Fly Facts: From Horseflies to Fruit Flies, We've Got You Covered

Enlarge Image Image Gallery Found in Hawaii, the melon fly is not present in the continental United States, but what a beautiful member of the fruit fly family. The melon fly is one of the most important pests with which vegetable growers have to contend. courtesy Scott Bauer/Agricultural Research Service

Wouldn’t harm a fly, huh?! Bet you would if you knew just how unpleasant these insect pests can be. Like mailboxes and roadside weeds, flies come with the territory when you live in the country. And if you have livestock around, your odds of encountering them increase sharply. You can swat at ’em, spray ’em, chase ’em out of the house or hang pest strips in the barn, but you’ll never get rid of ’em. So, from horseflies to fruit flies, you might as well learn a few fly facts and learn to coexist.

Ever wonder why it’s so difficult to hit a fly with a swatter? It’s their incredible eyesight. Flies have compound eyes containing more than 4,000 photo receptors, or lenses, in each eye, allowing them to see in almost any direction at once. Couple that with their ability to switch flying directions in midair, and your poor batting average begins to make sense. 

Entomologists say more than 18,000 fly species are found in North America, with 120,000 species worldwide. They divide flies into five groups, including biting flies, filth flies, small flies, overwintering flies and gnats. 

Biting flies are the vampires of the fly world. The more than 300 species include the black fly, stable fly and deer fly, though the bite of the horsefly is one of the most painful. Horseflies are equipped with scissor-like mandibles that tear and cut. While the males feed on pollen, the females feed on blood to assist in egg development. Left uncontrolled, swarms of deer flies and horseflies can madden horses and cows. In as little as six hours, 20 to 30 of these flies can withdraw almost a third of a pint of blood from their victims, causing cattle to lose as much as 100 pounds of weight in a single season, and reducing milk production by up to 30 percent. 

Biting flies can spread diseases such as conjunctivitis, poliomyelitis, typhoid fever, tuberculosis, anthrax, leprosy, cholera and dysentery. In sub-Saharan Africa, tsetse flies are responsible for transmitting the protozoa that can cause sleeping sickness. According to the World Health Organization, as many as 70 million people are threatened by the disease, with tens of thousands of new cases each year. Black flies are also responsible for causing River Blindness, affecting millions of people in Africa and Latin America.