How long does the average fly live? Well, the lifespan of a fly differs depending on the type of fly. The average lifespan of a housefly is about 28 days, so 4 weeks. There are many different species of fly with lifespans ranging from just a few days to almost two months. By far though, the most commonly encountered fly is the common housefly, Musca domestica.

Like most common pests, houseflies are looked upon with disdain by humans. As known carriers of disease and connoisseurs of carrion, they are often picked to represent decay and rot, and the fly has been used in literature and poetry to symbolize the brevity and fragility of human life.

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“Flies are so mighty that they win battles, paralyse our minds, and eat up our bodies.” — Blaise Pascal

Despite the negative associations, houseflies are actually extremely useful in the laboratory, where their ease of culturing, fast reproduction cycles, and long-lived somatic cells make them ideal model organisms for studying the effects of DNA manipulation and hereditary mechanisms. In particular, houseflies have robust genetic sex-determination mechanisms, which makes them of interest to researchers investigating reproduction.

Anatomy Of A Housefly

Like most insects, the common housefly consists of a segmented body, with a head, abdomen, and thorax. In addition to their 6 legs and 2 wings, they are most distinguished by their relatively large compound eyes. The fly’s mouth consists of non-functional mandibles and maxillae and a long flexible proboscis. A fleshy sponge-like appendage at the end of the proboscis is used to lap up material on surfaces.

Each leg of the fly contains chemoreceptors for taste, and at the end of each leg is a pair of claws for grasping and climbing. At the terminal end of each leg are two adhesive pads known as pulvilli. The pulvilli allow the fly to adhere to smooth surfaces by exploiting what are known as Van der Waals forces, the attractive forces that arise when the electrons of molecules are brought extremely close together.

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“A spoon full of honey gets more flies than a barrel full of vinegar.” — St. Francis de Sales

Houseflies have an eye shape and function that is disanalogous to that of the human eye. Where the human eye is composed of a single smooth lens that focuses light onto photo-receptor cells, housefly compound eyes are made out of thousands of tiny individual receptors called ommatidia (ommatidium sing.). Each ommatidium is attached to a nerve receptor cell which relays light information to the fly’s brain. So while the human eye relays more or less a single image to the brain, housefly eyes relay thousand of individual images to the fly’s brain at the same time. In other words, human vision is like a single painting, while the housefly’s vision is like a mosaic where each piece is a smaller image.

As a result of their eye structure, houseflies cannot focus on an image as humans can. But, they have an almost 360-degree field of vision and can perceive individual images at a rate of up to 300 frames per second. For reference, the average human vision has only a 6-degree field of vision on the macula and can only discriminate between individual images at a rate of ~16-24 frames per second. Any higher framerate is perceived as a continuous motion by humans. Their extraordinarily quick perception is one reason why flies are notoriously hard to catch, Karate Kid notwithstanding.

Life Cycle/Behavior

The average housefly has a lifespan of about 4 weeks. Due to this relatively short lifespan, a large amount of a housefly’s life is spent attempting to reproduce. In a single lifetime, one female housefly can lay approximately 500-600 eggs in batches of 75-150. The eggs are typically laid in decaying organic material which acts a food source for the newly hatched maggots. In about two weeks the maggots fully mature into adult flies and set off to mate and reproduce.

Houseflies mainly subsist on an all-liquid diet, as they lack functioning mandibles to masticate solid material. The sponge-like appendix at the end of the proboscis, known as the labellum, secretes saliva to soften up solid material and gather food particles. Because their diet consists of mostly dead or decaying organic material, houseflies play an important ecological role as nature’s waste dispensers. Houseflies also form the base of the food chain for a number of predators, including reptiles, birds, spiders, and amphibians.

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Additionally, their favoring of dead and rotten material makes houseflies strong potential vectors for disease. Houseflies have been shown to carry over 100 different parasites and diseases, ranging from roundworm to tuberculosis and anthrax. In fact, houseflies are such strong potential disease vectors that they have historically been used as a form of biological warfare among humans. In the medieval period, fleas and flies were used to spread the plague, and entomological warfare (the use of insects as weapons of war) has been utilized by a number of modern nations as recently as World War II.

Uses In The Laboratory

Houseflies are relatively easy to culture, have fast reproductive cycles, and have long-lived post-mitotic cellular tissues. These properties make houseflies ideal model organisms for use in the laboratory. Houseflies and the closely related fruit fly Drosophila melanogaster are used extensively in laboratories to study DNA manipulation and its effects on hereditary patterns. The biologist Vincent Dethier in his biography To Know A Fly (1962) quipped that the lack of sympathy for flies made them ideal test subjects, as, “he whose heart shudders at the thought of working with vertebrates has little compunction when swatting a fly.”

According to studies, houseflies exhibit multiple mechanisms for sex determination in reproduction. Generally, the mechanisms for determining the sex of offspring are the same with respect to a certain species, but houseflies seem to use multiple mechanisms. Interestingly, these mechanisms display such a flexibility that artificial sex determination mechanisms can be created in the laboratory.

“An optimist is a fellow who believes a housefly is looking for a way to get out.” — George Jean Nathan

Additionally, housefly larvae have sometimes been used in medical environments as maggots eat decaying flesh that may be susceptible to infection. Maggots can be used to clean open wounds of dead and decaying tissue, and there is evidence that suggests maggot-based therapies are more effective for open wound treatment than conventional methods. Perhaps most incredibly, there is evidence that maggot secretions are effective antimicrobial agents against drug-resistant staph infections.

So despite their sinister reputation and potential health risks, houseflies actually serve a number of unique purposes. They are effective at removing waste from environments and they show promising futures in biological and medical research. So next time you see a housefly before you swat it thank it and its kin for all they do for us.