Unlike many other insects, houseflies have one pair of wings attached to their thorax, or midsection. (Photo: Muhammad Mahdi Karim via Wikimedia Commons)

Did you know that a female housefly (Musca domestica) can lay 500 eggs over several days? Legless white larva (affectionately known as maggots) emerge from eggs in fewer than 24 hours. Maggots avoid light and begin feeding immediately on the material in which the eggs were laid (manure, rotting flesh, compost). The maggots develop for four to 30 days depending on temperature, then crawls to a dry, cool place to begin its pupal stage. A reddish-brown pupal case protects the developing fly for several days until it emerges as a fully grown adult with reddish eyes. The fly does not grow larger after it emerges – the size of an adult fly just indicates how well it ate as a maggot, with larger flies having eaten better than smaller ones.

At this point, female flies need protein to produce eggs. Scientists have calculated that a pair of flies that begins to reproduce in April could have 191,010,000,000,000,000,000 flies by August if all were to live. Of course, most flies do not live to reproduce, for flies are an important food source for other species, including birds, reptiles, amphibians, insects and spiders.

The common housefly, such as this one in Groton, has two large red eyes that help it see in all directions. (Photo: Tom Murray)

According to one study, flies prefer to lay eggs in manure, fermenting vegetable matter and kitchen waste (in that order). Warm summer weather creates the perfect conditions for houseflies to grow. A fly can complete its life cycle in as little as 10 days if conditions are good. Over the winter, houseflies become dormant, emerging again as the weather warms. There may be a dozen generations of houseflies per year in our region.

Adult flies generally live two to three weeks. They are carnivores that feed on carrion and manure as well as milk or sugary foods. They do not have mouthparts to chew, so their diet is a liquid one: The fly proboscis has a spongelike tip that sucks up liquids by capillary action. The fly also uses its proboscis to regurgitate digestive juices onto solid food to break it down until it is liquified enough to be sponged up.

A housefly alights in Lancaster in September. (Photo: Robert Gessing)

Flies can pick up bacteria from garbage, manure, sewage and carrion. They can transfer these bacteria to human food when they land on it, when they regurgitate to break down food or when they poop. External bacteria on houseflies survive for only a few hours, but bacteria in the gut can live for days and is the most likely to be transferred.

Houseflies have rapid metabolisms that take in and process visual information about seven times faster than we do. This means that a moving flyswatter, which is a blur to our eyes, moves in slow motion to a fly’s eyes. (Side note: Dogs take in visual information at least 25 percent faster than humans do. For this reason, they are not usually interested in watching television. What we see as a smoothly moving image, they see as a series of jerky images because the frame rate is too slow for their eyes.)

In addition, the compound eyes of a housefly have 3,000 or more facets, and they can see in 360 degrees. Flies, however, cannot focus on a single object like we do; they see the world in a mosaiclike pattern.

Housefly eggs, white larvae (maggots), reddish pupae and adult flies. (Photo: Clemson University’s USDA Cooperative Extension Slide Series, Bugwood.org)

Houseflies have taste receptors on their feet and legs. They identify foods by walking on them. To keep these taste receptors clean, flies can often be seen combing their legs and feet. Houseflies walk up windows and walls or crawl across the ceiling. They can do this because at the end of each leg is a pair of claws and two adhesive pads that use weak intermolecular forces to attach to other surfaces. The claws help each leg detach to take a step. At night, flies rest on ceilings, overhead wires, trees and grasses. To stick to a ceiling, a housefly flies toward it, rolls over and sticks all six legs at once onto the ceiling.

Did you know that during World War II, the Japanese made bombs containing houseflies and a slurry of cholera bacteria? In 1942 and 1943, bombers dropped these bombs on two Chinese cities. The ensuing cholera epidemics killed about 400,000 people.

The common housefly drinks liquids using its spongelike mouthparts. (Photo: Muhammad Mahdi Karim via Wikimedia Commons)

Even though flies are pests, and humans have used them in inhumane ways, houseflies are not inherently evil. They play an important role in ecosystems as composters that break down waste. Without flies breaking down our farm manure and human garbage, the world would be a fetid and unsanitary place to live. Flies are also superb pollinators, second only to bees.

Houseflies can even help on farms by recycling animal waste. Placing housefly eggs in poultry manure, for example, has been shown to reduce the amount of waste by 50 percent. This waste is also less smelly. The maggots can then be harvested and used as animal feed. Chickens fed diets containing 10 percent to 15 percent maggots grew larger and faster and were healthier than chickens that did not have maggots in their diet.

So even though houseflies are pests, they are here because they have followed humans and their waste around the planet. Houseflies can be found anywhere humans live, including the arctic; our lives are intertwined with theirs. As the poet William Blake realized, houseflies and humans have many traits in common:

Am not I
A fly like thee?
…
For I dance
And drink & sing:
Till some blind hand
Shall brush my wing.

Seen nearby

A male tree swallow guards a nest box in Wayland in May in a photo by Gretchen Sterling.

Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.

Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.

This post was updated April 26, 2023, to correct the spelling of Jeffrey Offermann’s name.