Black Flies

Black horse flies are large enough to be seen—their deep black color also makes them easy to spot.

Black horse flies are notoriously aggressive, therefore, you will know when they are around. Even if you swat them away, they will return with a vengeance to inflict deep and painful bites on humans and animals.

Black horse flies are all flat-black in color.

Black horse flies are typically about 1/2 to 1½ inches in length.

Black horse flies also have 2 black non-transparent wings that are attached to their shoulders in a wide V-shape.

They have 6 legs and 2 short splayed antennae at the very top of their heads originating between black eyes.

Their frontal abdomen has horizontal black and hairless bands and their dorsal abdomen is covered with black hair.

Horse flies are larger than deer flies. Horse flies have large heads, large eyes, and bodies that are much stouter than deer flies.

Horse flies wings usually have clear or cloudy wings (as opposed to black wings) and deer flies have dark bands or spots on their wings.

Horse flies typically bite humans on the legs or body when not in motion and deer flies attack humans higher on the body usually on the head or neck usually when in motion. 

Studies of black horse flies show no evidence that horse flies transmit diseases to humans, however, investigators have isolated viruses, bacteria, and protozoa from the female mouthparts and their digestive systems.

When black horse flies bite humans, the bites are much more painful than mosquitos because black horse flies use their jaws to rip into human flesh so they can feed on blood. As a result, black horse fly bites can cause rashes, dizziness, wheezing, swelling and infection, however, humans typically recover from black horse fly bites.

While black horse flies have not been known to cause noticeable damage to homes, they do cause damage to livestock and by-products (for example, beef and milk production).

Diseases agents, viruses, and parasites can be transmitted to animals thus causing a variety of symptoms in animals, including lethargy, weight loss, and even death.

Black horse flies are attracted to shiny surfaces, carbon dioxide, warmth, and movement.

Female black horse flies must feed on protein based blood before they are able to reproduce. They typically bite nonmoving mammals on the legs or body.

Male black horse flies eat pollen and plant nectar.

From longer distances, carbon dioxide released from warm-blooded animals attract black horse flies by providing long-range cues.

From shorter distances, black horse flies are attracted by size, shape, dark colors and motion.

Adult male black horse flies usually live a few days. Without food, they survive only about two to three days.

Female black horse flies live longer and into the fall.

Female horse flies need a blood meal in order to reproduce eggs and can lay 100-800 eggs per year. 

Females usually deposit eggs in clusters on wet soil or the undersides of leaves and vegetation overhanging water.

Horse flies overwinter in 1 of 6-13 larval stages, pupate in the spring, and then emerge as adults in late spring or early summer.

Horse flies are located around the world (except the extreme northern and southern latitudes).

There are about 3,000 species of horse flies.

Horse flies can be up to 1 ½ inches in length making them the world’s largest flies.