The common house fly, an insect found globally, leaves behind a tell-tale sign of its presence: tiny, dark specks on surfaces. While it may seem like a fly is leaving behind waste with every landing, they do not defecate every single time they perch. The frequent spotting is actually the result of two distinct biological processes happening in rapid succession—defecation and regurgitation (fly vomit). This constant marking behavior is a direct consequence of the fly’s specialized feeding method and fast-paced metabolism.
Understanding Fly Defecation
House flies have a digestive system built for speed, which necessitates frequent excretion of waste. Since their diet consists primarily of liquids, or food they have liquefied externally, nutrients move through their system at an accelerated rate. The entire digestive process, from ingestion to excretion, can be completed in as little as four to six hours. This rapid transit time means they must constantly eliminate waste products, sometimes defecating up to 300 times in a single day. This constant need to process food results in the deposition of small, dark, semi-liquid droplets, which are the final waste product of their digested meals.
The Regurgitation Factor
The spots left by a fly are not solely feces; a large portion is actually regurgitated material, or vomit. Flies lack the necessary mouthparts to bite or chew solid food, so they must use a process of external digestion. When a fly lands on a solid food source, it extends its proboscis and deposits a mixture of saliva and digestive enzymes onto the surface. These enzymes break down the solid material into a liquid form that the fly can then suck up through its straw-like mouthpart. If the fly consumes a large amount of liquid food, it is stored temporarily in a specialized organ called the crop. To concentrate the nutrients or to make room for a new meal, the fly may expel some of this stored, partially digested food, resulting in a vomit spot.
Anatomy of a Fly Spot
The dark specks we see are typically a combination of both defecation and regurgitation, often deposited at the same location. The spots created by regurgitation tend to be larger, paler, and more circular than the specks of feces. Regurgitation spots are essentially the remnants of a previous meal, which can appear light brown, yellowish, or sometimes reddish, depending on what the fly has recently consumed. Fecal spots, on the other hand, are generally smaller and much darker, appearing as black or deep brown dots. A fly landing to feed on a surface will often simultaneously vomit to liquefy a new meal and defecate to expel the waste from a previous one, ensuring that almost every landing spot becomes a biological marker.