Flies, which belong to the order Diptera, represent a massive and diverse group of over 125,000 described species. This variety is mirrored by a wide spectrum of dietary habits. While many flies are known for their attraction to fermenting or decaying matter, their food sources extend far beyond what is commonly observed. The food consumed by a fly is tied to its specific species and life stage, resulting in a complex web of ecological roles.
The Common Adult Fly Diet and Feeding Mechanics
Common nuisance flies, such as house flies and fruit flies, subsist on a liquid-only diet, necessitating a unique feeding mechanism. These flies possess a specialized mouthpart called a proboscis, which terminates in fleshy, sponge-like organs known as the labella. The labellum contains numerous fine channels, or pseudotracheae, that function like tiny sponges, drawing up liquid food through capillary action.
Since they lack mandibles for chewing, adult flies must liquefy solid food before ingestion. They accomplish this by regurgitating a mixture of saliva and digestive juices onto the solid substrate, a process often referred to as “fly puke.” This fluid contains enzymes that rapidly break down the material into a digestible, soupy mixture, which the fly then sponges back up with its labella.
This feeding method dictates their attraction to high-energy liquids like sugars, fruit juices, and honeydew. They are also drawn to products of fermentation, such as overripe fruit, and soft organic waste, including animal feces or spilled food. This preference for consuming food externally is why common flies are frequently found on human food and waste, serving as mechanical vectors for bacteria and pathogens.
Specialized Diets of Predatory and Parasitic Flies
Beyond the generalist feeders, many adult flies have evolved specialized diets supported by distinct mouthpart structures. Blood-feeding (hematophagous) flies, such as mosquitoes, horse flies, and stable flies, use piercing and sucking mouthparts to penetrate skin and access a host’s blood vessels. In many species, the female requires protein from a blood meal to produce eggs, though both sexes feed on nectar and plant juices for energy.
Other flies are active predators, like robber flies (family Asilidae), which ambush and capture other insects mid-air. Robber flies use bristly legs to grasp prey in flight, then inject them with neurotoxic and proteolytic saliva using a short, stout proboscis. This injection paralyzes the victim and begins to digest its internal tissues, which the robber fly then sucks out.
A different specialization is found among flies that act as pollinators, such as flower flies (Syrphidae) and certain bee flies (Bombyliidae). These species feed primarily on floral nectar and pollen. Some anthophilous flies have developed elongate mouthparts several times the length of their body, allowing them to reach nectar deep within a flower’s tubular corolla, similar to a hummingbird.
The Larval Diet and Its Role in Decomposition
The juvenile stage of a fly, known as the larva or maggot, often has a diet separate from the adult, focused on rapid growth. Maggots are highly efficient recyclers, specializing in consuming decomposing organic matter. Their food sources include carrion, feces, and rotting vegetation, making them primary decomposers in most ecosystems.
The predictable pattern of fly colonization and larval development on decaying material is the foundation of forensic entomology. By studying the developmental stage of the maggots, which is influenced by temperature and food availability, forensic scientists can estimate the postmortem interval, or the time elapsed since death. Different species arrive at a carcass in a specific sequence as decomposition progresses.
In a medical context, the larvae of some fly species are utilized in maggot debridement therapy. These larvae consume only necrotic (dead) tissue from wounds without harming healthy tissue, effectively cleaning chronic ulcers and promoting healing. This selective feeding behavior highlights the specialization of the larval diet to the complex chemistry of decay.