Flies

Fly Facts

Classification:

Kingdom: Animalia

Subkingdom: Bilateria

Phylum: Arthropod

Class: Insecta

Order: Diptera

Suborder: Various (see below)

Geographical location: Every continent

Habitat: Grasslands, forests, near rivers and lakes; everywhere except the ice caps of the North and South poles

Gestational period: Eggs hatch in one to seven days; adulthood occurs in one to six weeks

Life span: Most species live between 7 days and six months

Special anatomy: Six legs; compound eyes; one pair of wings plus halteres, antennae, proboscises; pulvilli

Flies belong to one of the largest and most diverse animal orders, Diptera, which comprises 125,000 species of two-winged insects known as "true flies." The Diptera evolved from four-winged insects, and their vestigial rear wings are stalks that act as gyroscope balance organs, enabling the forewings to move the fly forward without causing nose dives and keeping it on a steady course.

Dipterans occur worldwide, including Antarctica, but are most plentiful in moist, warm climates. The main dipteran suborders are Nematocera and Brachycera. Nematocera are mosquitoes, slender, with long antennae. The other suborder has stout bodies and short antennae; among them are houseflies and tsetse flies.

Physical Characteristics of Flies

A fly body has three parts: a head, thorax, or middle part that holds six legs, and an abdomen or rear end. Two compound eyes containing up to four thousand facets cover most of the head. The eyes see light changes and sudden movement from many different directions at once. This is why flies are hard to catch.

Atop a fly’s head, paired antennae provide the senses of touch and smell. Dipterans have a mouth part called a proboscis. It is funnel-shaped, with a wide part at the bottom. The proboscis is like a straw, sucking up fluid via a pump in the head. The proboscis of a housefly is soft, so it cannot bite. Bee flies have a long proboscis that enters flowers for nectar. Biting flies, such as horseflies, have hard, sharp proboscises that pierce the skin of their victim.

Each foot on a fly is tipped by claws that grip rough surfaces. Under the claws are pads called pulvilli. On smooth surfaces, they flatten and grip tightly, allowing a fly to walk upside down on ceilings without falling. Flies breathe through openings called spiracles on each side of the thorax and abdomen.

Fly Life

The life of a fly begins when a female lays hundreds to thousands of eggs in manure, garbage, vegetable waste, fruit, plant stems, or stagnant water. Houseflies have telescoping ovipositors that place eggs in soft matter. Other species have stiff ovipositors that penetrate plant stems or fruit. Flies, like mosquitoes, also lay eggs on water.

The heat given off in these environments incubates the eggs. In one or two days, they hatch as white, legless larvae (maggots), which eat the material surrounding them. Maggots rapidly grow too large for their skins, which split and allow molting. After molting twice, the larvae find sheltered places to form pupariums, where they molt a final time and become pupae and then become winged flies. The process usually takes four to ten days. Adult flies emerge from the pupae full-sized. Most live for thirty days. In winter, most die, but larvae and pupae live to become adults in the spring. The development of a fly from egg to larva to pupa to adult is called metamorphosis.

Tsetse Flies, Horseflies, and Blowflies

Tsetse flies are around thirty-three species of the genus Glossina. About five of these species carry sleeping sickness. They live in grasslands, forests, and river and lake shores in Africa, between the Sahara and Kalahari Deserts. Tsetse flies suck blood with their sharp proboscises; the blood is digested with the assistance of a membrane that is secreted around the blood meal in the foregut. Tsetse flies mate year round. Females have one offspring per mating. After fertilization, an egg hatches in the mother and is later born as a full-sized maggot, which burrows into the soil, pupates, and becomes an adult in a month. The flies carry trypanosomes that can infect people with sleeping sickness. When the tsetse fly bites a victim, the trypanosomes enter, multiply, make them very tired, and quickly kill them.

The 4,500 Tabanidae species of horseflies and deer flies occur worldwide, mostly in tropical and temperate fields and forest areas near water. They have inch-long, stout bodies, large heads, short antennae, and iridescent compound eyes. Their mouths pierce the surfaces of plants and animals to suck fluids. Males eat nectar and plant juices, while females suck blood. The time of mating varies according to species and climate. Females must eat blood before laying eggs, or the eggs do not develop. A female lays one thousand eggs in damp sites such as rotten wood. Larvae hatch in two to three weeks, although some hibernate all winter. Before becoming adults, larvae pupate for about three weeks. Adults live for six weeks before mating and dying. Horsefly bites are painful and may serve as a vector for anthrax, transmitting it from animals to humans or causing a rare bacterial infection called tularemia.

Blowflies (family Calliphoridae) are metallic blue-green and are larger than houseflies. Females lay eggs on meat or in the open wounds of animals. Eggs or larvae that are in food swallowed by animals and humans cause pain and sickness when the maggots eat into the wound or tissue where they were laid. When the larvae enter an animal’s skin, puss-filled sores form. The screwworm, a blowfly larva, harms livestock. If screwworms are not controlled, animals die.

Flies and Disease

A housefly can carry pathogenic bacteria disease because it lives in manure and garbage. Thousands of related species transmit germs to whatever surface they land on, spreading disease. Often, it is best to kill flies and stop their reproduction. Flies destroy crops, parasitize animals, and carry typhoid and cholera. Other commonly carried diseases include inflammatory eye infections like trachoma and conjunctivitis, typhoid, cholera, dysentery, giardiasis, pinworm, roundworm, whipworm, hookworm, and tapeworm, among many others. Flies carry these pathogens on their mouths, leg hairs, sticky foot pads, or in their vomit or feces.

However, flies are also useful and are a critical part of the world's ecosystems. Some flies, such as hoverflies, pollinate plants. Larvae also eat aphids, which kill crops. Flies speed the decomposition of animal carcasses and manure. In addition, flies consume other harmful insects, controlling their numbers. Finally, flies are the food source for numerous insectivores higher up the food chain.

Principal Terms

Iridescent: Showing the colors of the rainbow depending on light reflection

Molt: Shed an insect shell

Symbiosis: Beneficial relationship between two organisms

Bibliography

Chapman, R. F. The Insects: Structure and Function. 5th ed. Cambridge, Cambridge University Press, 2016.

Fischer-Nagel, Heiderose, and Andreas Fischer-Nagel. The Housefly. Minneapolis, Carolrhoda Books, 1990.

"Flies 101: Different Types of Flies and the Threats they Pose." National Pest Management Association, www.pestworld.org/news-hub/pest-articles/flies-101-different-types-of-flies-and-the-threats-they-pose. Accessed 25 June 2023.

Kjer, Karl M., et al. “Progress, Pitfalls and Parallel Universes: A History of Insect Phylogenetics.” Journal of the Royal Society, Interface, vol. 13, no. 121, 2016. doi:10.1098/rsif.2016.0363.

Lawrence, Peter. The Making of a Fly: The Genetics of Animal Design. West Sussex, Blackwell Science, 1992.

Miller, Sara Swan. Flies: From Flower Flies to Mosquitoes. London, Franklin Watts, 1998.

"True Flies (Diptera)." Smithsonian, www.si.edu/spotlight/buginfo/true-flies-diptera. Accessed 10 Sept. 2024.