Introduction to How Houseflies Works

fly in soup
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Has this ever happened to you? A housefly may be a tasty meal for some animals, but offers only the risk of illness
for humans. See more
insect pictures.

What's that irritating buzz I hear? What's crawling across my television set? Waiter, what's this floating in my soup? Unless you live in polar regions, the answer to all these questions, sooner or later, is inevitably the same: the common housefly.

While there are certainly other flies and insects that make themselves at home in your home, 90 percent of all flies found inside are members of the specific species of fly musca domestica. That's right, the term "housefly" isn't just a generic term; it's a specific kind of fly that happened to find its evolutionary niche living among us.

Shoo Fly

But the problem with flies isn't just that they irritate us by trespassing. Houseflies carry and spread a number of potentially deadly diseases including typhoid fever, salmonella, tuberculosis, conjunctivitis, leprosy and cholera, as well as intestinal worms and the bacteria responsible for dysentery. They feed and reproduce on garbage, feces and rotting flesh. And then, after picking up millions of microorganisms, they fly into our homes and land on our food, bodies and personal belongings, leaving behind the same germs.

In this article, we'll take a close look at this garbage in, garbage out diet, the anatomy and life cycle it nourishes, and how you can best protect yourself.

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Housefly Anatomy: The Head

Though houseflies are rulers of the insect-scavenging world, they have many predators just waiting for a chance to gobble them up. Spiders, frogs, lizards, sparrows, wasps -- even members of the plant kingdom -- want a slice of the housefly pie. On top of that, disease-conscious humans try to shut houseflies out or exterminate them once they're inside. To help overcome all these obstacles, houseflies depend on excellent sensory awareness and top-notch aerial maneuverability.

Like all insects, a housefly's body is covered with a hard exoskeleton of chitin and is divided into three sections: head, thorax and abdomen.

Features of the housefly head
NHPLM/Stone/Getty Images
A face only a mother
could love.

A pair of large complex eyes covers most of the housefly's head. Each eye is composed of 3,000 to 6,000 simple eyes. Unlike the eyes of vertebrates, houseflies can't focus in on the particulars of the environment around them. Instead, they provide an excellent mosaic view of everything to the left, right, front and above the fly.

Imagine a casino security guard, constantly scanning a bank of video monitors to see who might be cheating, causing a drunken disturbance or suspiciously loitering. The guard scans key areas for potential threats. It's much the same with the eyes of the flies -- only they have thousands of monitors that allow them to detect even the slightest of movements from nearly every direction.

Houseflies also boast three additional simple eyes called ocelli, located between the two compound eyes. Think of the ocelli as a kind of navigational device or compass, letting the fly know which way is up. The fly accomplishes this by keeping track and moving towards sunlit areas. This is why you'll often find houseflies buzzing around windows.

housefly
Housefly head

Houseflies depend on their keen sense of smell, provided by their antennae. For tasting and consuming meals, the insect uses its proboscis, a plunger-like appendage that extends from the bottom of the head. Two small, antenna-like feelers called maxillary palps allow the fly to taste its food. The end portion of the proboscis, called the labellum, terminates in a spongy mouth, where the fly slurps its food.

But how does the housefly get to its food? On the next page, we'll examine its amazing legs and wings.

Did that housefly just bite me?
Nope, not if it was really a housefly. While some species of similar-looking flies (such as stable flies and horseflies) have the necessary mouth parts for biting, houseflies couldn't take a chomp out of you -- even if they wanted to.

Housefly Anatomy: Wings, Legs and Abdomen

The housefly's thorax features all of its limbs used for movement. Though flies appear to have only one pair of wings, closer inspection reveals the presence of tiny, secondary wings, called halters, located below the main pair.

Little Fly, Big Annoyance
While keeping an eye out for houseflies, you might run across one of musca domestica's smaller relatives, the lesser housefly. At first, they might look like small houseflies, but the lesser housefly is a completely different species, fannia canicularis. These little insects tend to prefer poultry barns to houses. At rest, the two species can easily be distinguished as the lesser housefly's wings fold back at a sharper angle than those of its larger kin, giving it a more streamlined appearance. A lesser housefly's wings form a tight "V" shape when folded; and the housefly's wings form a fatter "V".

While their ancient ancestors boasted two full sets of wings, houseflies have learned to do more with less. Over millions of years, the lower set evolved into the shrunken appendages you see today. They're far from useless, however. Houseflies flap their halters at high speeds during flight, using them to maintain balance in the air. If one halter is removed, the insect can only fly in circles. Without either halter, the fly can't take to the air at all.

housefly's wings
Peter Weber/Photographer's Choice/Getty Images
A housefly's wings are delicate instruments -- perhaps you've seen a housefly cleaning them. This is because even the smallest piece of food caught on the wing's thin surface can throw a fly off balance during flight.

The primary wings do the rest of the work, beating 200 to 300 times a second for an average speed of 4.5 miles (7.24 kilometers) per hour. The wings provide enough power and precision for instant liftoff and complicated flight movements, such as tight spirals, zigzags and even backward maneuvers.

And then there are the legs, which the fly uses to taste everything it lands on. Tiny hairs on the end leg segment, or tarsi, work like human taste buds. For this reason, you can often spot houseflies walking around in circles on potential meals.

housefly
Housefly anatomy

If you happened to observe the same fly walking on the wall or ceiling, then you saw the other portions of the tarsi in action. The bottom of the housefly's feet boast tiny, gripping claws and moist suction pads called pulvilli, which allow the fly to land almost anywhere.

The abdomen contains other key organs, including an egg-laying ovipositor (in females) and a sperm-depositing aedeagus (in males). Both remain retracted when not in use.

So houseflies seem to have all the equipment to find a meal, but how do they go about eating it? Find out on the next page.

Housefly Diet

Once a housefly sniffs something interesting and tastes it with its feet and palps, the time has come to gobble up the delicious morsel.

If the meal in question happens to be liquid already, then the task is easy. Since the housefly's mouth parts are designed to sponge and ingest liquefied food, it's just a matter of turning on the suction. The food goes straight into the fly's stomach.

housefly eating sugar
Oxford Scientific/Photolibrary/Getty Images
A housefly uses its proboscis to eat a piece of sugar.

Solid food is slightly more difficult. Houseflies can't bite or chew -- they simply don't have the proper equipment. So if forced to contend with something more solid, like a grain of sugar or dried blood, houseflies must employ a different tactic.

First, the housefly scrubs the dry food substance with the bristles on the end of its proboscis. This frees up food particles, if they're not already loose and crumbly.

The second step can be compared to what happens when you add hot water to instant oatmeal -- only instead of hot water, the housefly adds a mixture of saliva and digestive juices. The fly vomits saliva and digestive material onto its meal, and after a few seconds pass for the juices to break down the food, the fly sucks everything back up.

While this may sound rather bizarre to us, remember that the fly isn't adding anything to its food that we don't add to ours. The difference is, our teeth and jaws allow us to break down organic matter enough so it can mix with our stomach's digestive juices. The housefly is doing the same thing, just on the outside of its body.

Mark of the Fly
If you've ever noticed fly specks on walls or surfaces, then you've observed the telltale signs of a housefly's recent meal. Those little dots are leftovers of its regurgitative eating habits.

If the vomit can't adequately break down food enough for it to pass through a tube leading into the stomach, the fly sends this food down a different tube to an inner sack called the crop. The fly may pass the bubble of dissolving food multiple times between crop and mouth, regularly applying fresh saliva. Eventually, the liquefied meal will be ready to send down to the stomach.

If all this vomiting sounds unhygienic, that's because it is. Not only do potentially deadly germs cling to the very legs a housefly might use to walk on (and taste) your sandwich, if it chooses to feed, it may wind up regurgitating portions of a previous meal as well. This only multiplies the risk of disease, which is why you should keep houseflies away from food.

All this flying and eating has a purpose; the same purpose as with any animal. These insects seek to reproduce and send another generation out into the world. On the next page, we'll see how houseflies do just that.

Housefly Life Cycle

The housefly life cycle closely mirrors that of most insects: a basic cycle that begins with an egg, then develops through a larva phase, a pupa phase, and finally, into an adult. During a warm summer -- optimal conditions for a housefly -- the cycle, from fertilized egg to adult, spans a mere seven to 10 days.

After a male housefly chases down and fertilizes a female counterpart, she's ready to lay her eggs. Houseflies are solitary creatures. Like the rest of the insect world, males and females do not stick together after mating and, unlike nesting insects, females do not care for or protect eggs. Females simply leave the eggs where they will be safe from predators and have plenty to eat upon hatching.

housefly life cycle
The life cycle of a housefly

The female housefly deposits her eggs in the crevices and corners of the same kinds of decaying organic matter adults feed on. Within a day, the first larvae begin to emerge from the eggs. Also known as maggots, these worm-like creatures are little more than fleshy, sectionless tubes with hooked mouth parts used for feeding.

The maggots grow rapidly. In less than two days they've doubled in size and therefore must molt. Molting is a process common to many invertebrates through which a growing insect sheds its former exoskeleton and grows a new one. A maggot will molt twice more, emerging larger and more developed each time.

housefly laying eggs
Hedi Koch/Minden Pictures/Getty Images
A female housefly lays her eggs.

Following its third molt, larvae will burrow deep into the substance they've been feeding on. Their skins will darken and harden as they enter the pupa stage. Inside this protective shell, the larva will fully develop the body segments and appendages of an adult housefly.

The only visible addition to the emerging housefly is a swollen bump on the fly's head, used to break through the shell. Since the housefly doesn't have teeth or jaws to chew its way out, it uses this fluid-filled pouch to break through the pupae shell. Once fully emerged, the bump deflates back into the fly's head.

Suspended Animation
When threatened by cold temperatures or lack of food and moisture, the housefly's body can temporarily shut itself down in a process similar to hibernation, called diapause. This process, which comes on gradually and can last for months, can take place at any point in a housefly's life cycle.

A new adult housefly has, at most, three months to reproduce before it dies. With so many predators, a housefly's average lifespan is even shorter: 21 days. Luckily for the housefly, the phrase "breeding like flies" isn't just a figure of speech. Each female can lay up to 900 eggs during her brief life.

The very thought of a housefly infestation may prove too disturbing for many homeowners. However, the next page will describe how a manageable amount of houseflies helps regulate the local ecosystem.

Helpful Houseflies

So germ-carrying houseflies come into our homes uninvited and begin walking, feeding, vomiting and laying eggs all over our personal belongings. They bring the risk of disease and, in return, we do out best to wipe out any unwelcome visitor. But are they really that bad?

While there's not much good to be said for their presence inside the home, like all flies, they fulfill an important environmental role as scavengers. As houseflies and their larvae feed, they consume nutrients in rotting organic matter. They leave behind picked-over remains that other organisms, bacteria and enzymes can further break down.

maggots
China Photos/Getty Images
Though hospitals don't use housefly larvae to help clean wounds like blow fly maggots, some researchers believe houseflies have their uses. These larvae were reared at a Chinese housefly farm, where experts believe they may be used to develop cancer medication.

Flies and their larvae are also an important food source for a large number of predators. Housefly pupae contain large amounts of protein and are thus especially beneficial to the various birds, reptiles, and insects that prey on them. After all, there's a reason maggots are often purchased as bait by freshwater fisherman. Commercial tilapia farmers have considered feeding their fish housefly maggots exclusively. You can even purchase frozen or freeze-dried housefly pupae to feed pet spiders.

However, if you happen to be reading this, you're probably not a spider or an ecosystem, in which case you want to cut down on the local housefly menace. Learn all about housefly management on the next page.

Shoo Fly, Don't Bother Me
Humans have pretty much sought to keep houseflies out of their lives as long as they've had houses for them to fly into. Even before advances in science allowed them to understand the risk of infectious disease, many people saw the fly as a harbinger of sickness and death. Some ancient civilizations made regular sacrifices to their respective fly gods to keep the swarms out of their homes and temples.

Managing Houseflies

Waging war against houseflies can be as easy as keeping a clean kitchen or as complicated as introducing new predators and technologies into your home. Consider the following options:

  • Sanitation: The essential weapon in any war against houseflies is simply keeping your home clean. In doing so, you'll limit the places they can feed or breed. Don't leave uncovered food out, and keep all trash in sealed containers. Likewise, keep the area outside your home clear of uncovered trash, manure and decaying organic matter.

    Venus flytrap
    ©iStockphoto/Vickie Sichau
    A Venus flytrap clamps its jaws around a housefly. To learn more about this botanical choice for housefly management, read How Venus Fly Trap Works.
  • Biological control: Houseflies have no shortage of natural enemies, so why not follow the example of the old woman who swallowed a fly and introduce predators to deal with the problem? Naturally, you don't want to encourage any more fly eaters inside your home than you already have. The one exception is the Venus flytrap, which, if properly cared for, can cut down on your home's housefly population. Outdoors, the options depend greatly on where you live, but you might not want to rid your home of spider webs and wasp nests if you're concerned about flies. Some commercial firms sell a species of parasitic wasp called pteromalidae to farmers. The wasps' larvae feed on housefly pupae and leave the rest of the farm alone.

  • Exclusion: One of the key ways to cut down on the housefly problem is simply to prevent adult flies from entering your home. Keep doors, windows and vents closed, and use screens to let in fresh air. High-traffic buildings sometimes use air curtains in conjunction with automatic doors to keep flies out. When the doors are open, a steady buffet creates a wall of force houseflies can't push through.

  • Physical control: The simplest example of this would be killing flies with a swatter -- though this method is generally unsanitary due to the rupturing and smearing involved. Other methods include the use of fly paper or light traps. Fly paper offers houseflies what seems like a tantalizing, scented treat, forcing them to land and become trapped in the sticky glue. Likewise, water traps lure flies into a jug- or bag-like container. Once inside with the bait, houseflies can't crawl out and eventually fall into the water and drown. But traps can exploit more than just a housefly's sense of smell. Some traps use ultraviolet light to lure them in. The trapped fly is then typically electrocuted against a wire grid, sucked into a dehydration chamber or captured in glue.

  • Chemical control: Generally speaking, the use of pesticides should be used sparingly, especially inside the home. Before you start using poisonous chemicals against houseflies, make sure you've tried fighting the problem with sanitation, exclusion and physical control. A number of housefly pesticide products are commercially available, but be sure to follow instructions and keep in mind the health of family members and pets. Products such as hanging pesticide strips are generally intended only for unoccupied areas such as attics.

For more information about houseflies and how to live with (or without) them, follow the links on the next page.

Lots More Information

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