The common house fly (Musca domestica) frequently regurgitates onto food as a necessary step in its feeding process. This action is directly related to the fly’s anatomy and its inability to consume solid foods directly. House flies are physically structured to ingest only liquids, meaning they must pre-treat any solid meal before consuming it. The fly’s mouthparts act like a spongy straw, incapable of chewing.
External Digestion: Why Flies Regurgitate
Flies must liquify solid food to make it consumable, a process known as external digestion. When a fly lands on food, it extends its proboscis, the tubular mouthpart, and deposits a drop of fluid. This fluid is a mix of digestive enzymes and saliva that breaks down the complex carbohydrates and proteins in the food.
The “vomit drop” rapidly liquefies the solid material on the food’s surface, turning it into a digestible “soup.” The fly then sucks up this newly liquefied mixture, which contains the dissolved food and the regurgitated digestive juices. Sometimes, a fly may regurgitate a portion of its previous meal to make room for a more concentrated new meal, a process called “vomit spotting.” The fly often leaves behind a tiny, dark spot, which is a combination of this regurgitated material and its feces.
How Flies Contaminate Food Beyond Regurgitation
While regurgitation is unappetizing, the primary health risk lies in the pathogens the fly carries. House flies are known as “filth flies” because they frequently land and feed on decaying organic matter, garbage, and animal feces. When a fly lands on contaminated material, bacteria, viruses, and parasites adhere to its body, particularly the fine hairs on its legs, which act like tiny brushes.
This external carriage of germs is a form of mechanical transmission, where the fly acts as a flying fomite, or carrier of infectious material. A fly can pick up numerous pathogens from a single landing site and deposit them onto your food with a single touch. Pathogens like E. coli, Salmonella, dysentery, typhoid, and cholera can be transmitted this way.
The fly also contaminates food internally. When a fly ingests pathogens from a dirty source, these microbes can survive inside its gut for several days. These live pathogens are then expelled onto your food through the fly’s feces and within the regurgitated vomit drop. Since pathogens survive longer inside the fly than on its exterior, the digestive fluids and fecal spots are concentrated sources of contamination.
Simple Steps to Protect Your Food
Minimizing the risk of fly-borne contamination involves simple sanitation practices. The most effective step is to always cover food, especially during outdoor eating or meal preparation. Using fine-mesh food tents or tightly sealed containers prevents flies from accessing the meal.
Keep all garbage cans sealed with tight-fitting lids and empty them frequently, as refuse is a major fly attractant and breeding ground. Maintain screens on all windows and doors to block entry into your home. Promptly cleaning up food spills and pet waste eliminates potential fly feeding and breeding sites, reducing the fly population.