It is a common sight to see a fly land, and a widespread belief suggests they are “throwing up” every time they do so. Understanding the feeding mechanisms of flies clarifies this frequent observation and sheds light on their unique digestive processes.
How Flies Consume Food
Flies, such as the common housefly, possess specialized mouthparts adapted for a liquid diet. Their feeding apparatus, known as a proboscis, ends in a pair of spongy lobes called the labella. These structures are not designed for biting or chewing solid food. Instead, they function by dabbing and soaking up liquids.
When a fly encounters solid food, it cannot simply ingest it. To overcome this, the fly first secretes digestive enzymes, contained within its saliva, onto the solid surface. These enzymes begin to break down and liquefy the food externally. Once the food is sufficiently dissolved into a liquid form, the fly then sops it up through capillary action into its digestive tract.
The Regurgitation Process Explained
Flies do not “throw up” every time they land, but they frequently engage in regurgitation, an integral part of their digestive strategy. The fly’s digestive system includes a structure called the crop, which acts as a temporary storage sac for ingested liquids.
When a fly feeds, it stores liquid food in its crop. From this crop, the fly can bring up some of the stored liquid, often mixed with digestive enzymes, back onto a food source. This action helps to further liquefy solid food, making it easier to ingest. The resulting droplet, commonly referred to as “fly vomit,” is a mixture of partially digested food, enzymes, and crop contents.
Regurgitation also aids in concentrating nutrients. Flies can expel excess water from their crop contents, leaving behind a more nutrient-dense solution that they can re-ingest. While not occurring with every landing, this behavior is frequent, especially after feeding or when preparing to feed.
Why This Matters to Us
The regurgitation behavior of flies carries implications for human health and hygiene. As flies land on various surfaces, including decaying matter and waste, they can pick up numerous microorganisms, including bacteria and viruses, on their legs and bodies. When a fly regurgitates onto food or surfaces in our environment, these pathogens can be transferred.
The “vomit spot” left behind by a fly can contain these harmful microbes. This direct deposition of contaminated material contributes to the mechanical transmission of diseases. Understanding this aspect of fly behavior underscores the importance of food safety practices and maintaining clean environments to minimize health risks.