It is a common observation to see a fly land on a surface, seemingly pausing before taking flight again. This often involves the fly regurgitating a small fluid droplet. While seemingly unhygienic, this action is a fundamental aspect of a fly’s biology, tied to its survival and feeding strategy.
Why Flies Regurgitate
Flies possess unique mouthparts designed for a liquid-only diet, unable to bite or chew solid food. Their mouthparts function like a sponge, featuring two fleshy pads, called labella, and a straw-like proboscis that allows them to absorb liquids.
When a fly encounters solid food, it cannot simply ingest it. Instead, it employs a strategy known as external digestion.
This involves the fly releasing a mixture of saliva and digestive enzymes onto the solid food source. These enzymes break down solid food into a digestible liquid. This pre-digestion outside the fly’s body is a crucial adaptation, enabling them to consume nutrients from a wide range of food sources that would otherwise be inaccessible. The process transforms solid particles into a “soup” that the fly can then easily suck up through its proboscis.
The Regurgitation Process
When a fly lands on a food source, it first “tastes” the surface using chemoreceptors on its feet. If the fly determines the substance is edible but solid, it will initiate the regurgitation process.
The fly expels a droplet containing saliva and digestive fluids from a specialized internal sac called a crop. This crop acts as a temporary storage organ for ingested liquids and also plays a role in preparing food for digestion.
This fluid is deposited onto the food, beginning the external breakdown. After the enzymes sufficiently liquefy the solid, the fly re-ingests the dissolved food mixture. Sometimes, the fly also regurgitates “vomit bubbles” to concentrate its food by evaporating excess water.
Hygienic Concerns
The regurgitation behavior of flies carries significant implications for human health and hygiene. As flies move between environments like decaying organic matter, garbage, and feces, they pick up microorganisms on their bodies, mouthparts, and legs. When a fly regurgitates onto food, it can deposit not only its digestive fluids but also these accumulated bacteria, viruses, and parasites. This mechanical transmission of pathogens can lead to the contamination of food, utensils, and surfaces.
Flies are known to transmit over 65 infectious diseases, including common foodborne illnesses like E. coli, Salmonella, dysentery, and cholera. The pathogens can multiply on contaminated surfaces, increasing the risk of illness if the food is consumed.
To mitigate these risks, strict hygiene practices are important. Covering food, cleaning spills promptly, and regularly emptying and covering garbage cans significantly reduce fly attraction and contamination. Ensuring screens on windows and doors are intact also helps prevent flies from entering indoor spaces.