FLIES, MIDGES, AND MOSQUITOES: Diptera

PHYSICAL CHARACTERISTICS

Adult dipterans have large compound eyes, or eyes with multiple lenses, that often meet over the top of the head in males but are usually separated in females. There are two basic kinds of antennae (an-TEH-nee), or sense organs. Dipterans that have long antennae with six or more segments include mosquitoes, crane flies, midges, punkies, and no-see-ums. Those with short antennae include bee flies, flesh flies, horse and deer flies, house and stable flies, hover flies, and robber flies. The mouthparts are long and are used for sucking liquids. In predatory and blood-sucking species, such as robber flies and mosquitoes, the jaws form stiff, needle-like structures that pierce the exoskeleton or hard outer covering of other animals. They then form a straw to draw bodily fluids from the prey. In houseflies and others, the mouthparts are soft and fleshy with sponge-like structures at the tip and are used for sopping up liquids. Some flies with short antennae have a special air-filled sac. This sac is inflated only once and is used to help the young adult burst out of the pupa (PYU-pah).

An important feature that distinguishes adult flies from all other insects is the presence of only two wings. They are attached to the middle section of the thorax or midsection, which is enlarged to hold the flight muscles inside. The second pair of wings is reduced in size and resembles small clubs. They are used as balancing organs during flight. Flies that live on the bodies of animals, such as bat flies, as well as some other species, are wingless. In winged species, the bases of the wings may or may not have flaplike lobes at their bases.

The legs are variable, depending on the habits of the fly. Some are spiny and are used to capture insect prey while flying. The legs of some males are used to grasp females while mating or in elaborate courtship behaviors. Parasitic species have legs that help them cling to feathers or hair. Many species of dipterans have legs equipped with combs and brushes for grooming. The feet of all species are five-segmented and are incredibly sensitive. Some flies actually taste their food with their feet. House flies have oily and bristly pads on their feet allowing them to walk upside down on ceilings or climb smooth surfaces such as glass.

The fly abdomen has eleven segments and may be long and slender or shorter and thicker. The last two or three segments vary considerably in different species and are used for mating and egg laying.

The legless larvae (LAR-vee), or young, never resemble the adults. They are long and are either nearly cylinder-shaped or tapered at both ends. In mosquitoes the thorax is much larger than the head or abdomen. Across their bodies are swollen regions usually covered with short spines. These bumps and ridges are rough to help the larvae to get a grip and move through soil, mud, wood, water, or flesh. In species with adults that have long antennae, the larval head is distinct and nearly round and has jaws that chew from side to side. In all other flies the small pointed head is less distinct or not distinct at all and has jaws that move up and down. In these species most of the head can be withdrawn inside the thorax. Simple eyes, or eyes with only one lens, and antennae of fly larvae are greatly reduced in size or absent.

The thorax and abdomen are soft. They may or may not have spiracles along their sides, breathing holes that connect to the respiratory system. Aquatic species have only a single pair of spiracles located at the tip of the abdomen. These are sometimes mounted on a long, snorkel-like extension, allowing them to remain underwater as they breathe air directly from the surface. For example, rat-tailed maggots, larvae of drone flies, live in the bottom of ponds and breathe through a long tube resembling a rat's tail. Other aquatic species have snorkel-like extensions fitted with tiny saws used to tap into air pockets in the tissues of underwater plants.

The pupae of flies with long antennae show hints of adult features. Their legs and wing pads are clearly visible and are not completely attached to the body. In other groups of flies the pupae are smooth and resemble seeds because they are wrapped inside the old exoskeletons of the mature larvae.

GEOGRAPHIC RANGE

Flies, midges, and mosquitoes are found on all continents including Antarctica. There are about 124,000 species of flies worldwide, with about 5,127 species in the United States and Canada.

HABITAT

Adults and larval dipterans live in nearly all habitats on land and in freshwater. Almost all adults are found on land and move about freely. However, louse flies spend most of their lives on their host animals. Dipteran larvae are found in fresh and brackish, or salty, water, wet soil and leaves, and other wet places. Some species live as external parasites on animals or bore through the tissues inside leaves. Larval shore flies live along the edges of hot springs and geysers where temperatures are more than 112°F (44.4°C). Others develop in pools of crude oil or inside the nests of ants, bees, and wasps. The pupae are usually found in the same habitats as the larvae. Aquatic species usually become pupae in the water, but species found along the water's edge sometimes prefer pupal sites away from water.

DIET

The larvae of many species eat plant materials such as leaves, fruits, or roots of plants. Aquatic species filter bits of plant matter from the water or scrape algae (AL-jee) off leaves, rocks, and wood. Predators, parasites, and scavengers eat rotting plant materials and animals or animal waste. Those species that live as external parasites attack insects, spiders, and centipedes.

Adults suck or lap up all kinds of plant and animal fluids from living or dead organisms. Only female mosquitoes, no-see-ums, black flies, and horse flies bite humans and other animals because they need a blood meal to start the development of their eggs.

BEHAVIOR AND REPRODUCTION

Many species gather in mating swarms, usually around a large, stationary object such as a shrub, tree, boulder, or house. Others use open, well-lit areas such as a sunny patch in the forest or along a road. March flies, also known as lovebugs, form large mating swarms along roads in Central and South America, as well as in parts of the southeastern United States.

Many species of hover flies, bee flies, fruit flies, and robber flies mimic the distinctive colors and shapes of ants, bees, and wasps. These flies not only look like stinging insects, but they act like them too. Resembling harmful insects allows mimics to fool potential predators and live longer so they can mate and reproduce.

Before mating, many flies engage in courtship behaviors that include leg and body movements or wing flapping. In some dance flies, males offer females a dead insect as food. Mating usually starts with the couple facing the same direction but ends with the male and female facing opposite directions. A few species reproduce by parthenogenesis (PAR-thuh-no-JEH-nuh-sihs), a process where the larvae develop from eggs that have not been fertilized.

The life cycles of flies include four very distinct stages: egg, larva, pupa, and adult. Females lay their eggs on or near the right kind of food for the larvae. Fruit fly females use their long needle-like ovipositor, or egg-laying tube, to pierce the skin of fruit and lay their eggs inside. Parasitic species lay their eggs in, on, or near their hosts. Flies with aquatic larvae lay their eggs in the water or on nearby rocks and vegetation. Mosquitoes lay their eggs singly or in groups that form floating rafts on the surface of the water. The eggs usually hatch in a few days or weeks. The larvae molt, or shed their exoskeletons, four to nine times before reaching the pupal stage. The larvae may take just a few weeks to up to two years to reach maturity. The adults may live for several weeks or more.

DIPTERANS AND PEOPLE

Flies are extremely important animals in the environment. Many species of animals depend on both the adults and larvae as sources of food. Many flower-visiting species are significant as pollinators of plants. Some are considered directly beneficial to humans. For example, the larvae of some flies prey on aphids in gardens and crops, while others eat the tissues of plants that are considered weeds. The presence of certain kinds of flies can be used as indicators of water quality. The presence of midge larvae known as blood worms indicates a polluted environment. The fruit fly, Drosophila melanogaster, is the most intensely studied animal on Earth and is essential to genetic research. Several groups of flies that breed in decaying flesh have proven useful in helping police detectives to investigate human deaths. These flies have very specific food, temperature, and habitat requirements that can be used to establish not only the time of death but also if the body has been moved after death.

Flies are better known as pests because they are the most important carriers of disease that plague humans and other animals. These diseases have affected the movements of humans and changed the course of history. For example, tsetse flies prevented Europeans from colonizing parts of Africa because they spread deadly sleeping sickness in humans and nagana in cattle. Mosquitoes carry four different kinds of Plasmodium—protozoan single-celled animals that cause yellow fever, dengue fever, and malaria—and infect people with their bites. Even today more people die from malaria every year than all other diseases, car accidents, and wars combined. Until the use of insecticides, medicine, and the occasional window screen, humans were unable to live in some lowland areas without getting sick. Horse and deer flies infect wild and domesticated hoofed animals with several deadly diseases. In the tropics, blood-sucking black flies infect humans with parasitic worms that can cause blindness, while sand flies spread protozoans that, if left untreated, destroy all kinds of tissue and lead to death.

The mere presence of the larvae of bat flies, flesh flies, and bottle flies can cause health problems, especially in animals other than humans. Myiasis (my-EYE-ah-sis) is the infestation of an animal by fly larvae. The larvae of some species live in a wound and feed on the host's living or dead tissue and body fluids. Other species live inside the body where they feed on food inside the host's digestive system.

TAKE TWO MAGGOTS AND CALL ME IN THE MORNING

Military surgeons have long noted that untreated wounds infested with maggots healed faster than treated wounds. This is because the maggots only eat tissues infected with bacteria. Using germ-free maggots sometimes prevented having to cut off arms and legs to stop infection. Antibiotics replaced maggots during World War II for treating serious infections. Bacterial resistance to antibiotics has led to a comeback of "maggot therapy," especially for wounds that will not heal.

Still other species are attracted to eyes or food. Eye gnats (Hippelates) and face flies (Musca autumnalis) are attracted to the moisture produced around eyes. House flies (Musca domestica), little-house flies (Fannia), and latrine flies (Chrysomyia) breed in filth, such as animal waste and garbage. They are considered not only a nuisance but also a potential health hazard when the adults are attracted to food at outdoor parties and picnics.

The larvae of fruit flies chew their way through citrus and other fruit and vegetable crops and are among the most destructive of all agricultural pests. Millions of dollars are lost every year because of the damage they cause, and millions more are spent on efforts to control them. The larvae of other crop pests, such as gall gnats, leaf miner flies, and root miner flies, weaken plants by boring through stems, leaves, and roots.

CONSERVATION STATUS

The World Conservation Union (IUCN) lists seven species of dipterans. Three species are listed as Extinct, or no longer in existence. The sugarfoot moth fly from the United States and the giant torrent midge from Australia are both listed as Endangered, or facing a very high risk of extinction in the wild. The Tasmanian torrent midge from Australia is listed as Critically Endangered, or facing an extremely high risk of extinction in the wild, because of the construction of a hydroelectric dam in its habitat. Belkin's dune tabanid fly, a horse fly from Mexico and the United States, is considered Vulnerable, or facing a high risk of extinction in the wild, because its habitat is being destroyed by development. The Endangered Delhi Sands flower-loving fly is the only fly listed by the United States Fish and Wildlife Service. Its habitat is disappearing due to development and is also being destroyed by trash dumping and pesticide use.

YELLOW FEVER MOSQUITO (Aedes aegypti): SPECIES ACCOUNTS

Physical characteristics: This small mosquito measures 0.1 to 0.15 inches (3 to 4 millimeters) long. It is black with a u-shaped patch of white scales on the back of the thorax and white rings on the legs. The wings are clear with scales along the edges. The white eggs soon turn black after they are laid.

Geographic range: This species is originally from Africa but is now established in all tropical and subtropical regions of the world.

Habitat: Yellow fever mosquitoes live in hot, humid habitats and often breed near human dwellings, especially in towns and cities. Females search for blood meals early in the morning or late afternoon. They prefer human hosts and generally bite around the ankles. They rest in poorly lit cabinets, closets, and cupboards. The eggs are laid singly along the water's edge. The larvae develop in standing water.

Diet: Both males and females depend on plant juices for their own nutrition. Only the females need a blood meal so that their eggs will develop. The larvae strain tiny bits of floating plant material from the water.

Behavior and reproduction: When resting, the back legs are curled up. They often clean these legs by rubbing them against one another. They also raise and lower their back legs, as well as cross and uncross them.

Yellow fever mosquitoes and people: This species is the most important transmitter of viruses that cause human dengue fever and urban yellow fever. It also spreads chikungunya virus in Asia.

Conservation status: This species is not considered endangered or threatened. ∎

STALK-EYED FLY (Cyrtodiopsis dalmanni): SPECIES ACCOUNTS

Physical characteristics: This species is about the size of a house fly. The compound eyes, or eyes with multiple lenses, are located on the tips of horn-like structures, or stalks, that project from the sides of the head. The distance between each eye on a male is nearly equal to the length of its body. The eye-stalks of the females are much shorter.

Geographic range: This particular stalk-eyed fly is widespread in Southeast Asia.

Habitat: They live on damp, shady forest floors near streams.

Diet: The larvae eat plants, while the adults feed on nectar and other plant juices.

Behavior and reproduction: Males stake out rootlets on the ground and compete with one another for females. Males face one another and wrestle each other with their front legs. Eventually the male with the shortest eye-stalks backs down. Females prefer large-bodied males with long eye-stalks as mates. They are usually found in small groups. A single male will mate with up to twenty females in just thirty minutes.

Upon emerging from the pupa, stalk-eyed flies pump body fluids into both their wings and eye-stalks for up to fifteen minutes until they expand to their full lengths.

Stalk-eyed flies and people: These fascinating animals are observed by scientists who study courtship behavior.

Conservation status: This species is not considered endangered or threatened. ∎

TSETSE FLY (Glossina palpalis): SPECIES ACCOUNTS

Physical characteristics: Adult tsetse flies are yellowish to brown, with forward-projecting, piercing mouthparts. They measure up to
0.47 inches (12 millimeters) in length. There is a hatchet-shaped cell in the center of each wing.

Geographic range: They live in western Africa.

Habitat: Tsetse flies are found in patches of dense vegetation along banks of rivers and lakes in hot, dry habitats. They also live in dense, wet, heavily forested equatorial rainforest.

Diet: Adults feed on the blood of birds, mammals, and reptiles.

Behavior and reproduction: Host animals are located primarily by sight, rather than smell. The female keeps a single egg for nine to twelve days inside her body, where it molts three times. The larva is then deposited in the soil and pupates. The pupal stage lasts four to five weeks. The adult emerges from the pupa with the aid of a special, inflatable sac on the head. Females are ready to mate two or three days after emerging, but males may take up to several days more. Adults are long-lived, with males living six weeks and females up to fourteen.

Tsetse flies and people: This species transmits a protozoan, or one-celled animal, that causes nagana in horses and cattle, and sleeping sickness in humans.

Conservation status: This species is not considered endangered or threatened. ∎

SPIDER BAT FLY (Basilia falcozi): SPECIES ACCOUNTS

Physical characteristics: Adult spider bat flies have flat bodies, no wings, and resemble six-legged spiders. They have long legs with strong claws. The shiny pupae are black and flattened.

Geographic range: This species is found in Australia.

Habitat: They live on the bodies of cave-dwelling broad-nosed, bent-winged, and big-eared bats.

Diet: Spider bat flies feed on the blood of bats.

Behavior and reproduction: Adults spend most of their lives on the body of their bat host. Females leave the host to deposit a single, fully developed pupa on cave walls and trees near bat roosts. Females will deposit several pupae during their lifetime. Adults emerge from the pupae when they sense the body heat of a nearby bat.

Spider bat flies and people: This species does not impact people or their activities.

Conservation status: This species is not considered endangered or threatened. ∎

BIG BLACK HORSE FLY (Tabanus punctifer): SPECIES ACCOUNTS

Physical characteristics: Adult big black horse flies are large, heavy-bodied flies and measure 0.35 to 1.1 inches (9 to 28 millimeters) in length. Their wide heads have bulging, brightly colored eyes. The thorax or midsection is gray, while the abdomen is black. The wings are blackish. The larvae are cylinder-shaped and have fine wrinkles along the length of the body.

Geographic range: This species is found in western Canada and the United States, from British Columbia south to California, east to Kansas and Texas.

Habitat: Adults live near ponds, streams, and marshes, while the larvae develop in mud or moist soil along the edges of these habitats.

Diet: Adults eat mostly nectar and pollen. Females require a blood meal before they can lay eggs. They suck blood from livestock and humans. The larvae prey on other insect larvae, snails, and earthworms.

Behavior and reproduction: Females land on exposed skin to feed. They lay up to one thousand eggs in masses three or four layers deep. The masses are laid on leaves, rocks, or other objects near water or moist areas and are covered with a jellylike material. Hatching larvae fall into the water or on moist soil. They pupate at the margins of pools or other drier areas in the habitat.

Big black horse flies and people: Horse flies are a nuisance to horses and mules because of the painful bites of the female. They will also bite people.

Conservation status: This species is not considered endangered or threatened. ∎

MEDITERRANEAN FRUIT FLY (Ceratitis capitata): SPECIES ACCOUNTS

Physical characteristics: Adult Mediterranean fruit flies measure 0.14 to 0.2 inches (3.5 to 5 millimeters) in length. The eyes are iridescent (IH-rih-DEH-sent), or shiny and multicolored. The wings are broad with yellow patterns. The females have a distinctive egg-laying tube, or ovipositor, on the tip of the abdomen. The larvae are white and narrow toward the head, becoming wider toward the rear. The tip of their abdomen is broad and flat. The dark reddish brown pupae are cylinder-shaped and about 0.12 inches (3 millimeters) in length.

Geographic range: This species is native to Africa. During the past one hundred years it has become established in countries in the Mediterranean region, including Spain, Italy, Greece, Jordan, Turkey, parts of Saudi Arabia, and most countries along the North African coast. It is also found in Portugal and the Hawaiian Islands. It is occasionally found in California and Florida in the continental United States, as well as in Mexico, Guatemala, and Chile.

Habitat: The adult Mediterranean fruit flies are found wherever fruit trees grow. The larvae bore inside of fruit.

Diet: Larvae feed within the flesh of citrus, peach, and guava, among many other fruits. The adults sop up fruit juices, honeydew, and plant sap.

Behavior and reproduction: Adults fly only short distances, but winds may carry them up to several miles (kilometers) away.

Females lay one to ten eggs beneath the skin of ripening fruit. They may lay up to three hundred eggs in their lifetime. Eggs hatch after two or three days and molt twice within six to ten days. Mature larvae leave the fruit and burrow 1 to 2 inches (2.5 to 5 centimeters) into the soil to pupate. Adults emerge in about ten days. Males defend leaves on fruit trees as territories and release pheromones (FEH-re-moans), chemical scents attractive to females. Courtship includes brief wing flapping and head movements.

Mediterranean fruit flies and people: The Mediterranean fruit fly is a major agricultural pest in temperate and subtropical regions worldwide. It attacks over two hundred different kinds of fruit crops. Fly infestations in North, Central, and South America are eradicated by flooding the area with thousands of sterile males. Sterile males are exposed to low doses of radiation and cannot produce sperm. Although sterile, these males will still mate with females, but their eggs will not be fertilized.

Conservation status: This species is not considered endangered or threatened. ∎

EUROPEAN MARSH CRANE FLY (Tipula paludosa): SPECIES ACCOUNTS

Physical characteristics: Adult European marsh crane files resemble giant, grayish brown mosquitoes with brown legs. They measure about 1 inch (25 millimeters) in length. Their narrow wings span 0.7 to 0.9 inches (17 to 25 millimeters). The gray larvae are known as leather jackets because their exoskeleton is tough and leathery. Mature larvae measure 1.1 inches (30 millimeters). The brown, spiny pupae are about 1.3 inches (33 millimeters) in length.

Geographic range: Native to northern Europe, they are now found in western Canada and the United States.

Habitat: This species lives in areas with mild winters, cool summers, and rainfall averaging about 23.5 inches (600 millimeters) a year. They prefer wet lawns, pastures, hay fields, and grassy banks along drainage ditches.

Diet: The larvae eat rotting vegetable matter, grass seedlings and roots, and the bases of other young plants.

Behavior and reproduction: Adults are weak fliers and are attracted to lights at night. They may accidentally enter houses and buildings.

Adults are most common in late summer. Females lay up to 280 black, shiny eggs in the soil, usually at night. They hatch within two weeks and grow rapidly to a maximum length of 1.1 inches (30 millimeters). They pupate in the soil in mid-July. The pupal stage lasts about two weeks. The adults emerge at sunset, leaving the pupal case partially sticking out of the soil, and mate immediately. Males live about seven days; females, four to five. There is one generation per year.

European marsh crane flies and people: The larvae strip the root hairs and kill parts of small trees in nurseries by chewing all the way around some stems.

Conservation status: This species is not considered endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Conniff, R. Spineless Wonders, Strange Tales from the Invertebrate World. New York: Henry Holt and Company, 1996.

Hubbell, S. Broadsides from the Other Orders. A Book of Bugs. New York: Random House, 1993.

Oldroyd, H. The Natural History of Flies. New York: W.W. Norton and Company, 1965.

Tavolacci, J., ed. Insects and Spiders of the World. Volume 4: Endangered Species-Gypsy Moth. New York: Marshall Cavendish, 2003.

Periodicals:

Gerster, G. "Fly of the Deadly Sleep. Tsetse." National Geographic 170, no. 6 (December 1986): 814–832.

Moffett, M. W. "Flies That Fight." National Geographic 192, no. 5 (November 1997): 68–77.

Web sites:

"Beetles. True flies, including mosquitoes." BioKids. Critter Catalog. http://www.biokids.umich.edu/critters/information/Diptera.html (accessed on October 25, 2004).

"Diptera. Flies, mosquitoes." Ecowatch. http://www.ento.csiro.au/Ecowatch/Diptera/diptera.htm (accessed on October 25, 2004).

The Young Diptera Site. http://www.sel.barc.usda.gov/Diptera/youngent.htm (accessed on October 25, 2004).

Videos:

Bug City. Flies and mosquitoes. Wynnewood, PA: Schlessinger Media, 1998.