Horse Flies


Nothing says summer like swimming pools and horse flies.

Horse flies, deer flies, gad flies - whatever you like to call them, they’re all part of the same club of big, bulky, biting flies. Mosquitoes, ticks, and bed bugs are subtle when it comes to sneaking a blood meal from you, since getting noticed could mean getting slapped dead. That subtly arises from having small bodies, being light-footed, and secreting painkilling chemicals in their saliva. So, when it comes to being fed on by these bloodsucking bugs, at least we can say it doesn’t hurt during the act of the bite. Horse flies employ a different strategy, abandoning subtly for persistence. It seems to be working out for them, much to the dismay of anything with a blood supply.

Like mosquitoes, only the female bites and takes blood. This high-protein food source is needed for developing the eggs. The male feeds on nectar, as does the female. Horse flies can be important pollinators, but that little fact is obscured by their bloodthirsty habits.

Male horse flies have eyes so large that they touch at the top of the head; the females have eyes that are distinctly separated. In many species, the eyes are magnificently colored and look as if the fly is wearing reflective aviator sunglasses.

Horse flies locate their victims through body heat, carbon dioxide (from exhaled breaths), movement, and contrasting colors. A black rubber ball, dangled from a string and shifting in the wind, will attract these flies, as will a white truck driving through shady woods.

Upon finding a suitable victim, a horse fly will hastily land and drive its spearing mouthparts into the skin. These cutting parts are like steak knives that can be moved up and down to widen the wound and cause bleeding. This can be very painful, as horse flies don’t have analgesics in their saliva like most other bloodsuckers. Their saliva does contain anticoagulants which keeps the blood flowing, instead of clotting. The horse fly then utilizes the spongy part of its mouth to lap up the pooling blood. Swatting at horse flies is usually a fruitless endeavor, as they are very persistent and will keep making passes until they can land, drink, and satiate themselves.

Long sleeves and long pants provide protection from bites, as do repellents. I recommend DEET for skin and permethrin for clothing, because these active ingredients are easy to find and proven to be safe and effective when used according to the label. If you are going into horse fly territory, I highly suggest applying some sort of repellent. Their bites hurt, but they can also transmit diseases like anthrax and tularemia - diseases that you’d be much happier not having.

If you have a swimming pool and like to swim in the summer or early fall, then you may know that horse flies seem to love being by the pool as much as you do. They are attracted to the polarized light reflecting off the pool’s surface. There’s not much you can do to stop that, unless you don’t mind swimming at night, but you can divert their attention with a clever trick. Horse flies are highly attracted to the color blue. I am including a reference from the University of Florida that shows an example of a blue trap that you can make at home (click the link below and scroll down to the bottom). You can also attract horse flies with blue towels (these won’t catch and trap, but takes less effort), so try laying out some blue towels to distract them from bothering you while you swim. I haven’t tried this myself, but it may be worth a try!


Horváth, G; Majer, J; Horváth, L; Szivák, I; Kriska, G. (2008). “Ventral polarization vision in tabanids: horseflies and deerflies (Diptera: Tabanidae) are attracted to horizontally polarized light.” Naturwissenschaften, 95 (11): 1093–1100.

Featured Creature. University of Florida. Department of Entomology and Nematology. Website: Accessed July 2017.