Flies, which belong to the order Diptera, possess some of the most varied diets in the insect world, making the simple label of “herbivore” insufficient. The order contains over 100,000 described species, and their feeding habits change dramatically between the larval and adult stages, often serving different ecological roles. While some flies are true herbivores, the majority of species encountered are liquid feeders or scavengers. Classifying the entire group requires considering their entire life cycle, not just plant consumption.
Defining Fly Diets and Trophic Roles
An animal is defined as a herbivore if it feeds on living plant material, such as leaves, roots, or fruit. This is distinct from a saprophage, which obtains nutrients by consuming dead, decaying, or non-living organic matter. The saprophagic diet includes carrion, feces (coprophagy), and rotting plant tissue, which is a major food source for many common fly species. An omnivore consumes both plant and animal matter, while a carnivore feeds primarily on other animals.
The Primary Role of Flies as Decomposers and Scavengers
The vast majority of fly species are decomposers and scavengers, particularly during their larval stage. These larvae, commonly known as maggots, feed on decaying organic material to accelerate biodegradation. Common house flies (Muscidae), blow flies (Calliphoridae), and flesh flies (Sarcophagidae) are prime examples of this trophic role. Blow fly larvae are frequently found processing animal carrion, which quickly reduces the mass of dead matter in the environment.
Drain flies illustrate this scavenging role by having their larvae consume the organic film that collects in plumbing. Similarly, the black soldier fly (Hermetia illucens) is an efficient saprophage whose larvae grow on a wide variety of decaying materials, including fruit and vegetable waste. The adults of these species often feed on liquids like nectar, honeydew, or the liquefied remnants of their larval food source. They process solid material by regurgitating saliva onto it to dissolve it before consumption.
Examples of Truly Herbivorous Flies
A significant number of fly species fit the definition of a true herbivore, relying on living plant tissue for their larval development. One of the most well-known groups are the fruit flies in the family Tephritidae, sometimes called “true fruit flies” to distinguish them from the common laboratory fruit fly (Drosophila). The larvae of Tephritid flies, such as the Mediterranean fruit fly (Ceratitis capitata), consume the flesh of living fruits and vegetables. These larvae are phytophagous, feeding directly on plant tissue, including fruits, flowers, stems, or roots, often causing significant agricultural damage.
Another notable group is the family Agromyzidae, commonly known as leaf miners. The larvae of these flies burrow directly into the tissue of leaves, creating distinctive tunnels as they consume the mesophyll cells. Species like the American serpentine leaf miner (Liriomyza trifolii) are polyphagous, meaning they attack many different plant species. These species, along with others that induce and feed within plant galls, demonstrate the existence of true plant-eating specialists within the fly order.