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Flies: Personally a Nuisance, Scientifically a Wonder

They ruin your picnics, land in your drinks and can even strike you with painful bites. But, flies aren’t just a nuisance; they’re a fascinating piece of our environment.

A surprisingly wide array of insects fall into the same family as flies. Flies are members of the insect order diptera. There are over 20,000 species of flies in North America alone. They have mobile heads with a pair of compound eyes. Their mouths have proboscises that are adapted for piercing, sucking or lapping up food.

Fly History and Biology

Flies are found in nearly all environments on Earth and on every continent except Antarctica. They are a hugely diverse family, with many members that do not resemble the others at all.

The fly family includes around a million species such as horse flies, mosquitoes, house flies and fruit flies. The name of their order, diptera, means “two-winged.” Their front set of wings are used for flying, while the back wings have evolved into a pair of club-shaped organs that help the balance. This wing arrangement makes them highly maneuverable. Their claw-like feet make it possible for them to grip a number of surfaces.

Fly Anatomy

Flies come in all sorts of shapes and sizes. The largest fly, Guaromydas heros, can attain several inches in length. Many live underground, but others consume the insides of rotted trees. The smallest fly, the phorid fly Euryplaeta nanaknihali, is five times smaller than fruit flies. This fly, discovered in Thailand, decapitates ants and lays its eggs inside them.

A number of flies, including mosquitoes, Syrphid flies and flower flies are helpful pollinators, second only to the family that includes bees. The flowers that they are attracted to include ones that are pale and dull in color, with hues from purple to dark brow. In many cases, such as skunk cabbage or corpse flower, they will have a putrid smell that mimics the carrion that many flies eat. Flies are typically more important pollinators in areas that are too cold for other pollinators to live in. A number of flowers that only produce low amounts of pollen use what is known as trap pollination; these flowers draw the fly in and trap it with the pollen from another specimen. It is thought that flies may be the earliest pollinators of flowers.

Fly on Leaf

Flies, depending on species, have a wide range of food sources. Some feast on the blood of animals, including people. Others live on the nectar of flowers. Still others eat rotted fruit and decaying carcasses left behind by other predators. So-called carrion flies can smell dead flesh from as far as seven kilometers away. These flies consume the dead animals and eliminate the rot from the environment. Many scientists have said that, without flies, we’d be covered in the planet’s waste before too long.

Flies are the food source of a number of animals that include birds, lizards and other insects.

A number of flies are parasitic in nature. They have been found to infest other insects, mammals, amphibians, mollusks, millipedes and woodlice.

Fly Reproduction and Lifecycle

Flies go through a complete metamorphosis that includes egg, larval, pupal and adult stages. Eggs are laid on a food source that the larvae can consume. The larvae, which lack limbs, are then able to develop in a protected environment inside the food source. Anyone who has picked up a piece of fallen fruit and seen maggots inside is familiar with this.

The number of eggs a fly lays depends on the species. The common housefly lays around 500 eggs, deposited in batches of 75 to 150. A female mosquito may lay anywhere from 100 to 300 eggs.

Fly Life Stages

Other types of flies will lay their eggs inside of the leaves of plants. These eggs will cause blisters in the leaves when the eggs hatch.

Other flies’ larvae feed on fungus. These flies can be found in fungally infected male flowers. When the fly lands there, they pick up pollen which is brought to the female flower.

During the pupal stage, the fly is protected inside a hard capsule. They emerge as winged adults. The adult phase of a fly’s life is typically very short, usually lasting a few days at most. The adult fly’s purpose is to mate and lay eggs for the next generation. The shape of fly genitals usually involves torsion so that the sex organs are easily accessible. The large swarms that make up most fly populations mean that adults have ample opportunities to mate.

In mosquitoes, only females developing eggs bite and suck blood. During the rest of their life cycle, and throughout the entirety of the male’s life, they feed on nectar.

Human Relationships with Flies

Fruit flies are used in scientific research because it is easy to rear them and they have a very rapid life cycle. Since they share about 75% of the genes that can cause disease in humans, they are very useful for studying human genetics.

Certain flies with large larvae are useful for bait when fishing. Some farmers also deliberately grow maggots to make up basically free, protein rich food for chickens and other avian livestock.


Military surgeons observed that soldiers whose wounds became infested with maggots had better outcomes than those whose did not. In the early 20th century, a surgeon at Johns Hopkins University experimented with deliberately applying maggots to wounds. More recently, blow fly maggots raised in sterile conditions have been used for debriding infected wounds. The maggots eat only the necrotic flesh, leaving behind the living and healthy tissue.

As most people know, however, our relationships with most fly species are not positive. Many species of mosquitoes bite humans to drink blood. When these mosquitoes feed on other people who have diseases, they can pass the infection along. Mosquito-borne diseases include malaria, yellow fever, several types of encephalitis, West Nile virus and Zika fever.

Other flies can transmit disease, as well, or even become parasitic on humans. Bot fly eggs, for instance, can be deposited in humans and other animals by mosquitoes. The bot fly larvae develop under the skin and can be quite painful.

Tsetse flies and screw worms can transmit trypanosomes, which can infect both humans and livestock such as cattle. The disease can be deadly and often the only way to stop the transmission of the disease is to slaughter infected livestock.

What to Do When You Are Bothered by Flies

At our homes and when we go out in nature, we can often be plagued by mosquitoes, horseflies, fruit flies and common houseflies.

Mosquitoes can be discouraged by ensuring that all standing water around your home is drained on a regular basis. This can include shallow baby pools, rainwater in flower pots and standing water that gathers under an air conditioning unit. Mosquito larvae grow in water, so eliminating this can cut down on the local population.

However, since mosquitoes can travel between one and seven miles in their lives, this is usually not sufficient alone. A number of mosquito repellents, containing everything from rosemary oil to deet are available to keep mosquitoes from biting. Consumer Reports found that the repellents containing either deet or picaridin were the most effective.

There are also mosquito traps on the market that use propane to produce carbon dioxide. Since mosquitoes are attracted to the carbon dioxide we exhale, these can effectively attract and trap mosquitoes.

Fruit flies can invade the home and will hover around trash cans, kitchen sinks and over ripe fruit in your fruit basket. To dissuade them, empty trash regularly and keep fruit in the refrigerator. The flies can be effectively trapped using fly paper in the areas where they gather. You can also rig a homemade trap by putting fruit scraps at the bottom of a bread bag. When flies enter the bag, seal the top and discard. It may take many traps to eliminate the flies in your house.

Dead Fly

Houseflies can be controlled by removing their food and water sources. Empty trash cans regularly. Dry the sink after use. If these efforts do not work, areas where flies congregate, such as under the eaves of your home, can be sprayed with a liquid insecticide or covered with insecticide dust. When you visit restaurants, you may see bags of water hung around, often with a penny inside. While this is a common folk remedy, scientific research has not found it to be an effective fly repellent.

Horse flies can be more difficult to deal with. There are few repellents that are effective, although some people report success using repellents that contain deet. A better solution is to avoid the flies themselves. There are a number of tent-style screen rooms on the market that can keep the flies from getting near you and attacking.

Flies and Us

Flies are ubiquitous in the environment. Eliminating them completely may have unforeseen consequences, such as build up of rot and failure to pollinate flowers. To ensure that we can live with them more peacefully, we should do what we can to keep them away from our campsites and out of our homes.

More About Flies: Personally a Nuisance, Scientifically a Wonder