Flies belong to the insect order Diptera, representing an incredibly diverse group of organisms with nearly 160,000 species worldwide. Accurately identifying a specific fly is the first step toward effective management, especially when they become household pests. Only true flies possess the specialized anatomy that defines this order. Learning the visual distinctions between these insects allows for targeted control.
Key Anatomical Features for Identification
The most fundamental feature identifying a true fly is the possession of only a single pair of functional wings (forewings) used for flight. The hindwings are reduced to small, club-shaped appendages called halteres. Halteres function as gyroscopes, vibrating rapidly to provide stability and balance, enabling the fly’s characteristic agility in the air.
Differentiation between fly families often begins with examining the antennae structure, which is categorized into two types. Flies in the suborder Nematocera (e.g., mosquitoes and crane flies) typically have long, slender antennae with many segments. In contrast, flies in the suborder Brachycera (e.g., house flies and fruit flies) possess short, stubby antennae, usually with only three segments and sometimes a bristle-like arista.
The visual system is another significant identifier, characterized by large, prominent compound eyes. Mouthparts vary dramatically and indicate feeding habits. These range from the sponging mouthparts of the common house fly, used to lap up liquid food, to the piercing-sucking mouthparts found on biting flies like stable flies or horse flies.
Identifying Common Household Flies
House Flies
The common house fly (Musca domestica) is medium-sized, typically measuring between ⅛ and ¼ inch long, with a dull gray coloration. A distinctive visual marker is the presence of four dark, longitudinal stripes running down the thorax. The head features large, reddish-brown compound eyes, and the mouthparts are adapted for sponging liquids, meaning they cannot bite.
Fruit Flies
Fruit flies, primarily belonging to the genus Drosophila, are noticeably smaller than house flies, measuring about ⅛ inch in length with a compact, oval shape. Their bodies are generally tan or brownish-black, and a distinguishing feature is their bright red eyes. These tiny flies are frequently seen hovering in the kitchen around fermenting fruit, old spills, or discarded bottles.
Drain/Moth Flies
Drain flies, also referred to as moth flies, are small insects ranging from 1.5 to 5 millimeters long. Their bodies and wings are densely covered with long hairs, giving them a moth-like look. When at rest, they hold their large, rounded wings pitched over their body in a characteristic roof-like manner. They are weak fliers and are often observed crawling or resting on walls near plumbing fixtures.
Blow/Bottle Flies
Blow flies (including bluebottle and greenbottle flies) are generally larger than house flies, measuring up to half an inch long. They are easily identified by their striking metallic sheen and vibrant, iridescent coloration that can be blue, green, or bronze. These robust flies produce a loud, low-pitched buzzing sound during flight. Their larvae, known as maggots, are commonly found in decaying organic matter or carrion.
Habitat and Behavior as Clues
Observing the fly’s location and movement can often narrow down the identification, providing context that physical appearance alone may not offer. Fruit flies are almost exclusively found near sugary, fermenting materials, such as overripe produce or discarded juice containers. Their flight pattern is typically slow and deliberate, often described as hovering lazily near their food source.
Drain Flies
Drain flies are strongly associated with standing water and the gelatinous film that accumulates in plumbing, septic tanks, or moist compost. These flies are notoriously poor fliers, usually taking short, hopping flights or crawling along walls. This behavior indicates a breeding source is close by, often within the drain itself.
Blow Flies
The presence of large, metallic blow flies indoors usually signals a more severe issue. They are drawn to carrion for breeding, suggesting a dead animal, like a rodent or bird, may be present in a wall void or attic.
House flies are less selective, frequently found near garbage, animal manure, or decaying organic matter, and their flight is fast and erratic. The sluggish movement of cluster flies, which enter homes in the autumn to hibernate in attics, contrasts with the agitated behavior of other household flies. Identifying the breeding material is an effective way to confirm the species and locate the source of the infestation.
Differentiating Flies from Look-Alikes
Many other small insects are mistakenly identified as flies. Gnats and midges, while also belonging to Diptera, are often confused with fruit flies, but many gnats have longer, more delicate bodies and legs. Fungus gnats, a common type, typically have dark eyes, unlike the red eyes of the common fruit fly, and are found near moist soil in houseplants rather than fermenting fruit.
Flies that mimic bees or wasps often possess similar black and yellow warning coloration. The distinction is reliably made by counting the wings: true flies have only one pair of forewings and reduced halteres, while bees and wasps have two full pairs of membranous wings. Furthermore, wasps have a distinctive, narrow “waist” connecting the thorax and abdomen, a feature generally absent in most flies. Beetles are differentiated by their forewings, which are hardened into protective covers called elytra, contrasting sharply with the membranous wings of a fly.