Flies often exhibit a common habit of rubbing their front legs together, a behavior many people observe and wonder about. This action, sometimes misinterpreted as a fly “spitting on its hands,” sparks questions about its purpose. This repetitive motion, seen across various surfaces, leads to curiosity about its biological reasons.
Unpacking the Behavior
The common perception of flies “spitting on their hands” is a misunderstanding. When a fly performs this action, it is regurgitating a small droplet of digestive fluid, often referred to as a “vomit droplet,” from its mouthparts onto a surface. The “hands” people observe are the fly’s forelegs. The fly then rubs these forelegs together or against other body parts like its head and eyes, coating them with this fluid. This process begins with the fly extending its proboscis, a tube-like mouthpart, and expelling the fluid. Flies lack teeth, so they cannot chew solid food. Instead, they rely on external digestion, liquefying their meals before ingestion.
The Multifaceted Reasons Why
Flies engage in this behavior for several reasons, centered on maintaining their sensory capabilities and facilitating digestion.
Cleaning
One purpose is cleaning. Flies possess sensory organs like antennae, compound eyes, and receptors on their bodies. These structures can easily become clogged with dust, pollen, moisture, and other debris as the fly moves through its environment. By rubbing their forelegs together and using them to wipe other body parts, flies remove these particles, ensuring their sensory systems remain clear and functional.
Tasting and Sensing
Another reason involves tasting and sensing their surroundings. The forelegs of flies are equipped with chemoreceptors, which are essentially taste buds. When a fly lands on a surface, it can “taste” it by touching it with these chemoreceptors. The regurgitated fluid helps dissolve substances on the surface, allowing the chemoreceptors to detect food sources or chemical cues. This allows the fly to determine if a substance is edible before it fully commits to feeding.
Feeding and Digestion
This behavior also plays a role in their feeding and digestive processes. Flies often regurgitate digestive enzymes onto a food source to pre-digest it, breaking down solid food into a liquid form that they can then consume through their proboscis. This external digestion is a necessary step for flies, as they are adapted to a liquid diet. The fluid droplet can also be used to concentrate a meal by evaporating excess water, allowing the fly to consume a more nutrient-dense meal.
Beyond the Forelegs: What This Means
This behavior of regurgitation and foreleg rubbing is crucial for a fly’s survival. The grooming ensures sensory organs remain sensitive, important for detecting food, navigating, and identifying mates or avoiding predators. The tasting function of their forelegs allows them to locate and process food sources.
For humans, these survival strategies of flies have important implications. Because flies frequently land on various surfaces, including decaying matter, waste, and food, their cleaning and feeding behaviors can inadvertently lead to the mechanical transmission of disease-causing organisms. Pathogens picked up on their body hairs or within their digestive systems can be deposited onto new surfaces, including human food, through this regurgitation. This positions flies as vectors for illnesses, highlighting the importance of sanitation in environments shared with these insects.