**Deer Flies** (genus *Chrysops*) are blood-feeding insects and highly annoying pests of humans and livestock. They belong to the family Tabanidae (horse flies and deer flies). Only the **female** feeds on blood, which is required for egg development. Deer flies are characterized by their painful bite and distinctive banded wings. They are also medically significant as vectors for the organism that causes the parasitic disease **Loiasis** (African eye worm) in Africa, and they can mechanically transmit diseases like anthrax and tularemia.
Taxonomy and Classification
Deer Flies belong to the family Tabanidae in the order Diptera (true flies). They undergo complete metamorphosis. The genus *Chrysops* is distinguished from its close relative, the larger horse fly (*Tabanus*), by its smaller size and the presence of colorful, patterned eyes and dark bands across the wings. They are known as **pool feeders** because they use their mouthparts to slice the skin and lap up the pooling blood, which results in a highly painful bite.
Physical Description
Adult Deer Flies are medium-sized flies, about $1/4$ to $1/2$ inch long. They are generally black or brown with distinct, often brightly colored, iridescent green or gold stripes on their eyes, and a characteristic **dark, zigzag or banded pattern** on their clear wings. Their flight is fast, darting, and highly persistent once they target a host.
The larvae are aquatic or semi-aquatic, whitish-yellow, maggot-like creatures found in mud or moist soil near water.
Distribution and Habitat
Deer Flies are found worldwide, particularly in warm, moist climates. Their breeding habitat is wetlands, marshy edges of ponds, and stream banks, where the larvae live in the saturated soil or mud. The adults are terrestrial, often found near wooded areas, moving water, and in open fields, where they search for suitable hosts (mammals, including deer, livestock, and humans).
Behavior and Life Cycle
Deer Flies typically have one generation per year, with the larval stage often lasting up to 12 months. Females lay masses of up to 1,000 eggs on emergent vegetation near or over water. The larvae drop into the mud and prey on small invertebrates while they develop. Adults emerge in late spring or early summer, and the adult population peaks in mid-summer.
The females are diurnal and are highly attracted to large, dark, moving objects (like a passing human or animal). They are known for their persistence and often repeatedly circle the host before landing and biting, usually attacking the head, neck, and shoulders.
Feeding and Conflict
Only the **female** feeds on blood. The male feeds on nectar and pollen. The bite is extremely painful, often causing an immediate, sharp sting and persistent, itchy wheals due to the insect’s pool-feeding method (slicing the skin and consuming the blood that pools up).
- **Disease Vector:** While rare in North America, they are mechanical vectors for **Tularemia** (Rabbit Fever) and **Anthrax**, transmitting the bacteria from one infected host to a healthy one through blood.
- **Livestock Impact:** In large numbers, feeding can cause significant annoyance, leading to reduced grazing, decreased milk production, and sometimes severe blood loss (anemia) in livestock.
Management and Prevention
Control is challenging due to the flies’ large range and aquatic larvae.
- **Repellents:** Traditional insect repellents containing DEET are often only mildly effective against determined deer flies.
- **Physical Barriers/Trapping:** Wearing light-colored, long-sleeved clothing and a hat is recommended. Specialized **sticky traps** (often black or blue and sticky, as they mimic a target host) placed around property or hats are the most effective way to reduce localized populations.
- **Breeding Site Treatment:** Treating the vast, inaccessible larval breeding sites is usually impractical.
Conservation and Research
Deer Flies are significant pests. Research focuses on developing better attractant traps that exploit their visual and chemical cues, as well as understanding their role in disease ecology, particularly in Africa where they vector Loiasis.