Post Register July 18, 2013


Without warning, a fiery bite began on my arm. Moving faster than Chuck Norris, I swatted the huge fly before she had finished making a hole in me. I wasn’t so lucky with the one that targeted my scalp in that annoying gap above the adjustment strap of my ball cap. My slap was too late, and all I did was add insult to the painful bite by smacking myself in the head and knocking my hat off.

The offending insects were horseflies, likely named because they can seemingly bite as hard as a horse. They attack utilizing sharp mandibles shaped like knives to tear through flesh so they can feast from the pooling blood. It feels rather like getting stuck with a large red hot needle and can leave an itchy welt for several days. This effective tool can even be deployed through clothing.

Horseflies are members of the true fly or Diptera Order of insects and are first cousins to that other irritating biting fly, the deer fly. They have about 4500 related species worldwide.

Horseflies are large as flies go. They are more than three times the length of a common housefly with robust bodies to match, and clear wings (sometimes black) that fold over the back. The wings alone distinguish them from their evil cousin, the housefly-sized deer fly, whose patterned wings form a triangle when at rest.

If there is a redeeming feature about the adult horsefly, it is its eyes. They occupy most of the outside portion of the insect’s head and can be strikingly beautiful. Most commonly they are an iridescent green with horizontal stripes. Since these are compound eyes, the surface of the eye resembles a dance hall mirror ball.

All biting horseflies are female. The males mind their business sucking nectar from plants and aiding in pollination. Females on the other hand, need a blood meal in order to produce eggs.

For humans, keeping horseflies at bay is a challenge. They live around most bodies of water and are attracted to heat (especially sunshine), carbon dioxide, shiny objects, color, and movement. You may not be able to do much about body heat and CO2, but you can control what you wear. Loose neutral-colored clothing will help, but isn’t a perfect defense. Even insect repellants containing DEET (N-diethyl-meta-toluamide) don’t prevent flies from landing or buzzing annoyingly around you. However, they do deter them from biting. If they find even a small weakness in your defenses though, they will attempt to take advantage.

For animals, protecting themselves from horseflies is an even bigger challenge. Where infestations are high, blood loss to horseflies may exceed half a pint in a single day. To beat the flies, animals seek shade, move to higher ground away from water and choose windy locations, often at the cost of spending less time eating.

Swatting a horsefly before she can make her first cut is a uniquely satisfying outdoor encounter.  After you’ve experienced your first bite, you’ll understand why.