It is a common human reaction to find flies unpleasant, often eliciting a sense of revulsion. This article explores the scientific reasons underpinning this common perception, delving into the specific traits and behaviors of flies that contribute to their “gross” reputation.
Unsettling Physical Traits and Behaviors
Flies possess physical attributes and exhibit behaviors that inherently contribute to their perceived uncleanliness. Their bodies are covered in tiny hairs (setae) and sticky footpads (tarsi) that readily pick up particles from every surface they land on. This anatomy makes them efficient transporters of microscopic debris and pathogens.
Their feeding process is particularly unsettling. Flies lack teeth, so they liquefy meals by regurgitating digestive enzymes and saliva onto food. This “vomit” breaks down solid matter into a digestible liquid, which the fly then sucks up using its sponge-like mouthparts. This repeated process means a fly landing on your food deposits partially digested previous meals and microbes directly onto your plate.
Flies frequently rub their legs together. This grooming action cleans their taste sensors located on the bristles of their feet, allowing them to better “taste” surfaces they land on. This behavior also means they are constantly redistributing contaminants picked up from previous landings onto new surfaces, including human food.
Vectors of Disease
Flies are significant carriers of pathogens, acting as mechanical vectors that can transmit over sixty-five different infectious diseases to humans. They pick up bacteria, viruses, and parasites on their bodies, including their legs, mouthparts, and even internally, from unsanitary sources like decaying matter, feces, and contaminated surfaces. When a fly lands on food, utensils, or human skin, these pathogens can be transferred.
The transfer of pathogens occurs through several mechanisms. Microbes cling to the external surfaces of the fly’s body and the fine hairs on its legs. Additionally, as flies regurgitate digestive fluids onto food to liquefy it, they deposit pathogens from their previous meals. They also defecate as they feed, further contaminating surfaces with microorganisms.
Common diseases that flies can transmit include food-borne illnesses caused by bacteria like Escherichia coli and Salmonella. They can also spread the bacteria responsible for cholera (Vibrio cholerae) and various forms of dysentery (Shigella species and Entamoeba histolytica). While flies do not cause these diseases themselves, their ability to transport harmful microorganisms from contaminated environments to human spaces makes them a public health concern.
Attraction to Decomposition
Flies are inherently drawn to decaying organic matter, a primary reason for their association with unsanitary environments. This attraction is fundamental to their ecological role and life cycle. Flies possess a highly developed sense of smell, allowing them to detect volatile organic compounds released during decomposition from long distances. These compounds, such as putrescine and cadaverine, signal suitable feeding and breeding sites.
Decaying substances like rotting food, animal waste, and carrion provide sustenance for adult flies and an ideal environment for laying eggs. Female flies deposit eggs directly onto or within this decomposing material. Upon hatching, the larvae, commonly known as maggots, feed voraciously on the decaying tissue, accelerating decomposition.
This behavior is a natural part of their contribution to nutrient cycling, breaking down organic matter and returning nutrients to the soil. However, for humans, this association with putrefying substances for their survival and reproduction contributes significantly to the perception of flies as “gross” creatures.