When people refer to “regular flies,” they typically think of the common house fly (Musca domestica) or the smaller fruit fly (Drosophila melanogaster). A common misconception is that all flies bite. However, the ability to bite is not universal among these insects. This article clarifies which types of flies possess the necessary mouthparts and behaviors to inflict a bite, and which do not.
Common Flies: The Non-Biters
Many flies encountered in homes, such as the common house fly and fruit fly, do not possess mouthparts designed for biting. House flies (Musca domestica) lack the physical capability to pierce skin; their mouthparts are adapted for sponging and lapping up liquids. They have a soft, fleshy proboscis with two spongy lobes called labella, which contain tiny grooves. This structure allows them to soak up liquid food by capillary action. If food is solid, a house fly regurgitates saliva and digestive juices onto it, dissolving it into a consumable liquid before sponging it up. While house flies can be a nuisance and transfer bacteria, their landing on skin is typically an attempt to explore or seek moisture, not to bite.
Flies That Do Bite: The Blood Seekers
Several fly species do bite, typically to obtain a blood meal necessary for the female to produce eggs. These biting flies possess specialized mouthparts that are either piercing-sucking or scissor-like, allowing them to cut or puncture skin. When they bite, they often inject saliva containing anticoagulants to prevent blood clotting, which can cause irritation, swelling, or allergic reactions in humans.
Examples of biting flies include stable flies (Stomoxys calcitrans), which closely resemble house flies but have a pointed, bayonet-like proboscis used for piercing skin. Both male and female stable flies feed on blood. Horse flies (Tabanus spp.) are large and inflict painful bites by using blade-like mouthparts to slice into the skin, creating a wound from which they then lap up blood. Deer flies (Chrysops spp.), smaller relatives of horse flies, also use scissor-like mouthparts to cut skin for a blood meal. Black flies (Simulium spp.) are small and hump-backed, using blade-like structures to cut a hole in the skin, allowing blood to pool for feeding.
Telling Them Apart
Distinguishing between non-biting and biting flies often comes down to observable physical characteristics and behavior. Non-biting flies, like house flies, are typically 1/8 to 1/4 inch long, gray, and have four dark stripes on their thorax. Their mouthparts appear soft and spongy, not rigid or pointed. They tend to land softly and explore surfaces without causing any sensation of pain.
Biting flies often have more prominent or distinct mouthparts, visible as a stiff, forward-pointing proboscis or a blade-like structure. They vary significantly in size; horse flies, for instance, can be up to an inch long, while stable flies are similar in size to house flies but possess a noticeable pointed proboscis. Biting flies are generally more aggressive and persistent in their pursuit of a host, often attracted by movement, warmth, carbon dioxide, and dark colors. A distinct, sharp pain or stinging sensation upon landing is a clear indicator of a biting fly, unlike the gentle landing of a house fly.