The term “gnat” is a colloquial label applied to many different species of small flying insects, making the question of blood attraction complex. Scientifically, “gnat” is not a single classification but a common name for various tiny flies within the suborder Nematocera. The answer depends on the specific insect, as some seek blood while the vast majority do not. Understanding the difference between these groups explains their feeding behaviors and what they are truly seeking.
Defining the Term Gnat
The word “gnat” is a non-specific term often used for any small, annoying fly, causing confusion about their biological function. Entomologically, these insects belong to a broad group of slender-bodied flies with long, segmented antennae. This group includes species like the fungus gnat and the drain fly, which are household pests but do not bite.
The insects that do bite are often misidentified as gnats, such as biting midges (known as “no-see-ums”) and black flies (sometimes called “buffalo gnats”). These biting flies share the small size of non-biting gnats, but their biological imperative is fundamentally different. This distinction is based entirely on their diet and whether they possess the specialized mouthparts necessary for piercing skin and drawing blood.
What Non-Biting Gnats Are Attracted To
Most insects commonly called gnats, such as fungus gnats and drain flies, are attracted to decaying organic matter rather than human or animal hosts. Fungus gnat larvae thrive in the moist soil of overwatered houseplants, feeding on fungi and decomposing plant material. The presence of these gnats indicates high moisture levels or rotting material nearby.
These non-biting species are also drawn to the byproducts of fermentation and sweet, sugary residues. Traps using apple cider vinegar, which mimics the smell of rotting fruit, are effective because of this attraction. Drain flies are specifically attracted to the gelatinous film of organic sludge that builds up inside sink and shower drains. In all these cases, the insects seek a food source or a moist breeding site, not a blood meal.
The Blood Feeders Misidentified as Gnats
The small flies that are attracted to blood are primarily female biting midges and black flies. Their behavior is driven by reproduction, as females require a protein-rich blood meal to develop their eggs, a process known as oogenesis. Male midges and black flies feed exclusively on plant nectar and do not bite.
These blood feeders locate hosts by sensing physical and chemical cues emitted by mammals. The primary long-range attractant is carbon dioxide (\(text{CO}_2\)) exhaled in breath, which they detect from significant distances. Closer to the host, they are guided by body heat and moisture, along with specific chemical compounds in sweat and body odor. Black flies are also attracted to dark-colored clothing. Ultimately, the female seeks the nutritional components within the blood, making the attraction a biological necessity.