 27: labellum 28: pseudotracheae 29: tip. (Georg-Johann / Wiki; cc by 3.0)

Flies are very diverse in appearance and range in size from minuscule to 2 inches long. The feature that most distinguishes flies from other insect groups is their number of wings. Almost all other insects have two pairs of wings, but flies have one pair, the forewings. Their back pair doesn’t mature and, instead, forms a knobby-looking “haltere” behind each forewing. The halteres don’t go unused, they serve as stabilizers to keep the insect level in flight. A very few fly species have neither wings nor halteres, but are classified as flies because of other subtle physical similarities. The wings of flies may be transparent or patterned. Flies reportedly beat their wings up to 1,000 times per second, making them the champion wing-beaters of the world.

A fly’s head is large and movable, with huge compound eyes (which consist of hundreds of individual photoreceptors) that are sensitive to movement. Many also have light-sensing organs, called ocelli (oh-CELL-ee) on their heads. Some parasitoid flies have tiny external ears they use to very accurately locate singing prey, such as crickets.

Close up of the compound eyes of a long-legged fly.

Head of a long-legged fly (family: Dolichopodinae). (Donald Jusa / Flickr; cc by 2.0)

The antennae of flies range from threadlike to feathery. Their mouthparts are adapted to sucking, lapping or piercing, depending on what they eat. Flies don’t have stingers, but, some species can deliver a painful “bite,” which is really a stab or a sawing movement. In some species, the female’s ovipositor (egg-laying organ) extends a noticeable length out from the end of her abdomen.

The body color of most flies is a shade of brown, gray or black, but there are many exceptions. For instance, some flies are pale. Flies in the family Bombyliidae (bom-buh-LIE-uh-dee) look like bees. Syrphidae (SURF-uh-dee), or flower flies, have striking black-and-yellow patterns on their abdomen. Long-legged flies, family Dolichopodidae (doll-uh-co-PODE-uh-dee), are iridescent green or orange.

Close up image of Picture-winged fly, Chetostoma curvinerve. They are in the fruit fly family.

Tephritidae. Shown here: Chetostoma curvinerve. (gailhampshire / Flickr; cc by 2.0)

Flies, like other insects, have six legs. A particular talent of flies is their ability to walk up walls, cling to window glass and hang upside down. They manage this with 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, do six tiny tootsies become unglued when the fly wants to fly? It turns out that each gluey foot is also equipped with two claws that are used to help it get unstuck. (With the right equipment, scientists can see the footprints left behind by flies.)

There’s hardly any environment or organic matter that flies don’t live in or on. Larvae (LAR-vee; an immature stage) of aquatic fly species are found in lakes, ponds, rivers, puddles, birdbaths, swamps, marshes — anywhere there’s water, 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 the soil, in fungi, on plants, under bark, in garbage, and in dead or alive animal tissues. The larvae of one species, the Petroleum Fly, Helaeomyia petrolei, live in pools of crude oil, feeding on insects who fall in.
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 prey. (It would only take 1,200,000 mosquito bites to totally drain our blood, so be careful out there!)
Close up image of robber fly, . Robber flies feed on insects.

Powerful and aggressive, robber flies feed on other insects. Shown here: Efferia aestuans. (WW)

Food sources

Adults of most species are active in daytime and feed on a wide variety of foods. At night, they rest, usually clinging to something several feet off the ground near their food source. Adults, depending on species, feed on nectar, sap, living or decaying plant or animal matter and dung. Some, like mosquitoes, feed on blood. Others, like crane flies, don’t feed at all as adults. Fly larvae feed on a wide variety of food sources, 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.

Flies taste with their mouth and their feet, which explains why some species walk on our picnic foods. Reportedly, if both their feet and mouth like the taste, they chow down. Flies that feed on garbage and carcasses inadvertently collect disease organisms on their bodies as they go about their daily business. The germs are then transferred to whatever else the flies touch or feed on. This becomes a potential problem for humans if, for example, the fly that landed on a hot dog was previously foraging in a dead animal. Flies can also spread germs to humans when they bite us. Fortunately, most people in the U.S. don’t develop symptoms greater than a feeling of disgust at the thought of it. 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 suffering and millions of deaths each year.

Life cycle
Flies undergo complete metamorphosis. This means they progress from an egg to larva to pupa to adult. Female flies announce their sexual willingness by releasing enticing chemical odors called pheromones. Some species have complex courtship rituals. Take fruit flies, for example: First, a male must tap a female on the abdomen with his foreleg. Then he must “sing” to her by vibrating his wings. As a final show of his ardor, he must touch his mouthparts to her genitals. If these actions don’t take place in the proper order, the poorly organized little fellow is out of luck.

Close up image of mosquito larvae, Culex sp., hanging head down in water.

Mosquito larvae live in water. Shown here: Culex sp. hanging head-down from the surface. (James Gathany / CDC; cc by 2.5)

A fly’s eggs are laid in masses of dozens to hundreds. For example, a single House Fly, Musca domestica, may lay 500 or more eggs in her lifetime and there may be as many as 10 or more generations in a season.¹ Hatching occurs in warm weather, anywhere from half-a-day to weeks later, depending on the species.

The larvae have highly varied coloration, depending on the species. They feed nonstop. As they get larger, they outgrow their skin. Depending on the species, they shed it (molt) three to nine times.² When they’ve molted for the last time they migrate away from their food source to pupate in a hardened outer skin. The complete life span of some species is several years long, with most of their life spent in the larval and pupal stages. When flies emerge as adults, they live only a short time, from a few hours up to about a month, depending on the species. House Flies go through their complete life cycle in about 30 days.

The larvae of most flies are tiny, soft-bodied and legless, like grubs. Except for aquatic species, the larvae don’t have a well-developed head. Aquatic larvae breathe through a tube located at the tip of the abdomen, raising it above the surface of the water, like a snorkel. Non-aquatic larvae, which is most of them, live in either the soil or in decaying matter or are parasites of other wildlife.   If Mosquitoes Love You, It’s in Your Genes

Flies mostly overwinter as larvae or pupae. Most adults die, but some survive by hibernating in a warm place, including homes or attics from which they may make surprise winter appearances.

Adult flies are preyed on by birds, bats, dragonflies and other insects, and spiders. Larvae are prey for predator and parasitic insects.

*Curator of Diptera, Museum of Natural History, London.
¹ About 91 percent of all flies in human habitations are House Flies, Musca domestica.
 ² The stage between molts is called an instar. When an egg hatches, the tiny caterpillar is a first instar. After it molts for the first time it’s a second instar, and after the second molt it’s a third instar, and so on.

*Top photo: House Fly, Musca domestica. (Julia Wilkins / Wiki cc by-sa 3.0