Horse flies are a very large family of biting flies. Scientifically, horse flies belong to the family Tabaninae and the order Diptera – True flies. In the subfamily, Tabanidae are both deer flies and horse flies. Both groups are biting flies. The estimated number of species of biting fly worldwide is 4,500 species. 
How to Spot a Horse Fly
Horse flies are big like bumble bees, wasps, and honey bees. They range in size from 3/4 of an inch to 1 1/4 of an inch. They have bristle-like hairs, and their bodies are either all black or a mixture of black and yellow. One of the first things that you notice about horse flies is their very lovely eyes, which are a kaleidoscope of color. If you are close enough to see their eyes, then you might also know their other all-too-common trait – their bite.
Why Do Horse Flies Bite?
Not all horse flies bite. It is only the female. She bites because she needs the high-quality nutrients found in blood to finish her reproductive cycle. A female horse fly lays 100-800 eggs. A few species may lay clutches of eggs 203 times per year, but in general, most female horse flies lays one clutch of eggs per year.
For the female horse fly, reproduction is the biggest event of her life. It also takes a lot of energy to produce and lay 800 eggs. Blood, whether human or animal is a rich food source for her. In short, your blood supplies the energy that helps create the next generation of horse flies.
The Evolution of a Predator
In science, we look at the flow of energy and how organisms change to find the more efficient way to survive while meeting all of their needs to reproduce. Because biting female flies require a blood meal to finish their reproductive cycle, their mouthparts evolved into very efficient tools that meet their needs.
To feed is dangerous. When the female horse fly bites you or an animal, she risks her life and her contribution to the next generation of horse flies. Her evolution is remarkable because it takes the risk of death into account. In short, she is an efficient feeder.
The mouthparts of a female horse fly are a weapon-grade tool. They consist of a pair of scissor-like mandibles which she plunges into flesh. As she does, they open like a pair of scissors or shears, thus expanding the wound. Even before she stabs you, glands begin to secrete an anticoagulant so that your blood flows quickly to the surface.
She can stab you, drink your blood, and fly off in such an efficient way that she usually avoids your hand as it tries to smash her. Her evolution is remarkable, and her bite is quite painful. In fact, a few horse flies can create mass panic in livestock or groups of humans. An infestation of biting flies has no problem clearing a beach of people. In fact, the University of Kentucky’s Entomology Department  estimates that heavy infestations of biting flies can dramatically diminish the production of livestock, such as dairy cattle, beef cattle, and horses.
Despite Modern Technology, Biting Flies Are Still a Pest
Mankind has tried to eradicate biting flies and still, they persist in being a menacing pest. The biggest reason behind our failure to kill off biting flies is that in the early attempts, we failed to understand them enough to deal with them. We thought that they were a species of flies, like the mosquito, which spends part of their lifecycle as aquatic insects. So we poisoned the bogs and wetlands with pesticides hoping to kill them. We did not succeed because horse flies are dependent upon laying their eggs in bogs or streams. In fact, the larvae are found in a variety of aquatic habits including livestock water troughs, hot springs, and small ponds. While most species are purely terrestrial, some have an aquatic phase to their lifecycle. People invented traps to lure in the females and then destroy them. Many of those designs did not work either. These flies are not like scavenger flies – blow flies, flesh flies, and bottle flies. They do not eat the dead, but the living so they do not rely on smell to find food. They rely on sight to track and target their prey, so baited traps did not work to subdue this wretched pest.
A tool that does work is a new type of trap that uses the biting flies method of hunting to lure them in and then destroys them. Biting flies hunt by sight. They look for large, dark objects that move slowly. Think about how deer, cattle, and horses graze. Consider how we mend fences, plow fields, and enjoy the outdoors. We all – cattle, horses, deer, elk, and man – slowly meander while we eat, drink, and be merry. The Fly Cage mimics those slow moving, grazing-like motions to draw in the female horse flies. When she tries to investigate, she becomes trapped and then dies. This new way of controlling biting flies works because it uses the biting flies own evolutionary advantages against them. It does not rely on pesticides or smelly baits. It uses the subtle movement of grazing animals, and it does its job well.