When people encounter a fly larger than the common house fly, they often wonder what specific insect they are seeing. The label “large fly” is not a scientific classification but a descriptive term applied to several distinct insect families, leading to confusion about their identity and habits. These large insects, all belonging to the order Diptera, range from painful biters to harmless, clumsy fliers. They are identified by key physical features and behaviors, including the stout biters, the metallic scavengers, and the spindly, misidentified giants.
The True Giants: Horse Flies and Deer Flies
The most aggressive large flies belong to the family Tabanidae, commonly known as horse flies and deer flies. These are stout-bodied insects that can range in size from 5 to 27 millimeters, with horse flies generally being the larger and more robust of the group. They possess very large, brightly colored compound eyes, which are often iridescent green, blue, or purple.
Only the female tabanids bite, needing a blood meal to acquire the necessary protein for egg production. Their mouthparts are formidable, functioning like a set of scissor-like blades to create a wound in the host’s skin. They then lap up the flowing blood with a sponging structure, a process that is notably painful. These persistent biters are also medically significant because they can mechanically transmit pathogens, including the bacteria responsible for tularemia in humans and equine infectious anemia in livestock.
Large Metallic Nuisances: Blow Flies and Cluster Flies
Large flies often observed buzzing near garbage or windows belong to the family Calliphoridae, which includes blow flies, blue bottle flies, and green bottle flies. These species are characterized by their robust bodies and striking metallic sheen in shades of brilliant blue, green, or copper. Blow flies are scavengers, drawn to the odors of decomposition, and will lay their eggs on carrion, meat scraps, or decaying organic material.
The presence of these flies is often an indicator of nearby decaying matter, as they are among the first insects to arrive at a fresh carcass. A related species, the cluster fly (Pollenia rudis), is a major indoor nuisance but lacks the bright metallic coloration of its cousins. Cluster flies are known for their habit of aggregating in large numbers to hibernate within the sheltered spaces of homes, particularly in attics and wall voids during colder months.
Misidentified Giants: Crane Flies
The largest flies encountered by the public, often mistaken for giant mosquitoes, are the crane flies from the family Tipulidae. These insects are recognizable by their long, spindly, and fragile legs, which easily break off during clumsy flight. Adult crane flies can reach impressive sizes, with a body length that can extend up to 60 millimeters.
Despite their intimidating appearance, these insects are harmless and do not bite humans or animals. The adults are non-feeding or consume only nectar, and they lack the piercing mouthparts of mosquitoes and horse flies. Their larvae, sometimes called “leatherjackets” due to their tough skin, can cause damage by feeding on the roots of turfgrass and seedlings, making them an agricultural concern.