The Filthy House Fly: Spreading Illness and Disease

House fly

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As their name indicates, house flies are regularly found around the home. In fact, because they breed and feed on human food and waste, they are generally found about anywhere that humans are. The common house fly is often called the "filth fly" because it can contaminate foods and surfaces every time it lands. If a fly sets itself down on the edge of your glass, it may have just flown over from a pile of trash, manure, or other waste.

Even the U.S. Department of Agriculture's Agricultural Research Service (USDA/ARS) description shows how filthy and disease-ridden this common pest can be:

"It's common knowledge that house flies are carriers of disease. That's why there's such widespread effort to keep them out of our kitchens and away from our food."

Identifying House Flies

Although there are other large flies, the common house fly can be identified by a few distinguishing features:

  • House flies are about 1/4 inch long.
  • They are grayish in color.
  • They have broad, dark stripes down the mid-section of their bodies.
  • Their most dominant feature is their large, reddish, compound eyes.

In addition to these physical characteristics, house flies can also be identified by something they don't do: bite. The house fly does not bite because it can't. It has "sponging" mouthparts, so it can only suck up liquids. If you are bitten by a large fly, it is most likely a horse fly, deer fly, stable fly, or black fly.

Why Can't I Swat That Fly?

It is the fly's compound eyes, tiny body hairs, and speed that make it so difficult to swat. The eyes and hairs enable it to sense changes in airflow so that well before your fly swatter gets close, the fly has flown off. This, along with its ability to fly at speeds of 5 mph—with bursts of up to 15 mph when threatened—gives this fly the power to be well away before your swatter, hand, or newspaper can land.

House Fly Habits and Behavior

In addition to feeding on garbage, the house fly also lays its eggs on garbage, animal feces, and other decaying organic material, such as lawn clippings. The eggs hatch within a day and the emerging maggots (larvae) feed on the waste to develop into adult flies in less than a week. A fly can live about a month, and in this time, the female fly can lay about 2,000 eggs in batches of 75 to 100.

The house fly can travel up to 20 miles, but it generally stays within two miles of where it hatched. Most common in spring and summer, these flies often enter homes through open doors and windows, but they can also get in through cracks around doors and windows, torn screening, and similar small openings.

Why House Flies Are a Problem

  • House flies spit on your food. The house fly can only digest liquid food, so when it lands on your sandwich it immediately regurgitates saliva onto it and then sucks in the liquid this creates (or most of it anyway). Some of that saliva is likely to still be on the bread when you go to take your next bite.
  • House flies leave specks. Those little black dots that you see in places where flies have spent a bit of time are their feces.
  • House flies fly from trash to table. Flies care little whether they feast on food from the dumpster outside or from your table, and they will fly back and forth between the two with the ability to carry more than a million bacteria on their feet and body.
  • House flies can carry disease. According to university reports, including the University of Rhode Island and Penn State, at least 65 diseases have been associated with house flies. The bacteria and viruses they carry can cause food poisoning, diarrhea, eye infections, dysentery, cholera, and even tuberculosis.