Black flies are small insects often encountered near flowing water environments. These common flies can be a significant presence in outdoor areas, particularly where streams and rivers provide suitable habitats.
Physical Characteristics and Size
Adult black flies measure between 1 and 6 millimeters in length. This places them in a size range similar to or slightly smaller than a common mosquito, but they are often described as being more robust or stockier. Their appearance is characterized by a dark coloration, often black, gray, or even yellow or orange, depending on the species.
A distinctive feature of black flies is their arched thorax, which gives them a somewhat humpbacked profile, leading to their common name, “buffalo gnats.” They possess relatively large compound eyes, short antennae, and broad, fan-shaped wings that lack hairs or scales.
Life Stages and Size Variation
Black flies undergo complete metamorphosis, progressing through four distinct life stages: egg, larva, pupa, and adult. Significant size variation occurs throughout their development. Eggs are laid in or on flowing water and hatch into larvae.
Black fly larvae develop underwater, attaching themselves to submerged objects like rocks or vegetation. Mature larvae can reach lengths of 5 to 15 millimeters, making them larger than the adult flies they will become. The pupal stage, during which metamorphosis occurs, is around 2 to 3 millimeters long. The adult black fly is the final dimension after emerging from the pupal case.
The size of black flies can also vary among the numerous species. There are over 2,200 recognized species of black flies globally, with 255 species identified in North America alone. Differences in water temperature, food availability, and other environmental conditions during larval development can influence the final size of emerging adults.
The Impact of Their Size
The small size of black flies has several practical implications for humans and animals. Their compact bodies allow them to navigate and access small openings, often swarming around the head and face. They can crawl into ears, eyes, and noses, causing considerable discomfort.
Female black flies require a blood meal to produce eggs. Their small mouthparts are adapted for this purpose. They use blade-like structures to cut the skin, creating a small wound from which they feed on pooled blood. The saliva injected during a bite contains substances that prevent blood clotting and can also have a numbing effect, which can delay awareness of the bite.
The small dimensions of black flies contribute to their nuisance factor, especially when they are present in large numbers. Their ability to swarm in dense clouds can make outdoor activities unpleasant. Severe reactions to bites can occur, including swelling, itching, and in rare cases, symptoms like headache, nausea, or fever, sometimes referred to as “black fly fever.” Large swarms can also pose a threat to livestock, potentially causing suffocation or other issues.