"Housefly" is the common name of the fly Musca domestica, found in most parts of the world.

Musca domestica is a true fly, of the insect order Diptera. As the order name implies, flies have only two membranous forewings, unlike other flying insects, who have four wings, two pair. What would otherwise be hindwings are reduced in flies to small stalked knobs, the halteres, which serve as organs of equilibrium. Flies are the fourth largest order of insects, with about 85,000 species.

As everyone knows, however, this species is especially common, and especially obnoxious. It is a scavenger and does not bite living animals, but is nevertheless dangerous because it carries bacteria and protozoans that cause many serious human diseases.

It breeds in filth. After the female is fertilized by the male (which happens on the ground, not in flight) she deposits about 120 eggs in whatever rich organic matter she finds, such as garbage, sewage, manure or whatever. The larvae or maggots hatch, grow and pupate within the next twelve days. The adults live about a month. It has been variously calculated that if one starts with a single pair of houseflies at the beginning of spring, and if all offspring survive and breed, by late August their descendants would cover the entire earth to a depth of between 23 to 47 feet.

Yuck.

Of course it doesn't happen. Both maggots and adults are subject to heavy predation by birds, reptiles and other insects, and unless there is sufficient food, moisture and warmth throughout the development period of the maggots, they never become adults.

Houseflies are quite sensitive to temperature. At 53 degrees Fahrenheit (about 11 degrees C) they're airborne; at 48 degrees (9 C), they can only crawl (one has seen them at this, as the weather chills down in the fall); at 44 degrees (about 7 C), virtually comatose; at a few degrees above freezing, they die. At much past 102 degrees (39 C) heat paralysis sets in; at 116 degrees (47 C), death.

So, where do they go in the North American winter? (They are not, by the way, native to the Americas, but came over with the first Europeans.)

Well, no one is sure. They might migrate, though no one has ever caught them at it, or they might survive in warm places like dairy barns and niches in houses. (With their reproductive rate, it would only take a few!)

As noted above, they spread disease, not just from the circumstances of their childhood, but as adults as well. The housefly feeds by depositing a drop of digestive liquid on its food, which may be garbage, excrement, or other filth. Although most of the liquid drop is sucked back again through the insect's tubelike lower lip, or labium, a residue remains that may contain disease-causing organisms from previous meals. The fly's sticky foot pads and hairy body may also carry bacteria and other similar organisms. Some researchers claim that a single city slum fly can carry as many as 33 million bacteria internally and have another half billion on its body. Among the candidate diseases are typhoid fever, cholera, dysentery, salmonella, polio and several parasites, including tapeworm.

We have every motive to control them, but not too much success. DDT was widely used during and after World War II, but the flies developed resistance to it, while we were poisoning ourselves and many other more beneficial organisms. All insect sprays do other damage, though maybe not quite so much.

Since we can't get rid of them, we may as well look on the bright side. Think how much fun it is to catch one midair if you can do it!

PHYLUM: Arthropoda, SUBPHYLUM: Uniramia, SUPERCLASS: Insecta, SUBLCASS: Endopterygota, ORDER: Diptera, FAMILY: Muscidae

Pearse, Buchsbaum, Living Invertibrates, Blackwell Scientific Publications, Boston, Massachusetts 1987;

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