There are many different shapes of True Flies. They are soft-bodied insects, most are fairly small (less than 1.5 cm long) but a few can be larger (up to 4 cm!). Adult flies have only 1 pair of wings, unlike other insects. The second pair has evolved into small balancing organs that look like little clubs. Adult flies feed on liquids and have either thin sucking mouthparts (like Mosquitos) or sponging mouthparts, a tube with wider sponge at the end (like Flower Flies and House Flies). Most adult flies have large eyes, to help them see when they are flying. Many adult flies look like wasps or bees. Sometimes they look a lot like The larvae of True Flies all look like thick segmented worms, but they have many different shapes. They don't have jointed legs, unlike beetle larvae. Some have mouthparts and a distinct head, but most don't. The pupal stage of a True Fly is covered with tough skin. It may have some of its legs and body parts visible, or it may be hidden inside a larval skin, and just look like a brown capsule.
Flies are one of the most diverse groups of insects. There are over 150,000 species known from around the world, and there are certainly many still undiscovered. In the Great Lakes region there are probably over 2,000 species
True Flies can be found almost anywhere. Adults of many species are strong fliers, which helps them locate supplies of food for their larvae. Fly larvae are most common in damp habitats, and flies populations are largest in humid places with lots of moisture.
True Flies have complete metamorphosis. Adult female flies lay eggs, and then small larvae hatch from the eggs. The larvae are often worm-like, and do not have jointed legs. They molt (shed their whole skin) several times as they grow. Then they transform into a pupa, which is a resting stage that transforms into an adult.
Most female flies produce hundreds of eggs. They lay them on the food supply for their larvae. They are often very sensitive to the smell of the food, and can locate it from kilometers away.
True Flies usually don't have much parental care. The female puts her eggs in the right place, and that's it.
Most flies live less then a year. Many fly species survive the winter only as eggs. Others survive as pupae, and a few survive as larvae or adults. Unless they hibernate, adult flies don't usually live very long, often only a month or two, and sometimes just few days or weeks. Flies usually spend most of their lives as a larva or a pupa. Flies are eaten by many predators, so very few of them live as long as they can.
Adult flies are usually active during the day when it is warmer and they can see as they fly. Fly larvae often feed continuously, day and night.
Flies use vision more than most insects do. They also sometimes detect the vibrations of wingbeats. Like all insects, they use their sense of smell a lot.
Adult flies often drink nectar. Some feed on any liquid that has nutrients. They also can "spit" onto dry food and then suck up the spit and some extra nourishment from the dry food. This is how they contaminate human food. Some female flies drink vertebrate blood, such as from mammals to get the protein they need for their eggs. A few adults are predators, they grab other insects, stab them with their mouthparts and suck out their blood and organs.
Many flies do most of their feeding as larvae. Some eat fungi or plants, especially fruit. Some lay their eggs in the stems or leaves, and they larvae give off chemicals that make the plant swell up into a gall. This protects the fly larva and gives it plenty to eat. Other species eat dead animals, and many eat dung. Some filter microscopic food particles from freshwater water. One big group of flies is parasitic. They lay their eggs inside or on insects and spiders, and the larvae feed on the inside of their host while it is still alive! A few species are parasites of vertebrates, such as mammals and birds, and get in wounds or under the skin.
Adult flies avoid predators with their speed and alertness. Also, many flies mimic stinging insects such as wasps or bees, so predators will avoid them. Larvae often live in places that are hard to reach.
Some flies are imporant pollinators. Many fly larvae are part of the natural 'clean-up squad', helping get rid of dung and dead animals. Flies are important food sources for many other animals.
True Flies are the worst insect pests. Some bite us, some spoil our food, some carry diseases.
The biggest benefit from flies comes from the parasitic species. They attack caterpillars, grasshoppers, and other insects that eat our food plants. Some flies also help pollinate plants that we grow. Flies are also important food source for other animals that we value, like fish.
Very few fly species need conservation. The few that do live in rare habitats that are in danger.
Insects that are True Flies have their name in two parts: House Fly, Flower Fly, etc. But some other kinds of insects are called "flies" even though they aren't related to the True Flies. They should have the "fly" part of their name attached to the previous part. For example: Dragonfly, Damselfly.